Chemical

periodic table of chemical elements

Lanthanides

Actinides

Metal

Aluminum and aluminum alloys

Basic concepts

• Aluminum (Al):A lightweight metal with low density and corrosion resistance, commonly used in industrial and civil fields.
• Aluminum alloy:Adding other metals (such as magnesium, copper, zinc, etc.) to aluminum to improve strength, hardness and wear resistance.

Main differences

Classification of aluminum alloys

• Aluminum copper alloy (2XXX series):High strength, suitable for aviation and high-strength structures.
• Aluminum-magnesium alloy (5XXX series):It has good corrosion resistance and is often used in ships and chemical equipment.
• Aluminum-zinc alloy (7XXX series):Extremely strong, used in aerospace and high-performance sports equipment.

How to choose

If the requirements are lightweight, corrosion-resistant, and do not require high strength, aluminum can meet the requirements; if higher mechanical properties are required, aluminum alloys should be selected.

iron

Basic information

Iron (Fe) is a chemical element with the symbol Fe and atomic number 26. It is a transition metal and is the fourth most abundant element in the earth's crust.

physical properties

Pure iron is a silver-white solid with metallic luster, strong magnetism, ductility, and good thermal and electrical conductivity.

naturally occurring

Iron in nature rarely exists in a simple state, and mainly exists in the form of ores, such as hematite (Fe₂O₃), magnetite (Fe₃O₄), etc.

refining

In industry, the blast furnace reduction method is mainly used to refine iron ore into pig iron.

Application and importance

Iron is the cornerstone of modern industry and is primarily used to make steel. By adjusting the carbon content, steel materials with different hardness and toughness can be produced, which are widely used in buildings, bridges, automobiles and various mechanical equipment.

biological function

Iron is an essential trace element for most living things. In the human body, iron is a core component of hemoglobin, responsible for binding oxygen and transporting it to tissues throughout the body. Lack of iron can lead to anemia and affect the body's metabolic efficiency.

steel

definition

Steel is a metal material with iron (Fe) as the main component and containing carbon (C) and other alloying elements. Its carbon content is usually between 0.02% and 2.1%, which affects the hardness and strength of steel.

Classification of steel

Classified by carbon content

• Low carbon steel (mild steel):The carbon content is less than 0.3%, it has good toughness and is suitable for structural parts and pipes.
• Medium carbon steel:The carbon content is 0.3%~0.6%, the strength is high, and it is used in mechanical parts and automobile parts.
• High carbon steel:The carbon content exceeds 0.6% and the hardness is high, making it suitable for knives and springs.

Classification by alloy elements

• Carbon steel:Containing only carbon and a small amount of impurities, it is widely used.
• alloy steel:Contains manganese (Mn), chromium (Cr), nickel (Ni) and other elements to improve strength, wear resistance and corrosion resistance.
• Stainless steel:Containing chromium (Cr) above 10.5%, it has good corrosion resistance and is often used in tableware, construction and medical equipment.

Properties of steel

• High strength, suitable for structural and load-bearing applications.
• It has good plasticity and ductility and can be processed through forging, rolling and welding.
• It has excellent heat resistance and can be used in high temperature environments.
• Some alloy steels and stainless steels have good corrosion resistance.

Application areas

• Construction work:Steel bars, steel structures, bridges.
• Machinery manufacturing:Gears, bearings, cutting tools.
• auto industry:Car body structure, engine parts.
• Home appliances and daily necessities:Kitchenware, stainless steel kettle.

How to choose steel

When selecting steel, the use environment and needs should be considered. For example, stainless steel can be used for corrosion resistance, alloy steel can be used if high strength is required, and carbon steel can be used for general building structures.

Tungsten steel

definition

Tungsten steel (tungsten carbide) is a high-strength alloy material that combines tungsten (W) with carbon (C) to form tungsten carbide (WC) as the main component, and adds cobalt (Co) or nickel (Ni) as a binder. It has extremely high hardness, wear resistance and high temperature resistance.

Characteristics of tungsten steel

• Extremely high hardness:The hardness can reach HRA 89~95, second only to diamond.
• Strong wear resistance:It has extremely high wear resistance and is suitable for high load environments.
• High temperature resistance:It can maintain stable performance in high temperature environments above 1000°C.
• High strength and rigidity:It has excellent pressure resistance and is suitable for high-precision cutting.
• Good corrosion resistance:Not easily oxidized or corroded by chemicals.

Classification of tungsten steel

• YG type (tungsten cobalt type):Contains cobalt as a binder, suitable for metal cutting and cutting tools.
• YT type (tungsten titanium type):Contains titanium (Ti), resistant to high temperatures and suitable for high-speed cutting.
• YW type (universal type):It has good comprehensive performance and is suitable for processing different materials.

Application areas

• Tools and Drills:Used to manufacture high-precision cutting tools, such as tungsten steel blades and drill bits.
• Mold making:Used for high-strength mold materials such as stamping dies and cold extrusion dies.
• Mechanical parts:Such as wear-resistant bearings, valves, turbine blades, etc.
• Military and aerospace:Manufacture of armor-piercing projectiles and jet engine parts.
• Jewelry and Accessories:Because it is scratch-resistant and wear-resistant, it is often used in high-end jewelry such as watch cases and rings.

How to choose tungsten steel products

• If high hardness and wear resistance are required, tungsten steel with lower cobalt content can be selected.
• If impact resistance is required, choose tungsten steel with higher cobalt content.
• For high temperature environments, titanium-containing tungsten steel can be selected to improve heat resistance.

FeS iron sulfide

Basic information

FeS is the chemical formula of ferrous sulfide, an inorganic compound in which iron is in the +2 oxidation state and combined with sulfide ions (S²⁻).

physical properties

Appearance: Usually black, water-insoluble solid.

crystal structure

The nickel arsenide-type structure of the hexagonal crystal system is often used, but there are many allomorphs.

naturally occurring

Exists in mineral forms as stoichiometric troilite (FeS) and non-stoichiometric pyrhotite (Fe₁₋ₓS).

Preparation and reactivity

Preparation: It is often produced by the heating reaction of iron and sulfur, or by the precipitation of iron (II) salts and sulfides.

Reactivity: Reacts with acids to produce hydrogen sulfide gas (H₂S, with a rotten egg smell); easily oxidized.

Note: Iron can form many sulfides, such as pyrite FeS₂ (fool’s gold), but FeS specifically refers to the monosulfide.

gallium

basic properties

Gallium (element symbol Ga) is a silver-white metal that belongs to Group 13 elements. It is characterized by an extremely low melting point but an extremely high boiling point.

• Atomic number: 31
• Melting point: 29.76°C (can melt in hand)
• Boiling point: 2204°C
• Density: 5.91 g/cm³
• It has low hardness, soft texture and can be cut with a knife.

chemical properties

Gallium is stable at room temperature and is not easily oxidized, but it will react with oxygen, sulfur, halogen, etc. when heated.

• Reacts with oxygen to form gallium oxide (Ga₂O₃）。
• Can react with hydrochloric acid to produce hydrogen:
• 2Ga + 6HCl → 2GaCl₃ + 3H₂↑

Physical characteristics

• It has a small volume in the solid state and expands in volume when melted (opposite to water).
• Can "wet" glass and most metals, leaving traces behind.
• Good thermal and electrical conductivity.

Application areas

• Electronic industry:Manufacture of semiconductor materials, such as gallium arsenide (GaAs) and gallium nitride (GaN), for use in light-emitting diodes (LEDs), solar cells, high-frequency communication devices, etc.
• Medical applications:Gallium compounds can be used in research into antibacterial and cancer treatments.
• Temperature indicator:Because the melting point is close to human body temperature, it is often used to make melting point indicators.

Security and storage

• Metal gallium has low toxicity, but its compounds (such as GaAs) may be toxic and need to be handled with care.
• Avoid contact with metals such as aluminum during storage, as it will penetrate into the metal and damage the structure.

germanium

basic properties

Germanium (element symbol Ge) is a gray-white, shiny metalloid that belongs to the 14th group of elements, the same group as silicon and tin.

• Atomic number: 32
• Melting point: 938.3°C
• Boiling point: 2820°C
• Density: 5.32 g/cm³
• Semiconductors have obvious properties and are important materials in early electronic technology.

chemical properties

Germanium is stable at room temperature and does not easily react with oxygen in the air, but it can be oxidized at high temperatures.

• When heated, it reacts with oxygen to form germanium dioxide (GeO₂).
• Can directly combine with halogens (such as chlorine and bromine).
• Will be slowly dissolved in concentrated nitric acid.

Physical characteristics

• It has good infrared transparency and can be used in optical lenses.
• Pure germanium is a semiconductor material with good electrical properties.
• It is a hard and brittle solid at room temperature.

Application areas

• Semiconductor industry:Germanium was the main material for early transistors and diodes, and is now mostly used in high-speed electronic components and optoelectronic components.
• Optical device:Germanium has good infrared light transmittance and is widely used in infrared lenses, optical fibers and night vision devices.
• Solar technology:Germanium is used as a base material in high-efficiency multi-junction solar cells.
• Alloy additives:Germanium can be used to strengthen the properties of metal alloys such as aluminum and silver.

