There are many types of organic compounds, which can be divided into two categories: hydrocarbons and derivatives of hydrocarbons. According to the carbon framework structure of organic molecules, they can also be divided into three categories: open chain compounds, carbon ring compounds, and heterocyclic compounds. According to the different functional groups contained in organic molecules, they are further divided into alkanes, alkenes, alkynes, aromatic hydrocarbons and halogenated hydrocarbons, alcohols, phenols, ethers, aldehydes, ketones, carboxylic acids, esters, and so on.

According to the carbon skeleton
1. Chain like compounds
The carbon atoms in the molecules of these compounds are interconnected in a chain like structure, and because they were originally discovered in fats, they are also called aliphatic compounds. Its structural feature is that carbon is connected to carbon to form an open chain.

2. Cyclic compounds
Cyclic compounds refer to compounds in which atoms are arranged in a cyclic manner within a molecule. Cyclic compounds are further divided into cycloaliphatic compounds and aromatic compounds.
(1) Acyclic compounds: cyclic compounds that do not contain aromatic rings (such as benzene rings, fused rings, or certain heterocyclic rings with benzene or fused ring properties). Such as cyclopropane, cyclohexene, cyclohexanol, etc.
(2) Aromatic compounds: cyclic compounds containing aromatic rings (such as benzene rings, fused rings, or certain heterocyclic rings with benzene or fused ring properties). Such as benzene, its homologues and derivatives, polycyclic aromatic hydrocarbons and derivatives, pyrrole, pyridine, etc.

By constituent elements
1. Hydrocarbons
Organic compounds containing only carbon and hydrogen are called hydrocarbons, abbreviated as hydrocarbons. Such as methane, ethylene, acetylene, benzene, etc. Methane is the simplest hydrocarbon.

2. Derivatives of hydrocarbons
A series of compounds formed by replacing hydrogen atoms in hydrocarbon molecules with other atoms or atomic groups are called derivatives of hydrocarbons. Such as halogenated hydrocarbons, alcohols, amino acids, nucleic acids, etc.

According to functional groups
Functional group: The atoms or atomic groups that determine the specific properties of a compound are called functional groups or functional groups. Containing the same functional group
The chemical properties of the compounds are basically the same. Common functional groups include carbon carbon double bonds, carbon carbon triple bonds, hydroxyl groups, and carboxyl groups
Base, ether bond, aldehyde group, carbonyl group, etc.
Homologues: Organic compounds with similar structures but differing in molecular composition by one or several "CH2" atomic groups are called homologues. And it must be the same type of substance (containing the same and equal number of functional groups, except for hydroxyl groups, phenols and alcohols cannot be homologous, such as phenol and benzyl alcohol). Due to their similar structures, the chemical properties of homologues are similar; Their physical properties often change regularly with the increase of molecular weight.

According to structure and properties
Open chain hydrocarbon: A hydrocarbon in which carbon atoms in a molecule combine with each other to form a chain without a cyclic structure, known as an open chain hydrocarbon. According to the content of carbon and hydrogen in the molecule, chain hydrocarbons can be divided into saturated chain hydrocarbons (alkanes) and unsaturated chain hydrocarbons (alkenes, alkynes).
Fatty hydrocarbons: also known as "chain hydrocarbons". Because fat is a derivative of chain hydrocarbons, chain hydrocarbons are also known as aliphatic hydrocarbons.
Saturated hydrocarbons: Saturated hydrocarbons can be divided into linear saturated hydrocarbons, also known as alkanes (also known as paraffin hydrocarbons), and another type of cyclic saturated hydrocarbons containing carbon carbon single bonds, namely cycloalkanes (see closed chain hydrocarbons).
Alkanes: also known as saturated chain hydrocarbons or paraffin hydrocarbons. The general formula is CnH2n+2 (n ≥ 1), and the hydrogen content in alkanes has reached saturation. The simplest alkane is methane, which is the main component of natural gas and biogas. Alkanes are mainly derived from petroleum, natural gas, and biogas. Substitution reactions can occur, and methane can undergo substitution reactions with chlorine gas under light conditions, producing CH3Cl -- CH2Cl2-- CHCl3-- CCl4.
Unsaturated hydrocarbons: hydrocarbons containing "C=C" or "C ≡ C" in their molecules. This type of hydrocarbon can also be divided into unsaturated chain hydrocarbons and unsaturated cyclic hydrocarbons. Unsaturated chain hydrocarbons contain fewer hydrogen atoms than their corresponding alkanes, are chemically active, and are prone to addition and polymerization reactions. Unsaturated chain hydrocarbons can be divided into olefins and alkynes. Unsaturated cyclic hydrocarbons can be divided into cyclic olefins (such as cyclopentadiene) and cyclic alkynes (such as phenylacetylene).
Olefins: Hydrocarbons containing "C=C" in their molecules. According to the number of "C=C" in the molecule, it can be divided into monoalkenes and dienes. Single olefin molecules contain a "C=C" with the general formula CnH2n, where n ≥ 2. The most important monoalkene is ethylene H2C=CH2, followed by propylene CH3CH=CH2 and 1-butene CH3CH2CH=CH2. Monoalkenes, abbreviated as olefins, are mainly derived from petroleum and its cracking products.
Diene: a chain or cyclic hydrocarbon containing two "C=C" atoms. Such as 1,3-butadiene. 2-methyl-1,3-butadiene, cyclopentadiene, etc. The most important conjugated double bond systems in dienes, such as 1,3-butadiene and 2-methyl-1,3-butadiene, are monomers used in the synthesis of rubber.
Alkynes: Unsaturated chain hydrocarbons containing "C ≡ C" in their molecules. According to the number of carbon carbon triple bonds in the molecule, it can be divided into monoalkynes and polyalkynes. The general formula for monoalkynes is CnH2n-2, where n ≥ 2. Alkynes and dienes are isomers. The simplest and most important alkyne is acetylene HC ≡ CH, which can be produced by reacting calcium carbide with water.
Closed bond hydrocarbon: also known as "cyclic hydrocarbon". It is a hydrocarbon with a cyclic structure. It can be divided into two categories. One is cycloaliphatic hydrocarbons (also known as aliphatic hydrocarbons), which have the properties of aliphatic hydrocarbons. Cycloaliphatic hydrocarbons are further divided into saturated cycloalkanes, where n ≥ 3. Cycloalkanes and alkenes are isomers. Cycloalkanes are present in certain petroleum products, while cycloalkenes are commonly found in plant essential oils. Another type of cyclic hydrocarbon is aromatic hydrocarbons, most of which have a benzene ring structure and the properties of aromatic compounds.
Cycloalkane: In cyclic hydrocarbon molecules, the carbon atoms that are bonded to each other by a single bond are called cycloalkanes, which are saturated cyclic hydrocarbons. Cycloalkanes with tricyclic and tetracyclic structures have poor stability and are prone to ring opening under certain conditions. Cycloalkanes with five or more rings are relatively stable and have properties similar to alkanes. Common cycloalkanes include cyclopropane, cyclobutane, cyclopentane, cyclohexane, etc.
Aromatic hydrocarbons: generally refer to hydrocarbons with a benzene ring structure in their molecules. According to the number of benzene rings contained in the molecule and the bonding mode between benzene rings, it can be divided into monocyclic aromatic hydrocarbons, polycyclic aromatic hydrocarbons, polycyclic aromatic hydrocarbons, etc. The general formula for monocyclic aromatic hydrocarbons is CnH2n-6, where n ≥ 6. Among monocyclic aromatic hydrocarbons, benzene is important
Polycyclic aromatic hydrocarbons: molecules containing two or more benzene rings, which share two phases between them.
Heterocyclic compounds: Compounds in which carbon atoms and other atoms such as oxygen, nitrogen, sulfur form a cyclic structure are called heterocyclic compounds. Among them, heterocycles with five and six atoms are more stable. Aromatic compounds are called aromatic heterocycles, and compounds formed by replacing one or more hydrogen atoms in hydrocarbon molecules with halogen atoms are called halogenated hydrocarbons. According to the different halogen atoms substituted, they can be divided into fluorinated hydrocarbons, chlorinated hydrocarbons, brominated hydrocarbons, iodinated hydrocarbons, etc. According to the number of halogen atoms in the molecule, it can be divided into monohalogenated hydrocarbons and polyhalogenated hydrocarbons. According to the different types of hydrocarbon groups, they can be divided into saturated halogenated hydrocarbons, namely halogenated alkanes, unsaturated halogenated hydrocarbons, namely halogenated alkenes and halogenated alkynes, halogenated aromatic hydrocarbons, etc., such as chlorinated CH3-CHBr-CH2Br, etc.