Security and storage

• Metal germanium itself has low toxicity, but some germanium compounds (such as organic germanium) may be toxic to the liver and kidneys, and long-term intake should be avoided.
• It should be kept dry during storage and avoid contact with high temperatures and strong oxidants.

chemical reaction

definition

A chemical reaction is a process in which atoms or molecules between substances rearrange to produce one or more new substances. During a reaction, the atoms themselves don't change, but the way they combine changes, forming new chemical bonds.

reaction characteristics

• generate new substances
• Accompanied by energy changes (exothermic or endothermic)
• Gas, precipitation, color changes, temperature changes, etc. may occur

reaction type

• Synthetic reaction:Two or more substances combine to form a new substance
  Example: 2H₂ + O₂ → 2H₂O
• Decomposition reaction:The decomposition of one substance into two or more substances
  Example: 2H₂O₂ → 2H₂O + O₂
• Single displacement reaction:An element replaces a part of a compound
  Example: Zn + 2HCl → ZnCl₂ + H₂
• Double displacement reaction:Two compounds exchange ions to create a new compound
  Example: AgNO₃ + NaCl → AgCl↓ + NaNO₃
• Combustion reaction:Substances react violently with oxygen and release energy
  Example: CH₄ + 2O₂ → CO₂ + 2H₂O

energy changes

• Exothermic reaction:The reaction process releases heat energy (such as combustion)
• Endothermic reaction:The reaction process absorbs heat energy (such as electrolysis of water)

Factors affecting reaction rate

• Temperature (increasing the temperature speeds up the reaction)
• Concentration or pressure (increasing concentration or gas pressure usually speeds up reactions)
• Surface area (the finer the solid, the faster the reaction)
• Catalyst (can speed up or slow down the reaction rate)

Chemical reaction formula

Use chemical symbols to represent the changes between reactants and products, and must abide bylaw of conservation of mass, that is, the type and number of atoms before and after the reaction are the same.

Example: C + O₂ → CO₂

Practical application

• metal smelting
• Food Preservation and Cooking
• pharmaceutical manufacturing
• Energy generation (e.g. fuel combustion, battery reactions)

Principles and Applications of Catalysis

definition

Catalysis refers tocatalystTo change the rate of a chemical reaction (usually to speed it up) whileitself is not consumedphenomenon. Catalysts can lower the activation energy so that the reaction proceeds at lower energy.

Classification of catalysis

• Positive catalysis:Accelerates reaction rates and is the most common form of catalysis.
• Negative catalysis (inhibition):Reduce reaction rate.

Classification according to form of existence

• Homogeneous catalysis:The catalyst and reactants are in the same phase (as a gas or liquid) and the reaction proceeds uniformly.
• Heterogeneous catalysis:The catalyst and reactants are in different phases (e.g. solid catalyst and gaseous reactants) and the reaction occurs at the surface.

Catalytic mechanism

Catalysts provide aLower energy intermediate processes, thus accelerating the reaction.

For example, in a catalytic reaction:

1. Reactants combine with catalysts to form intermediates.
2. Intermediates are converted into products, simultaneously releasing the catalyst.

Common examples

• Ammonia production reaction (Haber process):Iron acts as a catalyst to promote N₂ + 3H₂ → 2NH₃.
• Decompose hydrogen peroxide:Manganese dioxide (MnO₂) catalyzes H₂O₂ → H₂O + O₂.
• Automobile catalytic converter:Platinum, palladium, rhodium, etc. catalyze the conversion of CO, NO and hydrocarbons.

Industrial and daily life applications

• Petrochemical industry (such as pyrolysis and isomerization reactions)
• Environmental protection technology (automobile exhaust treatment, industrial waste gas denitrification)
• Food industry (enzymes as biocatalysts)
• Pharmaceutical industry (selective synthesis, enzyme-catalyzed reactions)

biocatalysis

Catalysts in living organisms are calledenzyme (enzyme), has high selectivity and efficiency, and can carry out complex reactions under mild conditions.

in conclusion

Catalysis is an indispensable core technology in chemical and industrial reactions. It not only improves efficiency, but also contributes to energy conservation, emission reduction and environmental protection.

Acidity and alkalinity

Acids and bases

definition

• Arrhenius definition:Acids are substances that release H⁺ in water, and bases are substances that release OH⁻.
• Brønsted–Lowry definition:Acids are proton (H⁺) donors and bases are proton acceptors.
• Lewis definition (Lewis):Acids are electron pair acceptors and bases are electron pair donors.

Properties of acids and bases

• Sour tastes sour, while alkali tastes bitter; acid can turn blue litmus red, and alkali can turn red litmus blue.
• Acid and alkali can neutralize to form salt and water.
• Strong acids and strong bases are almost completely ionized in water; weak acids and weak bases are partially ionized.

Common examples

Acid-base strength and pH

• pH value:is the negative logarithm of the hydrogen ion concentration, pH = -log[H⁺].
• pH < 7 is acidic, pH = 7 is neutral, and pH > 7 is alkaline.
• Ka (acidity constant) and Kb (basicity constant):Used to express the strength of an acid or base.

Acid-base reaction

• Neutralization reaction:acid + base → salt + water
• Acids react with metals:Produce hydrogen
• Reaction of acids and carbonates:produce carbon dioxide gas

application

• Industry: Fertilizer making, detergents, electroplating, paper making
• Biology: Gastric acid (HCl), blood buffer system (H₂CO₃/HCO₃⁻)
• Daily life: vinegar, digestive medicine, soap, detergent

Acidic solutions are often used as cleaning agents

Dissolve inorganic dirt

Acids react chemically with inorganic substances, converting them into soluble compounds that can be easily removed.

• Remove scale:Scale is mainly composed of calcium carbonate (CaCO₃), which can be dissolved by acid, for example:
  CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑
• Remove rust:Rust (Fe₂O₃) can react with hydrochloric acid to form soluble ferric chloride:
  Fe₂O₃ + 6HCl → 2FeCl₃ + 3H₂O

Break down fats and organic matter

Certain acids can break down the molecular structure of oil, making it emulsified and easier to rinse away.

• Acetic acid (vinegar) removes oil and soap scum.
• Citric acid cleans calcium deposits in your coffee machine.

Sterilization and disinfection

Acidic environments can inhibit the growth of bacteria and mold, so acidic solutions are often used for disinfection.

• Acetic acid can effectively kill E. coli and salmonella.
• Dilute hydrochloric acid can be used to sterilize medical equipment.

Enhance surface finish

Acid can remove the oxide layer and mineral deposits on the surface and restore its luster.

• Citric acid can be used to clean glass and make it more translucent.
• Oxalic acid can clean stainless steel and marble surfaces.

Application scope

Acidic cleaners are widely used in domestic and industrial areas.

• Bathroom cleaner (removes limescale and soap scum).
• Rust remover (dissolves rust and oxide layers).
• Coffee machine descaler (removes calcium deposits).

Phosphoric acid

chemical properties

Phosphoric acid (H₃PO₄) is a moderately strong tribasic acid containing three ionizable hydrogen atoms in the molecule. It can release three protons respectively to form dihydrogen phosphate (H₂PO₄⁻), hydrogen phosphate (HPO₄²⁻) and phosphate (PO₄³⁻).

physical properties

Pure phosphoric acid is a colorless, odorless, viscous liquid or crystal, easily soluble in water, and hygroscopic. Common industrial phosphoric acid is an aqueous solution with a concentration of about 85%.

Preparation method

Phosphoric acid can be produced by two main methods:

• Thermal method: Burning white phosphorus produces phosphorus pentoxide, which then reacts with water to produce phosphoric acid.
• Wet method: reacts sulfuric acid with phosphate rock to produce phosphoric acid and by-product gypsum, which is suitable for the fertilizer industry.

use

Phosphoric acid is used in a variety of applications, including:

• Food additives: as sour agents and preservatives (common in carbonated drinks)
• Fertilizer raw materials: used to make fertilizers such as potassium dihydrogen phosphate and superphosphate.
• Surface treatment: used for rust removal and anti-rust treatment of metals
• Medicine and dentistry: used in the preparation of dental adhesives and cleaning agents

safety and environment

Although phosphoric acid is less toxic, concentrated phosphoric acid is corrosive and contact with skin or eyes should be avoided. Excessive use of phosphate fertilizers may lead to eutrophication of water bodies and affect the ecosystem.

Acetic acid removes oil and soap scum

The nature of oil pollution and the role of acetic acid

Oil stains are mainly composed of fats, oils (fatty acid esters) and other organic matter, usuallynon-polarSubstance that is difficult to dissolve in water.

Acetic acid (CH₃COOH) ispolarityWeak acid can remove oil stains through the following mechanisms:

• Emulsification:Acetic acid can change the properties of the grease surface, making it form an emulsion with water, making it easy to rinse.
• Acidolysis:Acetic acid can undergo a hydrolysis reaction with certain fats and oils to produce water-soluble fatty acids and glycerin, such as:
  (C₁₇H₃₅COO)₃C₃H₅ + 3CH₃COOH → 3C₁₇H₃₅COOH + C₃H₈O₃

Reaction of soap scum components with acetic acid

Soap scum is mainly composed ofinsoluble fatty acid salts(such as calcium soap, magnesium soap), these compounds are formed after reacting with soap in a hard water environment.

Acetic acid can undergo an acid-base neutralization reaction with these insoluble compounds, converting them into water-soluble substances for easy flushing.