Alcohol: The product obtained by replacing one or several hydrogen atoms in a hydrocarbon molecule with a hydroxyl group is called an alcohol (if the product obtained by replacing the hydrogen atom on the benzene ring with a hydroxyl group belongs to the phenolic class). According to the number of hydroxyl groups in alcohol molecules, they can be divided into mono -, di -, tri -, etc. According to the different alkyl groups in alcohol molecules, they can be divided into saturated alcohols, unsaturated alcohols, and aromatic alcohols. Due to the position of the carbon atom connected to the hydroxyl group, it can be further divided into tertiary alcohols such as (CH3) 3COH. Alcohols are generally neutral, lower alcohols are easily soluble in water, and polyols have a sweet taste. The chemical properties of alcohols mainly include oxidation reactions, esterification reactions, dehydration reactions, reactions with hydrogen halide acids, and reactions with active metals.
Aromatic alcohol: a substance in which the hydrogen atom on the side bond of the benzene ring in aromatic hydrocarbon molecules is replaced by a hydroxyl group. Such as benzyl alcohol (also known as benzyl alcohol).

Phenols: Compounds in which the hydrogen atom on the benzene ring of aromatic hydrocarbon molecules is replaced by a hydroxyl group are called phenols. According to the number of hydroxyl groups contained in phenolic molecules, they can be divided into monophenols, diphenols, and polyphenols, etc. If the solution undergoes a color change reaction. Phenols have weak acidity and can react with bases to form phenolic salts. The benzene ring in phenolic molecules is susceptible to substitution reactions such as halogenation, nitration, and sulfonation due to the influence of hydroxyl groups.

Ether: A compound formed by connecting two alkyl groups through an oxygen atom is called an ether. It can be represented by the general formula R-O-R '. If R is the same as R ', it is called a simple ether, such as methyl ether CH3-O-CH3, ether C2H5-O-C2H5, etc; If R is different from R ', it is called a mixed ether, such as methyl ether CH3-O-C2H5. If the number of aldehyde groups in the molecules of diols can be divided into monohydric aldehydes, dihydric aldehydes, etc; According to the different alkyl groups in the molecule, they can be obtained by oxidation of corresponding primary alcohols. Carbonyl groups in aldehydes can undergo addition reactions and are easily oxidized by weaker oxidants such as Fehling's reagent and Dorian's reagent to form the corresponding carboxylic acid. Important aldehydes include formaldehyde, acetaldehyde, etc.
Aromatic aldehyde: An aldehyde formed by directly connecting an aldehyde group to a benzene ring in a molecule, called an aromatic aldehyde. Like benzaldehyde.
Carboxylic acid: A compound formed by linking a hydrocarbon or hydrogen atom to a carboxyl group is called a carboxylic acid. Depending on the number of carboxyl groups in the carboxylic acid molecule, it can be divided into monobasic acids, diacids, polyacids, etc. Monobasic acids such as acetic acid, saturated acids such as propionic acid CH3CH2COOH, unsaturated acids such as acrylic acid CH2=CH-COOH, etc. Carboxylic acids can also be divided into fatty acids, cyclic fatty acids, and aromatic acids. Saturated fatty acids such as stearic acid C17H35COOH, etc.

Carboxylic acid derivatives: Compounds formed by replacing the hydroxyl group in the carboxyl group of carboxylic acid molecules with other atoms or atomic groups are called carboxylic acid derivatives. Such as acyl halides, amides, anhydrides, etc.
a. Acyl halide: a compound formed by replacing the hydroxyl group on the carboxyl group of a carboxylic acid molecule with a halogen atom.
b. Amide: a compound formed by replacing the hydroxyl group on the carboxyl group of a carboxylic acid molecule with an amino-NH2 or a hydrocarbon amino group (- NHR or - NR2); It can also be regarded as a compound formed by replacing the hydrogen on the nitrogen atom of ammonia or amine molecules with an acyl group.