• Calcium soap removal:Acetic acid reacts with calcium soap (such as calcium stearate) in hard water to form soluble calcium acetate:
  2CH₃COOH + Ca(C₁₇H₃₅COO)₂ → Ca(CH₃COO)₂ + 2C₁₇H₃₅COOH
• Magnesium Soap Removal:Similarly, acetic acid converts magnesium soap into magnesium acetate, making it soluble in water.

Enhanced cleaning effect

The decontamination ability of acetic acid can be further enhanced. If used in conjunction with hot water or other surfactants (such as dish soap), it can more effectively break down oil stains and soap scum.

Sodium dehydroacetate

chemical properties

Main purpose

• Food preservation:Used in pastries, soy products, pickles, jams, etc., it can effectively inhibit mold, yeast and bacteria.
• Daily chemical products:Added to shampoo, skin care products, toothpaste, etc. to extend shelf life and prevent microbial contamination.
• Medical use:Used in medicines, antiseptic wipes, eye drops and other products.

Regulations and usage regulations

• China:It is included in the "National Food Safety Standards - Standards for the Use of Food Additives" (GB 2760) and can be used as a preservative, with the limit usually being 0.3~0.5g/kg.
• European Union:It is not included in the list of food additives approved by the European Union and is not allowed to be used as a food additive.
• USA:The U.S. Food and Drug Administration (FDA) does not allow its use as a direct food additive, but only as an indirect additive (such as antibacterial ingredients in packaging materials).
• Japan:It is prohibited to be used as a food additive.

Carcinogenicity and safety

Basic information

• English name: Sodium dehydroacetate
• Chemical formula: C₈H₇NaO₄
• Molecular weight: 190.13
• CAS number: 4418-26-2
• Toxicity Class: LD₅₀(Rat, oral) approximately 2000 mg/kg

organic chemistry

Exploring the science of carbon compounds

hydrocarbon

Overview

Hydrocarbons are organic compounds composed of two elements: carbon (C) and hydrogen (H). They are one of the most basic compounds in organic chemistry. Hydrocarbons are classified into several types based on the type of bonding between carbon atoms.

Classification

Hydrocarbons are mainly divided into the following three categories based on structure and bonding:

• Saturated hydrocarbons:Contains only single bonds, such as alkanes (methane, ethane).
• Unsaturated hydrocarbons:Contains double or triple bonds, such as alkenes (ethylene) and alkynes (acetylene).
• Aromatic hydrocarbons:It has a conjugated π bond cyclic structure, such as benzene and toluene.

nature

The properties of hydrocarbons vary depending on their structure:

• Low molecular weight hydrocarbons are usually gases (such as methane, ethane), while high molecular weight hydrocarbons are mostly liquids or solids.
• Most hydrocarbons are insoluble in water but soluble in organic solvents.
• When burned, carbon dioxide and water are produced and a large amount of energy is released.

use

Hydrocarbons have a wide range of uses in industry and daily life:

• As fuel: such as natural gas, gasoline, kerosene.
• As chemical raw materials: preparation of plastics, rubber, synthetic fibers, etc.
• Used to synthesize other organic compounds: such as alcohols, ketones, and acids.

alkyl

definition

Alkanes are a type of saturated hydrocarbon compounds whose molecules contain only carbon-carbon single bonds (C–C) and carbon-hydrogen single bonds (C–H). Its general formula is C_nH_2n+2, the simplest alkane is methane (CH₄）。

Classification

• linear alkane: Carbon atoms are connected in an unbranched manner, such as ethane, propane.
• Branched alkanes: Carbon atoms form branches, such as isobutane.
• Naphthene: Carbon atoms form a ring structure, such as cyclopentane.

physical properties

• Low molecular weight alkanes are gases (such as methane, ethane), medium molecular weight alkanes are liquids (such as hexane), and high molecular weight alkanes are solid (such as paraffin).
• Insoluble in water, but soluble in organic solvents.
• Melting and boiling points increase with increasing molecular weight.

chemical properties

• The chemical properties are relatively stable, but can react under high temperature or catalytic conditions.
• Substitution reactions with halogens, such as chlorination, produce chloroalkanes.
• Under sufficient oxygen supply, it can be completely burned to produce carbon dioxide and water.
• It can be cracked to produce small molecular hydrocarbons, which can be used in petroleum cracking.

application

• Methane is the main component of natural gas and is used as fuel and to make hydrogen.
• Liquid alkanes such as hexane and octane are used as solvents and fuels.
• Paraffin wax is used to make candles, insulation materials and waterproof coatings.

Methane

chemical properties

Methane (CH₄) is the simplest alkane, colorless and odorless, a non-polar molecule, insoluble in water but soluble in organic solvents. When burned, carbon dioxide and water are produced, and a large amount of heat energy is released.

physical properties

Under standard conditions, methane is a colorless and odorless gas, less dense than air, with a melting point of about -182°C and a boiling point of about -161.5°C.

source

The main sources of methane include natural gas, decomposition of animal and plant organic matter, biogas fermentation and biological metabolic processes. The main industrial source is natural gas extraction.

use

Methane is widely used in the energy, chemical industry and fuel fields. It can be used for household gas, power generation, production of methanol, hydrogen and synthesis gas, etc.

environmental impact

Methane is a potent greenhouse gas that contributes far more to climate change than carbon dioxide. Reducing methane emissions is of great significance to environmental protection.

benzene

Basic properties of benzene

Benzene is a colorless, sweet-tasting and highly volatile liquid. Its chemical formula is C₆H₆, is the simplest aromatic hydrocarbon.

• Molecular structure: The benzene molecule is ring-shaped, with six carbon atoms forming a conjugated π bond structure.
• Melting point: 5.5°C
• Boiling point: 80.1°C
• Density: 0.8765 g/cm³
• Hardly soluble in water, but easily soluble in organic solvents.

Source of benzene

Benzene is mainly obtained through the following pathways:

• Petrochemical: Extracted from petroleum cracking gas or catalytic reforming.
• Coal chemical industry: obtained by fractionation of coal tar.

Uses of benzene

Benzene has a wide range of uses in the chemical industry:

• Synthetic raw materials: used to produce chemicals such as phenol, aniline, and styrene.
• Solvent: As an industrial solvent, it is used in products such as paints, glues, and cleaners.
• Fuel additive: once used in gasoline to increase the octane number (now restricted in most countries).

health and environmental impact

Benzene is potentially harmful to humans and the environment:

• Effects on health: Long-term exposure to benzene may cause blood diseases such as leukemia; inhaling high concentrations of benzene can cause headaches, dizziness and even poisoning.
• Environmental pollution: Benzene is volatile, easily causes air pollution, and may seep into groundwater sources.

Safe handling and storage

The following should be noted when handling and storing benzene:

• Store in a cool, well-ventilated place away from sources of fire.
• Protective equipment such as gloves and respirators should be worn during operation.
• Avoid direct contact or prolonged exposure to benzene.

ene

definition

Alkenes are a class of unsaturated hydrocarbon compounds containing carbon-carbon double bonds (C=C). Its general formula is C_nH_2n, the simplest alkene is ethylene (C₂H₄）。

Classification

• Linear alkenes: The carbon chain is not branched, such as propylene.
• Branched alkenes: Carbon chain with branches, such as isobutylene.
• Cyclene: Forms a cyclic structure, such as cyclopentene.

physical properties

• Low molecular weight alkenes are gases or liquids that are colorless and have a slight odor.
• Insoluble in water, but soluble in organic solvents.
• As the molecular weight increases, the melting point and boiling point increase.

chemical properties

• Alkenes are highly reactive due to their carbon-carbon double bonds.
• Can perform addition reactions, such as with hydrogen, halogens and water.
• Oxidation reactions can occur to produce epoxides or carboxylic acids.
• The arrangement of double bonds can cause cis-trans isomerism.

application

• Ethylene is an important raw material for plastics (such as polyethylene) and other chemicals.
• Propylene is used to make polypropylene, isopropyl alcohol and acrylonitrile.
• Cyclolefins are used in the synthesis of specialty polymers and drugs.

Styrene

chemical properties

Physical properties (common values)

• Molecular weight: 104.15
• Appearance: Colorless to light yellow oily liquid
• Boiling point: approx. 145°C
• Melting point: about −30°C
• Density: approx. 0.90–0.91 g/cm³ (20°C)
• Flash point: approximately 31°C (closed cup)
• CAS number: 100-42-5

Main purpose

• Polymerized into plastics and synthetic resins such as polystyrene (PS), styrene-butadiene-styrene copolymer (SBS), and ABS.
• Monomer raw materials for the manufacture of synthetic rubber, coatings, adhesives, latex (latex produced by emulsion polymerization) and engineering plastics.
• Used in the manufacture of insulation materials, packaging materials and various industrial intermediates.

Reactivity and storage

• Free radical polymerization is prone to occur; in order to prevent self-polymerization, inhibitors (such as tert-butyl catechol, etc.) are often added during storage and transportation.
• It is flammable in case of fire. Avoid fire sources, high temperatures and oxidants, and use explosion-proof ventilation equipment.

Toxicity and health risks

• Short-term exposure to high concentrations can irritate the eyes, nose, throat and respiratory tract, and may cause symptoms such as headache, dizziness, drowsiness and other central nervous system depression symptoms.
• Prolonged or repeated exposure has been associated with reports of hearing impairment, neurobehavioral changes, and systemic effects; skin contact may cause dryness or dermatitis.
• Carcinogenicity: The International Agency for Research on Cancer (IARC) has classified styrene as "possibly carcinogenic to humans" or "carcinogenic" in past assessments (depending on the year of assessment and data). In other words, most major agencies consider potential carcinogenic risks, with special concerns regarding occupational exposure and long-term low-dose exposure.
• Environment: Styrene is a volatile organic compound (VOCs) that contributes to air quality and ozone precursors, and its emissions need to be controlled.

Security Protection (Practical Focus)

• There should be good local exhaust or ventilation systems in the process and processing areas, and explosion-proof electrical equipment should be used.
• Wear appropriate personal protective equipment (protective gloves, protective glasses and respiratory protection, choose the type of mask depending on the exposure concentration).
• Avoid open flames, sparks and heat sources; storage containers must be sealed and placed in a cool and ventilated place, away from acids, oxidants and other incompatible substances.
• When handling spills, use absorbent materials for recovery and dispose in accordance with hazardous waste regulations.

Regulations and Occupational Exposure

• Countries have different regulations and standards for the use, emissions and occupational exposure of styrene (such as occupational safety exposure limits, environmental emission standards, etc.).
• Occupational exposure limits and environmental monitoring indicators vary by country/institution. In practice, local labor safety and environmental protection authority regulations should be followed, and a monitoring plan should be established to ensure worker safety.

Basic information

• English name: Styrene (or Ethenylbenzene, Vinylbenzene)
• Chemical formula: C₈H₈（C₆H₅-CH=CH₂）
• Molecular weight: 104.15
• CAS number: 100-42-5

Terpenes

definition

Terpenes are a class of isoprene units (C₅H₈) composed of organic compounds. They are found widely in plants and certain insects and often have a fragrant odor.

Classification

• monoterpenes: Consists of two isoprene units, such as eucalyptol.
• sesquiterpene: Consists of three isoprene units, such as farnesol.
• diterpenes: Consists of four isoprene units, such as vitamin A.
• Triterpenes: Consists of six isoprene units, such as squalene.
• Tetraterpenes: Consists of eight isoprene units, such as rubber.

physical properties

• Usually a volatile liquid with a strong odor.
• Insoluble in water, but soluble in organic solvents.

chemical properties

• Contains unsaturated bonds and is prone to addition reactions and oxidation reactions.
• The cyclization reaction can be carried out under acid or high temperature.

application

• Used in the manufacture of essential oils, perfumes and fragrances.
• Used in medicines as an antioxidant and anti-inflammatory agent.
• As biofuels and industrial solvents.

Terpene molecule 3D structure diagram

Taking terpinene α-Pinene as an example

Alkyne

definition

Alkynes are a class of unsaturated hydrocarbon compounds containing carbon-carbon triple bonds (C≡C). Its general formula is C_nH_2n-2, the simplest alkyne is acetylene (C₂H₂）。

Classification

• terminal alkyne: The triple bond is at the end of the molecule, such as 1-butyne.
• internal alkyne: The triple bond is in the middle of the carbon chain, such as 2-butyne.

physical properties

• Low molecular weight alkynes are usually gases or volatile liquids with a slight odor.
• Insoluble in water, but soluble in organic solvents.
• Triple bonds give the molecule a higher bond energy and linear structure.

chemical properties

• The π bond among the three bonds is easily broken and can undergo addition reactions, such as with hydrogen, halogen and water.
• Terminal alkynes are acidic and can react with strong bases to form alkyne anions.
• Polymerization reactions can be carried out under catalytic conditions to generate complex structures.

application

• Acetylene is used for metal cutting and welding because it produces high temperatures when burned.
• Alkyne compounds are important raw materials for the synthesis of organic chemicals such as plastics, rubber and pharmaceuticals.
• Certain alkyne compounds have anticancer or antibacterial activity in medicine.

alcohol

definition

Alcohols are a class of organic compounds that have one or more hydroxyl groups (–OH) attached directly to a carbon atom. Usually, the general formula for alcohol isR–OH, where R is alkyl or aryl.

Classification

• Monohydric alcohol: Contains only one hydroxyl group, such as methanol and ethanol.
• polyol: Contains multiple hydroxyl groups, such as glycerol (glycerol).
• fatty alcohol: Contains saturated or unsaturated long-chain alkyl groups, such as cetyl alcohol.

physical properties

• It is polar and its miscibility with water decreases as the molecular weight increases.
• The boiling point is higher than the corresponding alkanes because the hydroxyl groups form hydrogen bonds.

chemical properties

• Can be oxidized to aldehydes, ketones or carboxylic acids.
• Reacts with acidic substances to form esters.
• Dehydration can produce alkenes.

application

• As a solvent, used in industry and laboratories.
• Ethanol is a common ingredient in beverages and is also used in medicine and fuel.
• Polyols are used in cosmetics and food additives.

phenol

definition

Phenols are organic compounds whose molecules contain one or more hydroxyl groups (–OH) directly attached to the carbon atoms of an aromatic ring. The simplest phenol is phenol (C₆H₅OH）。

Classification

• Monophenol: Contains only one hydroxyl group per molecule, such as phenol.
• Polyphenols: Containing multiple hydroxyl groups per molecule, such as catechol and phloroglucinol.

physical properties

• Most phenols are crystalline solids with a distinctive odor.
• Solubility varies with molecular structure, and low molecular weight phenols are soluble in water.

chemical properties

• It is acidic and can react with alkali to form phenoxide.
• The presence of hydroxyl groups makes it prone to oxidation and nitration reactions.
• Phenol can enhance the electrophilic substitution reaction activity of aromatic rings through resonance structures.

application

• Used in the manufacture of plastics (such as phenolic resin) and adhesives.
• Phenol is an important raw material in the manufacture of drugs such as aspirin.
• Polyphenols have antioxidant properties and are widely used in food and cosmetics.

ether

definition

Ethers are organic compounds that contain an oxygen atom connecting two alkyl or aryl groups. Its general formula is R–O–R', where R and R' can be the same or different.

Classification

• Symmetric ether: Two groups are the same, such as diethyl ether.
• Asymmetric ether: Two different groups, such as methyl ethyl ether.

physical properties

• Usually a volatile liquid with a sweet or pungent odor.
• The boiling point is lower than that of the corresponding alcohol because of the lack of hydrogen bonding.
• Insoluble in water, but low molecular weight ether has certain water solubility.

chemical properties

• The chemical properties are relatively stable and do not react with acids or bases.
• It may react with oxygen to form peroxide under high temperature or light.
• Ring-opening reactions (epoxides) can be performed under strong acid catalysis.

application

• Used as a solvent, suitable for oils, resins and organic reactions.
• Diethyl ether is a type of anesthetic that has been widely used in medicine historically.
• Cyclic ethers (such as ethylene oxide) are intermediates in the synthesis of plastics and other organic compounds.

Ethylene oxide

Structure and properties

Ethylene oxide is an organic compound with a three-membered ring structure, with the molecular formula C₂H₄O, consisting of two carbon atoms and one oxygen atom forming an extremely tense ring structure. It is a colorless, sweet-smelling flammable gas that is gaseous at room temperature.

Due to its extremely high cyclic tension, it is highly reactive and is an important intermediate in many organic synthesis reactions.

application

• Disinfection and sterilization:Ethylene oxide can be sterilized at low temperatures and is widely used in medical equipment, plastic supplies and other equipment that cannot be sterilized at high temperatures.
• Chemical raw materials:It is an important raw material for the manufacture of ethylene glycol, surfactants, polyethers and other organic compounds.

security

Ethylene oxide is toxic and carcinogenic. Inhalation may cause effects on the nervous system and respiratory system, and long-term exposure may increase the risk of cancer. It is also highly flammable and can form explosive mixtures with air, so its use and storage must be strictly controlled.

Nitromethane

chemical properties

use

• Fuel additives:It is widely used in high-performance internal combustion engine fuels such as model aircraft, racing cars and drones to improve combustion efficiency and power output.
• Organic synthetic raw materials:Used to prepare intermediates such as nitro compounds, amines, and isocyanates.
• Solvent usage:It can be used as a solvent for cellulose ethers, polymers, fluorescent dyes, etc.
• Explosive ingredients:Mixed with other ingredients to create liquid explosives such as PLX, it is not considered a highly sensitive explosive by itself.

Toxicity and harm

Safety and handling

• Store in a cool, ventilated place away from fire sources.
• Avoid contact with strong bases, strong acids, reducing agents or oxidizing agents.
• Wear protective gloves and goggles when using, and ensure good ventilation.

regulations and environment

Basic information

• English name: Nitromethane
• Chemical formula: CH₃NO₂
• Molecular weight: 61.04
• CAS number: 75-52-5
• Boiling point: approx. 101°C
• Flash point: approximately 35°C (closed cup)
• Toxicity: LD₅₀(Rat, oral) approximately 940 mg/kg

Nitrogen-containing heterocycle

definition

Nitrogen-containing heterocycles refer to organic molecules containing one or more nitrogen atoms in cyclic compounds. Such compounds are widely found in natural substances, drugs and functional materials.