c. Anhydride: A compound formed by the dehydration of a monocarboxylic acid molecule between two molecules or the dehydration of a dicarboxylic acid molecule within two molecules, called an anhydride. If two acetic acid molecules lose one water molecule to form acetic anhydride (CH3COOOCCH3)
Ester: a compound formed by replacing the hydroxyl group on the carboxyl group of a carboxylic acid molecule with an alkoxy-O-R 'group
Lipids: a general term for higher fatty acid glycerides. Oil is called liquid at room temperature, and fat is called solid. It can be represented by the general formula: if R, R ', R "are the same, it is called monoglyceride; If R, R ', and R "are different, they are called mixed glycerides. Most natural oils and fats are mixed with glycerides.
Nitro compounds: Compounds formed by replacing hydrogen atoms in hydrocarbon molecules with nitro-NO2, which can be represented by the general formula R-NO2. R can be an alkyl group or a benzene ring. Such as nitroethane CH3CH2NO2
Amine: An organic compound formed by replacing the hydrogen atom in an ammonia molecule with a hydrocarbon group. According to the different hydrocarbon structures, they can be divided into fatty amines such as methylamine CH3NH2, dimethylamine CH3-NH-CH3, and aromatic amines such as aniline C6H5-NH2, diphenylamine (C6H5) 2NH, etc. It can also be divided into monoamines, diamines, and polyamines based on the number of amino groups. Monoamines such as ethylamine CH3CH2NH2, diamines such as ethylenediamine H2N-CH2-CH2-NH2, and polyamines such as hexamethylenetetramine (C6H2) 6N4. Most amines have weak alkalinity and can react with acids to form salts. Aniline is an important substance in amines and a raw material for synthesizing dyes and drugs.
Nitrile: A compound formed by connecting a hydrocarbon group with a cyanide group (- CN). The general formula is R-CN, such as acetonitrile CH3CN.
Diazo compounds: mostly organic compounds with the general formula R-N2-X, containing a type of diazo compound in their molecules, among which aromatic diazonium salts are the most important. It can be used as an intermediate for the production of azo dyes due to its chemical properties.
Azo compounds: Organic compounds containing azo groups (- N=N -) in their molecules. Represented by the general formula R-N=N-R, where R is a hydrocarbon group, azo compounds all have colors and some can be used as dyes. It can also be used as a pigment.
Sulfonic acid: A compound formed by replacing the hydrogen atom in a hydrocarbon molecule with a sulfonic acid-SO3H group, which can be represented by RSO3H. The preparation of aliphatic sulfonic acids is commonly done through indirect methods, while aromatic sulfonic acids can be directly prepared through sulfonation reactions. Sulfonic acid is a strong acid that is easily soluble in water, and aromatic sulfonic acid is an important intermediate for the synthesis of dyes and drugs.
Amino acid: A compound formed by replacing the hydrogen atom on the alkyl group of a carboxylic acid molecule with an amino group. According to the position of amino substitution, it can be divided into alpha amino acids, beta amino acids, gamma amino acids, etc. The amino group in alpha amino acids is located on the carbon atom adjacent to the hydroxyl group. Alpha amino acids are the fundamental units that make up proteins. Proteins can be hydrolyzed to obtain over twenty types of alpha amino acids, such as glycine, alanine, glutamic acid, etc., most of which are L-type alpha amino acids. Among the amino acids required by the human body, those supplied by proteins in food, such as lysine, tryptophan, phenylalanine, threonine, etc., are called "essential amino acids". Glycine, serine, alanine, glutamic acid, etc. can be obtained from other organic compounds in the human body, so they are called "non essential amino acids".
Peptide: A compound formed by the condensation of an amino group in one molecule of amino acid with a carboxyl group in another molecule of amino acid, resulting in the loss of water molecules. A peptide formed by two amino acid molecules is called a dipeptide, such as two amino acid molecules
Peptide: an organic compound obtained from natural products by condensing multiple alpha amino acid molecules to eliminate water molecules and form multiple peptide bonds
Protein: also known as prion protein. Generally, the molecular weight is greater than 10000. Protein is a major component of living organisms and the foundation of life activities. The composition, arrangement order, and three-dimensional structure of amino acids in various proteins are different. The amino acid sequence and three-dimensional structure of various proteins have been clarified. Proteins can be divided into fibrous proteins and globular proteins according to their molecular shape. Fibrin such as silk, hair, hair, skin, horns, hooves, etc., globulin such as enzymes, protein hormones, etc. According to their solubility, they can be divided into albumin, globulin, alcohol soluble protein, and insoluble hard protein. According to their composition, they can be divided into simple proteins and complex proteins. Simple proteins are composed of amino acids, while complex proteins are formed by the combination of simple proteins and other substances. For example, proteins bind to nucleic acids to produce nucleic acid proteins, proteins bind to sugars to produce glycoproteins, and proteins bind to hemoglobin to produce hemoglobin.
Carbohydrates: also known as carbohydrates. The general term for polyhydroxy aldehydes or polyhydroxy ketones, as well as compounds that can be hydrolyzed to form polyhydroxy aldehydes or polyhydroxy ketones. Sugar can be divided into monosaccharides, oligosaccharides, polysaccharides, etc. The ratio of hydrogen atoms to oxygen atoms in general carbohydrates is 2:1, but substances such as formaldehyde and CH2O are not carbohydrates; And rhamnose: C6H12O5 belongs to sugars.
Monosaccharides: are the simplest sugars that cannot be hydrolyzed, such as glucose (aldose)
Oligosaccharides: Polysaccharides that can generate 2-10 molecules of monosaccharides during hydrolysis are called oligosaccharides. Among them, disaccharides are the most important, such as sucrose, maltose, lactose, etc.
Polysaccharides: also known as polysaccharides. Polysaccharides that can produce more than 10 monosaccharides when hydrolyzed are called polysaccharides, such as starch and cellulose, and can be represented by the general formula (C6H10O5) n. N can be several hundred to several thousand.
Polymer compound: also known as "macromolecular compound" or "polymer". The molecular weight can reach thousands or even millions or more. It can be divided into two categories: natural polymer compounds and synthetic polymer compounds. Natural polymer compounds such as proteins, nucleic acids, starch, cellulose, natural rubber, etc. Synthetic polymer compounds such as synthetic rubber, synthetic resin, synthetic fiber, plastic, etc. According to their structure, they can be divided into chain like linear polymer compounds (such as rubber, fibers, thermoplastic) and network like polymer compounds (such as phenolic plastics, vulcanized rubber). Synthetic polymer compounds can be divided into adducts and condensates based on the different reactions they undergo during synthesis. Additive polymer is a high molecular weight compound generated through an additive polymerization reaction. Such as polyethylene, polyvinyl chloride, polypropylene, etc. Polycondensates are high molecular weight compounds generated through condensation reactions. Such as phenolic plastics, nylon 66, etc.