Classification

Depending on the size of the ring and the number of nitrogen, nitrogen-containing heterocycles can be divided into various types:

• Five-membered heterocycle: such as Pyrrole, Imidazole
• Six-membered heterocyclic ring: such as pyridine (Pyridine), piperidine (Piperidine)
• Polynitrogen heterocycles: such as Triazole, Tetrazole

nature

Nitrogen-containing heterocycles are often alkaline, can form salts with acids, and participate in hydrogen bonding, electron transfer and other functions in biological systems. Its chemical activity depends on the position and electron distribution of nitrogen atoms.

Biological and medical applications

Many biological molecules such as nucleic acid bases (adenine, guanine) and vitamins (such as nicotinic acid) contain nitrogen heterocycles. In drug development, such as antibiotics, anticancer drugs, antiviral drugs, etc., nitrogen-containing heterocyclic structures are also common.

industrial use

Nitrogen-containing heterocycles are widely used in the development of dyes, pesticides, polymers and catalysts, and are important basic chemical units for functional materials and high-tech products.

Tetrazole

Structure and properties

Tetrazole is a heterocyclic compound containing a five-membered ring, consisting of one carbon atom and four nitrogen atoms, with the molecular formula CH₂N₄. Its structure is similar to aromatic compounds and has resonance stability. Depending on the position of the nitrogen atom, tetrazole can be divided into multiple isomeric forms, the most common of which is1H-tetrazoleand2H-Tetrazole。

chemical properties

Tetrazole has carboxylic acid-like acidity (pKa ~4.5–5), can form hydrogen bonds, and can resonantly stabilize negative charges. This makes tetrazole often used as the bioequivalent of carboxylic acid functional groups in drug molecules to improve metabolic stability or bioavailability.

Application areas

• Medicinal Chemistry:Tetrazole is often used as a functional component of antihypertensive drugs such as losartan.
• High energy materials:The high nitrogen content makes it useful in explosives, rocket propellants, etc.
• Coordination Chemistry:The nitrogen atoms in the ring can form stable coordination bonds with metals.

safety and properties

Some tetrazole derivatives are thermally unstable and explosive and must be handled with care. Pure tetrazole itself is a white or light yellow solid, has certain water solubility, and is sensitive to light and heat.

Piperidine

chemical structure

Piperidine is an organic compound containing a six-membered saturated nitrogen-containing heterocycle with the molecular formula C₅H₁₁N. Its structure is similar to cyclohexane, with one carbon atom replaced by a nitrogen atom.

physical properties

Piperidine is a colorless liquid at room temperature with a strong ammonia smell. It is miscible with polar solvents such as water and ethanol. Its boiling point is about 106°C and it is weakly alkaline.

use

Piperidine is commonly used in the pharmaceutical industry and organic synthesis, and is an intermediate for a variety of drugs (such as pethidine, fentanyl, etc.). It can also be used as a catalyst, solvent or chemical reagent.

Application in medicine

The structure of piperidine is an important backbone for many opioids, antidepressants, and antipsychotics, and plays a key role in the synthesis of phenylpiperidines.

Toxicity and safety

Piperidine is an irritant and may cause discomfort if inhaled or in contact with skin. Wear appropriate protective equipment and follow chemical safety practices during use and storage.

Phenylpiperidines

chemical structure

Phenylpiperidines are compounds containing benzene ring and piperidine ring structures, usually composed of a phenyl-substituted piperidine skeleton. This structure gives it high activity on the central nervous system.

Pharmacological effects

Phenylpiperidines are mostly opioid drugs that mainly act on μ-opioid receptors and have analgesic, sedative and respiratory depression effects. Some compounds may also act as antispasmodics or sedatives.

Representative medicine

Representatives of this type of drugs include fentanyl, pethidine (also known as pethidine) and alfentanil, etc., all of which are powerful analgesics and are commonly used for severe pain control in medical treatment.

Purpose and application

Phenylpiperidine drugs are widely used in surgical anesthesia, cancer pain control and acute severe pain treatment. Due to their high potency, they are often designed as controlled-release dosage forms to reduce the risk of abuse.

Risk of abuse

Due to their powerful effects on the nervous system, phenylpiperdine drugs are highly addictive and overdose may lead to respiratory depression and death, so they are highly controlled drugs.

triazine ring

Structure definition

Triazine ring is a nitrogen-containing six-membered aromatic ring compound with the molecular formula C₃H₃N₃. It consists of three carbon atoms and three nitrogen atoms arranged alternately. The structure is similar to a benzene ring, but some carbon atoms are replaced by nitrogen.

three isomeric forms

• 1,2,3-Triazine:Three nitrogen atoms arranged in a row
• 1,2,4-Triazine:Nitrogen is located at positions 1, 2, and 4 respectively
• 1,3,5-Triazine:Nitrogen is staggered with carbon and is the most common and stable form

Aromaticity

The triazine ring is aromatic because its π electron system satisfies Huckel's rule (6π electrons) and has stability and resonance structure.

important derivatives

• Melamine:1,3,5-Triazine derivatives, containing three amino groups, are used in the manufacture of plastics and resins.
• Atrazine:Triazine herbicides for agricultural weed control.
• Sulfadiazine:Sulfonamide antibacterial drugs containing triazine structure.

Application areas

• Polymer materials (such as triazine resin, flame retardants)
• Pesticides (especially herbicides)
• Pharmaceutical synthesis (antibacterial, anticancer drugs)
• Dyes and Pigments Industry

Summarize

The triazine ring is a nitrogen-containing aromatic ring structure with high chemical stability. Its versatility makes it widely used in the fields of chemical industry, pesticides, medicine and materials.

melamine

Basic introduction

Melamine, with the chemical formula C₃H₆N₆, is a nitrogen-containing organic compound. It appears as a white crystalline powder, is insoluble in most organic solvents, is slightly soluble in hot water, and is alkaline.

chemical structure

Melamine is composed of three amino groups (–NH₂) and a triazine ring. It is a triazine compound with a high nitrogen content (about 66%). Therefore, it releases nitrogen when burned and is flame retardant.

Main purpose

• Melamine formaldehyde resin:Commonly used in the manufacture of tableware, furniture coatings, decorative panels, heat-resistant plastics, etc.
• Fireproof materials:Due to its high nitrogen content, it can be used as a flame retardant.
• Synthetic intermediates:Used in the preparation of pesticides, medicines and dyes.

Security and Controversy

• melamineNot for use in food。
• In 2008, an incident involving melamine mixed with infant milk powder broke out in China, causing a large number of kidney stones and health problems among infants and young children, and attracted global attention.
• If melamine is ingested into the human body, it will form insoluble crystals with uric acid crystals, causing kidney damage.

Relevant regulations

• Countries have strict limits on melamine content in food.
• The tolerable daily intake (TDI) set by the World Health Organization (WHO) and the Food and Agriculture Organization of the United Nations (FAO) is 0.2 mg/kg body weight.

Summarize

Melamine is a widely used industrial raw material, butImproper use in foodIt poses serious health risks and requires strict supervision.

localized chemical bonding

definition

Localized chemical bonding refers to a chemical bond in which the covalent bond between atoms is limited to two specific atoms. This description method is suitable for most molecules and helps to understand their structure and bonding properties.

feature

• Electron pairs are concentrated between two atoms to form a covalent bond
• Comply with Lewis structure and Valence Bond Theory
• Can clearly indicate the atomic correspondence of each pair of bonds

example

• Hydrogen (H₂): Two hydrogen atoms sharing a pair of electrons
• Water (H₂O): Oxygen atom and two hydrogen atoms each form a localized O-H bond
• Ethylene (C₂H₄): The double bond between carbon atoms consists of a σ bond and a π bond

Comparing localization and non-localization

Theoretical basis

The theoretical basis of localized bonding is valence bond theory, which emphasizes the overlap of atomic orbitals and electron pair sharing. Bond angles and molecular shapes are usually combined with the concept of hybrid orbitals (such as sp³, sp²).

delocalized chemical bonding

definition

Delocalized chemical bonding refers to a form of covalent bonding in which electrons are not limited to two specific atoms, but are shared between three or more atoms. This type of bonding is common in molecules that have resonant structures or overlapping common orbitals.

feature

• A cloud of electrons distributed among multiple atoms
• Cannot be represented by a single Lewis structure, a resonance structure must be used
• Usually have special stability, such as aromatic properties

example

• Benzene (C₆H₆): Six π electrons are evenly distributed on six carbon atoms, forming a stable aromatic ring
• Carbonate ion (CO₃²⁻): three C=O bonds of equal length, electrons delocalized between three oxygen atoms
• Nitrate ion (NO₃⁻): N-O bond lengths are equal and there are delocalized π electrons

Comparing localization and delocalization

Theoretical basis

Delocalized bonding can be explained by molecular orbital theory, in which π orbitals can be extended to multiple atoms, allowing electrons to move freely throughout the region. This phenomenon enhances the stability and symmetry of molecules.

A bond weaker than a covalent bond

definition

Bonds that are weaker than covalent bonds refer to non-covalent interactions that exist within or between molecules. Their bond energy is lower than that of covalent bonds, but they still play an important role in biological, material and chemical systems.

Common types

• Hydrogen Bond:The attraction between hydrogen atoms and high electronegativity atoms (such as O, N, F) is commonly found in water molecules, proteins and DNA structures.
• Van der Waals Forces:The weak attraction between molecules caused by instantaneous dipoles, including dispersion force and induced dipole.
• Ion-Dipole Interaction:The attraction between charged ions and polar molecules is common in the dissolution of salts.
• π-π stacking effect:Parallel superposition attraction between aromatic rings is widely found in DNA base pairing and organic materials.
• Hydrophobic Interaction:The tendency of non-polar molecules to aggregate in aqueous solutions promotes the folding and stabilization of biomolecules.

Bond energy comparison (rough range)

Application and importance

Although these bonds are weaker than covalent bonds, they are extremely important in maintaining protein structure, stabilizing nucleic acid double strands, binding drugs to receptors, and self-assembly of nanomaterials.

stereochemistry

definition

Stereochemistry is the branch of chemistry that studies the arrangement of atoms or functional groups in molecules in three-dimensional space. It not only focuses on the composition of molecules, but also on the shape and mapping relationship of its structure in space.

Main types

• Configurational Isomers:Isomers that cannot be freely converted due to different arrangements of bonds, such as enantiomers and geometric isomers.
• Enantiomers:Isomers that are mirror images of each other and are non-superimposable, with chiral centers.
• Diastereomers:Configurational isomers that are not mirror images.
• Geometrical Isomers:Occurs in compounds with double bonds or cyclic structures, such as cis/trans or E/Z isomerism.

Chirality and asymmetric centers

Chiral molecules have at least one chiral center (usually a carbon atom connected to four different substituents), and their mirror image isomers are called enantiomers.这些异构体在物理性质相似，但在生物活性与旋光性上常有显著差异。

Stereospecificity and selectivity

• Stereospecificity:Certain reaction pathways produce only products with specific stereoconfigurations.
• Stereoselectivity:The reaction may produce multiple stereoisomers, but one is preferred.

application

Stereochemistry is critical to fields such as drug design, biochemistry, materials science, and more. For example, one enantiomer of a certain drug may have a therapeutic effect, while another enantiomer may be ineffective or toxic.

Commonly used notations

• Wedge-Dash: represents the direction of bonds in three-dimensional space
• Fischer projection: used to represent the configuration of sugars and amino acids
• CIP system (R/S nomenclature): naming of absolute configurations of chiral centers according to order of preference

Carbocation, carbanion, free radical, carbene and nitroso group

Carbocations

Carbocations are positively charged carbon atom intermediates that usually have six electrons and do not comply with the octet rule. Common types include tertiary, secondary and primary carbocations, with the order of stability being tertiary > secondary > primary > methyl.

• For example:(CH₃)₃C⁺(tertiary carbocation)
• Reaction characteristics: Easily participate in nucleophilic substitution and addition reactions

Carbanions

Carbanions are negatively charged carbon atoms that usually have a lone pair of electrons and obey the octet rule. Carbon atoms exist in sp³ or sp² hybridized states.

• For example:CH₃⁻、PhCH₂⁻(Benzyl anion)
• Reaction characteristics: Strong nucleophilicity, participating in substitution and addition reactions

Free Radicals

Free radicals are neutral molecules or atoms containing unpaired electrons that are extremely unstable and highly reactive. Carbon free radicals can be divided into three levels, two levels, and one level according to their stability.

• For example:CH₃•、(CH₃)₃C•
• Reaction characteristics: Participate in free radical polymerization, chlorination reaction, etc.

Carbenes

Carbenes are intermediates containing neutral carbon atoms with two bonds and a pair of lone electrons. They are commonly used in cycloaddition or insertion reactions. It can be divided into two electronic states: singlet (single state) and triplet (three states).

• For example:CH₂(methene)
• Reaction characteristics: easily reacts with double bonds to form cyclopropane structure

Nitrenes

Nitroso is an intermediate containing neutral nitrogen atoms, similar to carbene, but with a nitrogen center and a hexavalent electron structure. Can be in the singlet or triplet state, usually from thermal or photolysis of nitrogen-containing compounds.

• For example:NH、RN:
• Reaction characteristics: Can insert C-H bonds and add to double bonds

Reaction mechanism and its judgment method

Reaction Mechanisms

A reaction mechanism describes the detailed steps of how a chemical reaction proceeds, including the breaking and forming of bonds, and the generation and transformation of reaction intermediates. It reveals how molecules are converted into products and describes the speed, selectivity, and stereochemical consequences of the reaction.

Common types of institutions

• Nucleophilic substitution reaction (SN1/SN2):Depending on the reactant structure and solvent, SN1 involves a carbocation intermediate and SN2 is a single-step mechanism.
• Elimination reaction (E1/E2):Removal of hydrogen and leaving groups creates a double bond, E1 undergoes a carbocation, and E2 is a cooperative mechanism.
• Free radical reaction:Involves free radical intermediates, such as halogenation, polymerization and other reactions.
• Addition reaction:Commonly seen in alkenes and alkynes, such as electrophilic addition and free radical addition.
• Rearrangement reaction:Rearrangement of atoms or functional group positions during a reaction, such as the Wagner–Meerwein rearrangement.

How to determine response mechanisms

1. Kinetic studies

By measuring the dependence of the reaction rate on the concentration of the reactants, the rate law of the reaction can be known and the mechanism can be inferred. For example, SN1 is a first-order reaction and SN2 is a second-order reaction.

2. Intermediate detection

Use spectroscopic techniques (e.g., NMR, IR, EPR) or capture reagents to detect transient intermediates present in reactions, such as carbocations, free radicals, or carbenes.

3. Isotopic labeling

Stable or radioactive isotopes such as hydrogen, carbon, or oxygen are used to label specific locations on the molecule and their distribution in the product is observed to determine bonding changes.

4. Stereochemical observation

Observe whether the reaction causes a change in stereoconfiguration. For example, the SN2 reaction causes a complete inversion (Walden inversion), while the SN1 reaction may produce a racemic mixture.

5. Organizational changes and product analysis

Analyzing the type, proportion, selectivity, etc. of products produced under different conditions can indirectly determine the reaction path and possible mechanisms.

6. Computational Chemistry Simulations

Use quantum chemistry and molecular simulation to calculate energy changes in transition states and reaction pathways, predict possible mechanisms, and compare with experimental data.

Conclusion

Correctly grasping the reaction mechanism helps control and design chemical reactions, and is of extremely high value in drug development, organic synthesis and catalysis research. Cross-validation of multiple methods is key to inferring institutions.

Photochemistry

definition

Photochemistry is the branch of science that studies chemical reactions caused by the interaction of light and matter. When a molecule absorbs photons of a specific wavelength, it may enter an excited state, and then undergo reactions such as bond breaking, addition, rearrangement, and polymerization.

Basic principles

• Light absorption:Molecules absorb photon energy (usually ultraviolet or visible light) and enter an excited state.
• Excited state:An electron jumps from the ground state to a higher energy orbital (such as from π to π* or n to π*).
• Possible paths:Including fluorescence, phosphorescence, internal conversion, intersystem conversion or chemical reaction.

Common photochemical reactions

• Photodissociation:For example, Cl₂ → 2Cl•, light energy breaks bonds to generate free radicals.
• Photoaddition reaction:For example, cyclohexene and aromatic alkenes undergo [2+2] addition under light.
• Photoisomerization:Such as conversion between cis and trans alkenes.
• Light-induced free radical reaction:Such as the anti-Markovnikov product of the addition of HBr to alkenes.

application

• biology:During photosynthesis, chlorophyll absorbs light and triggers an electron transfer reaction.
• Materials Science:Light-curing resin, photoconductor, photosensitive materials (such as photographic films, solar cells).
• Organic synthesis:Building complex molecules using photocatalytic reactions.
• medicine:Photodynamic Therapy treats cancer and skin diseases.

laws of photochemistry

• Grothus–Draper law:Only light absorbed by molecules can initiate chemical changes.
• Stark–Einstein law:Each absorbed photon excites at most one molecule into the reaction.

experimental techniques

• Ultraviolet-Visible Spectroscopy (UV-Vis)
• Time-resolved spectroscopy (e.g. femtosecond laser)
• Light reactor and photocatalytic system

Effect of structure on reactivity

1. Electronic effects

• Inductive Effect:Atoms with high electronegativity will pull electrons through the σ bond, reducing the electron density of neighboring atoms and affecting reactivity.
• Resonance Effect:The delocalization of π electrons can stabilize reaction intermediates or transition states and enhance or inhibit certain reactions.
• Hyperconjugation:The interaction between σ bonds and π systems or empty orbitals can stabilize free radicals or carbocations.

2. Spatial effect (stereo effect)

• Steric Hindrance:Bulky substituents will prevent reactants from approaching the reaction center and slow down the reaction rate.
• Stereoselectivity and stereospecificity:Reactions may favor the formation of specific stereoisomers, affecting the configuration and conformation of the products.

3. Heterocyclic and aromatic structures

• Aromatic rings have special stability due to the delocalization of π electrons, making substitution reactions more common than addition reactions.
• Heterogeneous atoms (such as N, O, S) in heterocyclic rings can provide lone electron pairs or electron-withdrawing effects, changing the reaction site and mechanism.

4. Hybridization state

• The sp hybrid orbital of carbon has a high electronegativity, and the conjugate acid of the sp atom is more stable, so it is more acidic.
• The reactivity of sp² and sp³ carbon centers also differs due to differences in electron density and bond angles.