Category heterogeneity
Organic compounds have the following class heterogeneity relationships:
1. Molecular composition conforms to the class isomers CnH2n (n ≥ 3): olefins and cycloalkanes;
2. Molecular composition conforms to the class isomers CnH2n-2 (n ≥ 4): alkynes and dienes;
3. Molecular composition conforms to the class isomers of CnH2n+2O (n ≥ 3): saturated monoalcohols and saturated ethers;
4. The molecular composition conforms to the class isomers of CnH2nO (n ≥ 3): saturated monoaldehydes and saturated monoketones;
5. The molecular composition conforms to the class isomers of CnH2nO2 (n ≥ 2): saturated monocarboxylic acids and saturated monoesters;
6. Molecular composition conforms to the class isomers of CnH2n-6O (n ≥ 7): homologues of phenol, aromatic alcohols, and aromatic ethers;
If n=7, there are five types: o-cresol, m-cresol, and p-cresol; Benzyl alcohol; Benzyl ether
7. Molecular composition conforms to the class isomers of CnH2n+1O2N (n ≥ 2): amino acids and nitro compounds
Structural characteristics

Organic compounds: There are a wide variety and a large number (known to be over 30 million species, and still increasing at a rate of millions per year).
But the constituent elements are rarely C, H, O, N, P, S, X (halogens: F, Cl, Br, I), etc.
1. The bonding characteristics of carbon atoms in organic compounds
The outermost layer of carbon atoms has four electrons, which are not easily lost or acquired to form cations or anions. Carbon atoms form covalent compounds with various non metals such as hydrogen, oxygen, nitrogen, sulfur, phosphorus, etc. through covalent bonds.
Due to the bonding characteristics of carbon atoms, each carbon atom can not only form four covalent bonds with hydrogen atoms or other atoms, but also bond with each other through covalent bonds. Carbon atoms can not only form stable single bonds, but also stable double or triple bonds. Multiple carbon atoms can combine with each other to form carbon chains of varying lengths, which can also have branched chains or combine to form carbon rings. Carbon chains and carbon rings can also combine with each other. Therefore, molecules containing the same type of atoms and the same number of each atom may have multiple different binding modes to form molecules with different structures.

2. Isomerization phenomenon of organic compounds
Compounds have the same molecular formula but different structures, resulting in differences in properties. This phenomenon is called isomerism. Compounds with isomerism are isomers of each other. In organic compounds, as the number of carbon atoms increases, the number of isomers also increases. The phenomenon of isomers is very common in organic compounds, which is also one of the reasons why there are a large number of organic compounds in nature.