5. Influence of functional groups

• The contribution of different functional groups to the reactivity of a molecule can be attributed to its polarity, resonance and electron donating or electron pulling abilities.
• For example: the carboxyl group in carboxylic acids is acidic, while the carbonyl group in aldehydes and ketones is highly electrophilic.

6. Interaction between environment and conditions

• The influence of structure on reactivity may determine the actual reaction pathway together with conditions such as solvent, temperature, and catalyst.

Examples

• The tertiary carbocation has high stability due to hyperconjugation and induction effects, and the SN1 reaction rate is fast.
• Aniline with a carboxylic acid in the ortho position reduces the nucleophilicity of the amine group due to hydrogen bonding and induction effects.

analytical chemistry

definition

Analytical chemistry is a branch of chemistry that studies the composition, structure and content of substances. The purpose is to identify the types of substances and determine their content. It is widely used in the fields of environment, food, medicine, materials and other fields.

Classification

• Qualitative analysis:Used to confirm whether a sample contains a specific element or compound.
• Quantitative analysis:It is used to measure the content of specific ingredients and can be further divided into gravimetric analysis, volumetric analysis and instrumental analysis.

Traditional analysis methods

• Gravimetric analysis:Calculate the component content based on the mass of precipitated or volatile products.
• Capacity analysis method:The concentration is determined by calculating the volume of reagent consumed through a titration reaction.

modern instrumental analysis

• Spectral analysis:Such as ultraviolet-visible spectroscopy (UV-Vis), atomic absorption spectroscopy (AAS), infrared spectroscopy (IR), nuclear magnetic resonance (NMR), etc.
• Chromatographic analysis:Such as gas chromatography (GC), high performance liquid chromatography (HPLC), etc.
• Electrochemical analysis:Such as potential method, current method, conductivity method, etc.
• Mass spectrometry (MS):Used to determine molecular weight and structural information.

application

Analytical chemistry plays a key role in:

• Drug Purity and Ingredient Analysis
• Detection of additives and pesticide residues in food
• Monitoring of heavy metals and pollutants in the environment
• Forensic Identification and Toxic Testing
• Composition Analysis in Materials and Nanotechnology

Development trend

Modern analytical chemistry is developing towards high sensitivity, automation, real-time monitoring and green analysis, combining artificial intelligence and miniaturization technology to improve efficiency and precision.

chromatography

definition

Chromatography is an analytical technique that separates components in a mixture. The components are separated from each other based on their different distribution behaviors in the stationary phase and the mobile phase.

Basic principles

The mixed sample is driven by the mobile phase. When passing through the stationary phase, different components have different forces with the stationary phase, resulting in differences in moving speed, and thus separation.

Main types

• Paper Chromatography:Paper is used as the stationary phase and is often used for analysis of pigments and small molecules.
• Thin Layer Chromatography (TLC):Using silica or alumina-coated glass plates as the stationary phase, observation is quick and easy.
• Gas Chromatography (GC):Use gas as the mobile phase to analyze volatile compounds.
• Liquid Chromatography (LC):Using liquid as the mobile phase is suitable for non-volatile or thermally unstable samples.
• High Performance Liquid Chromatography (HPLC):It is a highly sensitive version of liquid chromatography and is used for quantitative analysis of pharmaceuticals and biochemical samples.

Application scope

Chromatography is widely used in chemistry, food, pharmaceuticals, biotechnology and environmental sciences, such as:

• Isolate and purify compounds
• Detect poisons or drug ingredients
• Analyze colorings, preservatives or pesticide residues in food
• Separation and characterization of biomolecules such as proteins, amino acids, and nucleotides

advantage

Chromatography has the advantages of good separation effect, wide application, relatively simple operation and high sensitivity. It is an indispensable tool in modern analytical chemistry.

quantum chemistry

Quantum chemistry is a discipline that applies the principles of quantum mechanics to study chemical structures and reactions. It is based on the quantum behavior of electrons and atomic nuclei, and uses mathematical models and calculations to predict physical and chemical properties such as molecular structure, bonding characteristics, and energy level distribution.

Quantum chemistry is based on the Schrödinger equation and treats chemical bonds and the electronic structure of molecules as quantum states described by wave functions:

Ĥψ = Eψ

in:

• Ĥis the Hamiltonian operator, describing the total energy of the system.
• ψis the wave function, describing the quantum state of the system.
• Eis the energy eigenvalue, corresponding to the steady state energy of the system.

Quantum chemistry usually uses a variety of numerical calculation methods, mainly including:

• molecular orbital theory: Assume that electrons occupy molecular orbitals in molecules and are used to describe bonding and electron distribution.
• Density Functional Theory (DFT): A theory that uses electron density instead of wave functions to describe systems. It has high computational efficiency and is widely used in macromolecular systems.
• Configuration Interaction (CI): Describe the interaction between electrons through linear combinations of multiple electronic configurations to improve accuracy.

• Molecular structure prediction: Calculate geometric structures such as molecular shape, bond length, bond angle, etc.
• Reaction mechanism research: Analyze energy changes and transition states during reactions and predict chemical reaction rates.
• Materials Science: Simulate the electronic structure of materials and design materials with specific optical, magnetic or conductive properties.
• drug design: Design more selective and effective drug molecules by simulating intermolecular interactions.

A major challenge in quantum chemistry is how to deal with many-body problems in complex systems. As the size of molecules increases, the interaction of electrons rapidly increases computational requirements, rendering traditional methods ineffective. Therefore, the study of quantum chemistry usually requires efficient numerical methods and powerful computing resource support.

With the development of computing technology, quantum chemistry has become an important tool in modern chemistry and materials science, promoting the advancement of many cutting-edge technologies.

Atomic structure

Basic composition

Atoms are made up of three basic particles:

• Proton (p⁺):It is positively charged, located in the nucleus, and has a mass of approximately 1 atomic mass unit.
• Neutron (n⁰):It is uncharged and is also located in the nucleus of an atom. Its mass is similar to that of a proton.
• Electronic (e⁻):It is negatively charged, orbits around the nucleus, and has a negligible mass.

Atomic number and mass number

• Atomic number (Z):The number of protons in an atom determines the chemical properties of an element.
• Mass number (A):The sum of the number of protons and neutrons.

electronic configuration

Electrons are distributed in energy levels (or electron shells) outside the nucleus, and each layer can accommodate a specific number of electrons. The way electrons are arranged affects the reactivity of elements and the formation of chemical bonds.

isotope

Isotopes are atoms with the same atomic number (same number of protons) but different numbers of neutrons. For example, carbon-12 and carbon-14 are both isotopes of carbon.

Development of the atomic model

• Dauton model:Atoms are viewed as indivisible solid balls.
• Thomson model:Also known as the "Plum Pudding Model", the electrons are embedded in a positively charged sphere.
• Rutherford model:Atoms have a positively charged, extremely dense nucleus surrounded by electrons.
• Bohr model:Electrons orbit the nucleus in specific orbits, each orbit corresponding to a fixed energy.
• Quantum mechanical model:Electrons are no longer considered to be in fixed orbits around the nucleus, but instead exist in a region of probability called an "orbital."

Conclusion

Atomic structure is the basis for understanding chemical reactions, element properties and the nature of matter. The evolution from classical models to quantum mechanical models reflects the deepening of human understanding of the microscopic world.

Atomic and molecular properties

atomic properties

• Atomic radius:The average distance from the nucleus to the outermost electron. It decreases from left to right along the periodic table and increases from top to bottom.
• Ionization energy:The energy required to remove the outermost electron from a gaseous atom. The further to the right and upward, the higher the ionization energy.
• Electron affinity:The energy released or absorbed by an atom when it accepts an electron. Nonmetals (such as halogens) generally have higher electron affinities.
• Electronegativity:The ability of atoms to attract shared pairs of electrons in chemical bonds. Fluorine has the highest electronegativity of all elements.

molecular properties

• Molecular geometry:The arrangement of atoms in a molecule in three-dimensional space depends on the valence shell electron pair mutual repulsion theory (VSEPR). For example: CH₄ is ​​a tetrahedron, and H₂O is a curved shape.
• polarity:分子是否具有永久偶极，取决于键的极性与几何对称性。 Like CO₂, it is non-polar, but H₂O is polar.
• Intermolecular forces:Including van der Waals forces, hydrogen bonds and dipole-dipole forces, which affect the boiling point, melting point and solubility of substances.
• Resonance structure:Some molecules cannot be represented by a single Lewis structure, and multiple resonance formulas are needed to describe their electron distribution, such as O₃, NO₃⁻.

Conclusion

Understanding the properties of atoms and molecules helps predict the behavior of chemical reactions, the physical properties and structure of matter, and is the core foundation for learning chemistry.

chemical bond formation theory

Valence Bond Theory

Valence bond theory states that chemical bonds are formed by overlapping atomic orbitals between two atoms. When atomic orbitals of electrons with opposite spins overlap, a covalent bond is formed. This theory emphasizes the directionality of bonds and electron distribution, and can well explain the geometric structure of molecules.

Main point

• Bond formation comes from orbital overlap (such as s-s, s-p, p-p overlap)
• The strength of the bond is related to the degree of orbital overlap
• Each atom provides one unpaired electron

Hybrid Orbital Theory

Mixed orbital theory is an extension of valence bond theory. In order to explain the actual geometric structure of molecules, it is proposed that the s, p, d and other orbitals within atoms can be linearly recombined to form new "mixed orbitals" and participate in bonding.

Common blends

• sp blended into:Linear structure (such as BeCl₂)
• sp² blends into:Planar triangular structure (such as BF₃)
• sp³ blends into:Tetrahedral structure (such as CH₄)
• sp³d blend:Triangular bipyramid (such as PCl₅)
• sp³d² blends into:Octahedron (such as SF₆)

Compare and apply

Conclusion

Valence bond theory and mixed orbital theory are important tools for describing the formation of covalent bonds and provide a basis for understanding molecular shape, bond energy and reactivity. Although they may not be complete in complex systems, they are extremely valuable in teaching and preliminary analysis.

Molecular orbital theory and spectral properties of diatomic molecules

Introduction to Molecular Orbital Theory

Molecular Orbital Theory (MO theory) believes that when atoms form molecules, the orbitals of the atoms will be reorganized into the molecular orbitals of the entire molecule. These molecular orbitals span the entire molecule, where electrons are viewed as distributed throughout the molecule rather than limited to bonds between two atoms.

The formation of molecular orbitals

• The combination of atomic orbitals will produce two kinds of molecular orbitals:bonding orbitalandantibonding orbital。
• For example, two 1s orbitals combined will form a σ₁s (bonding) and a σ*₁s (antibonding) orbital.
• Electrons preferentially fill the bonding orbitals with lower energy.

Molecular orbital arrangement and electronic configuration

For diatomic molecules (such as O₂, N₂, F₂, etc.), the arrangement of molecular orbitals is usually as follows (for those with atomic number Z ≤ 7):

σ1s < σ*1s < σ2s < σ*2s < π2p < σ2p < π*2p < σ*2p

And Z ≥ 8 (such as oxygen) is changed to:

σ1s < σ*1s < σ2s < σ*2s < σ2p < π2p < π*2p < σ*2p

Key levels and magnetism

• Bond Order:Bond level = (number of bonding electrons − number of antibonding electrons) ÷ 2.
• Bond level can be used to judge the strength and stability of a bond. The higher the bond level, the stronger the bond.
• magnetic:If there are unpaired electrons in the molecular orbitals, the molecule is paramagnetic (such as O₂); if all electrons are paired, the molecule is diamagnetic (such as N₂).

Spectral properties of diatomic molecules

The spectral properties of diatomic molecules (such as ultraviolet-visible light absorption, infrared light absorption) are closely related to their electronic structures:

• Electronic transition:Electrons can jump from the bonding orbital domain to the antibonding orbital domain, absorbing energy of a specific wavelength, corresponding to the ultraviolet-visible light region.
• Vibrational and rotational spectra:Diatomic molecules can absorb energy in the infrared range, resulting in bond vibration or overall rotation of the molecule. These absorptions correspond to specific frequencies and energy level differences.

application

Molecular orbital theory can effectively predict the following properties:

• Bond length, bond energy and stability of diatomic molecules
• Paramagnetism and Diamagnetism
• Electronic transition and absorption spectral characteristics
• Unusual bond order (e.g. double bond behavior of O₂)

Conclusion

Molecular orbital theory describes the behavior of electrons in the entire molecule more deeply than valence bond theory. It is especially valuable for the magnetic and spectral interpretation of diatomic molecules and is one of the important theoretical foundations of modern quantum chemistry.

Electronic structure, photoelectron spectroscopy and frontier orbital theory of polyatomic molecules

electronic structure

Electronic structure describes the arrangement of electrons in an atom or molecule. Electrons are distributed in different energy levels and orbitals, and these distributions determine the chemical properties and reactivity of molecules. In polyatomic molecules, electrons are usually distributed in molecular orbitals (MOs) rather than limited to single atoms.

• Electrons first fill the orbitals with the lowest energy (the principle of lowest energy)
• Satisfies the Pauli Exclusion Principle and Hundt's Law

Photoelectron Spectroscopy (PES)

Photoelectron spectroscopy is an experimental technique used to measure the binding energy of electrons in atoms or molecules. When a photon irradiates the sample, if the energy of the photon is enough, the electron will be excited and break away from the molecule, producing photoelectrons.

principle:

According to Einstein's law of photoelectric effect:

E_photon = E_binding + KE_electron

in:

• E_photon：photon energy
• E_binding：electron binding energy
• KE_electron：Kinetic energy of the released electron

application:

• Qualitative and quantitative analysis of the electronic structure of molecules
• Determine the energy levels and electron distributions of different molecular orbitals
• Identify isomers and functional group positions

Frontier Orbital Theory

The frontier orbital theory emphasizes that the most active in chemical reactions are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). These two orbitals are called frontier orbitals.

HOMO（Highest Occupied Molecular Orbital）：

Donate electrons and become a nucleophilic center.

LUMO（Lowest Unoccupied Molecular Orbital）：

It accepts electrons and is an electrophilic center.

Theoretical applications:

• Predicted reactivity: The reaction mainly occurs through the interaction between HOMO and LUMO.
• Stereoselectivity: Predicts the stereochemical outcome of a molecule, such as a cycloaddition reaction.
• Reaction mechanism analysis: such as Diels–Alder reaction, nucleophilic substitution, free radical reaction, etc.

Conclusion

Electronic structure determines the chemical nature of molecules, photoelectron spectroscopy provides experimental methods to probe these structures, and frontier orbital theory provides in-depth explanations of reaction behavior from the perspective of orbital interactions. The three complement each other and are the core foundation of modern organic and quantum chemistry.

Transition metal complexes

definition

Transition-Metal Complexes are compounds formed by transition metal ions and one or more ligands through coordination bonds. These complexes have diverse geometric configurations, magnetic and optical properties, and are widely used in catalysis, materials science and biological systems.

composition structure

• Center metal:Usually d-zone metals, such as Fe, Co, Ni, Cu, Pt, etc., are positively charged.
• Ligand:Atoms or molecules containing lone pairs of electrons can provide electrons to metals to form coordination bonds. Common ones include H₂O, NH₃, Cl⁻, CN⁻, EDTA, etc.
• Coordination number:Indicates that the central metal can bond with several ligands, usually 4 (tetrahedron or planar square) or 6 (octahedron).

Naming method (IUPAC)

The naming order is: ligand (in alphabetical order) + central metal (including oxidation number)

For example:[Cu(NH₃)₄]²⁺Named "tetraammine copper(II) ion"

Electronic structure and magnetism

• The d orbital of the central metal will be split due to the electric field of the ligand, producing orbitals of different energies, which are calledCrystal Field Splitting。
• If electrons fill high-energy orbitals, it is calledhigh-spin; If only low-energy orbitals are filled in, it islow-spin。
• The magnetism can be judged based on the number of unpaired electrons: those with unpaired electrons areParamagnetic, if all pairs areDiamagnetism。

Common geometric configurations

• Tetrahedron (e.g. [NiCl₄]²⁻)
• Plane square (such as [Pt(NH₃)₂Cl₂])
• Octahedron (e.g. [Fe(CN)₆]³⁻)

Color and spectral properties of complexes

Complexes often have bright colors resulting from d-d electronic transitions or charge transfer from ligands to the metal. These optical properties can be analyzed by ultraviolet-visible spectroscopy (UV-Vis).

application

• catalyst:For example, palladium and rhodium complexes are used in organic reactions (cross-coupling, etc.).
• medicine:For example, cisplatin is an anticancer drug.
• Materials Science:It can be used to manufacture conductive polymers, electrochromic materials, etc.
• Biological systems:For example, heme and vitamin B₁₂ contain metal complex structures.

Conclusion

Transition metal complexes exhibit rich chemical properties and application potential. Its unique structure and electronic properties enable it to occupy a key position in the fields of catalysis, biomedicine and materials, and is one of the core research objects of modern inorganic chemistry.

Bonding in Solids and Liquids

bonding in solids

Solids can be divided into types with different structures and properties based on their bonding types:

• Ionically bonded solids:Formed from cations and anions, such as NaCl. It has a high melting point and hardness and conducts electricity when melted or dissolved in water.
• Covalently bonded solids:All atoms are connected by covalent bonds, such as diamond and quartz. Extremely hard and low conductivity.
• Metal bond solid:Metal cations such as Fe and Cu are immersed in a sea of ​​freely moving electrons. It is ductile and has good electrical and thermal conductivity.
• Molecular solids:The molecules are maintained by Van der Waals forces or hydrogen bonds, such as ice and iodine. It has a low melting point, is volatile, and usually does not conduct electricity.

bonding in liquids

The molecular bonds in liquids are mainly intermolecular forces. These forces determine the physical properties of liquids, such as viscosity, volatility and surface tension.

• Hydrogen bonding:Strong and directional forces exist between polar molecules such as water, alcohol, and hydrofluoric acid.
• Dipole-dipole action:The force between polar molecules caused by electrostatic attraction.
• Van der Waal force (dispersion force):The instantaneous dipole attraction exists between all molecules, and non-polar molecules such as CH₄ and CCl₄ mainly rely on this to maintain their liquid state.

comparison table

Conclusion

The type of bond determines the structure, melting point, electrical conductivity, and other properties of solids and liquids. Understanding these bonds helps materials design, chemical synthesis, and prediction of physical properties.

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