Organic compounds are classified into various groups and sub-groups.

They may be broadly classified into the following classes:

1. Open chain or acyclic compounds.
2. Closed chain or cyclic (or ring) compounds.

Open chain or acyclic compounds

These compounds contain an open chain system of carbon atoms. The chains may be either straight chains (non-branched) or branched. The open chain compounds are also called aliphatic compounds. Straight chain (or non-branched) compounds

CH₃-CH₂-CH₂-CH₃ CH₃-CH₂-CH₂-CH₂OH H₂C = CH₂

Butane 1- butanol ethene

Branched Chain Compounds

These compounds contain one or more closed chains (rings) of atoms and are known as cyclic or ring compounds.

These are of two types:

1. Homocyclic compounds, or carbocyclic compounds
2. Heterocyclic compounds.

 

Homocyclic or Carbocyclic compounds

The compounds in which the ring consists of only carbon atoms are called homocyclic or carbocyclic compounds. Homocyclic compounds or carbocylic compounds are further divided into:

(a) Alicyclic compounds (b) Aromatic compounds

Alicyclic compounds

When a ring of three or more carbon atoms resembling aliphatic compounds are contained in homocyclic or carbocyclic compounds, they are called alicyclic compounds.

The saturated alicyclic hydrocarbons have the general formula C_nH_2n. Typical alicyclic compounds are given below.

Cyclopropane                   cyclobutane                       cyclohexane

cyclopropene                     cyclohexene             cyclohexa-1,4-diene             cyclopentene

Aromatic compounds

These carbocylic compounds contain at least one benzene ring i.e., a ring of six carbons atoms with alternate double and single bonds.

 Aromatic compounds are called so because many of them possess a fragrant smell.

Typical examples of aromatic compounds are given below:

Benzene

The aromatic compounds may have a side-chain or a functional group attached directly to the ring. For example,

Toluene                         Phenol             Nitrobenzene             Benzaldehyde

The aromatic compounds may also contain more than one benzene rings fused together

Naphthalene

Anthracene

Heterocyclic compounds

When atoms of more than one kind make up the ring in the compounds, they are known as heterocyclic compounds or heterocycles. In these compounds one or more atoms of elements such as nitrogen 'N', oxygen 'O', or sulphur 'S' are present. The atom other than that of carbon viz., N, O or S, present in the ring is called hetero atom. Heterocyclic compounds with five and six atoms in the ring are termed as five-membered, and six-membered heterocycles respectively.

Pyridine                                   Furan                           Thiophene                 Pyrrole 

Heterocyclic compounds may be further classified as

1. Monocyclic
2.  Bicyclic and

Tricyclic depending on the presence of one, two or three rings in their molecules, respectively.

Compounds, which contain nitrogen atom 'N' in their ring, have names usually ending with 'ole', if five-membered and with 'ine', if six-membered.

Hetero atom is always numbered as atom number 1, (except in one or two cases).

Hydrocarbons can be further classified into four types on the basis of their structures. These are:

1. Alkanes: Hydrocarbons that contain only C-C single bonds in their molecules are called alkanes. These include open chain as well as closed chain (cyclic) hydrocarbons. For example, ethane, propane, cyclopentane.

Alkanes are further divided into:

1. Open chain or acyclic They have the general formula C_nH_2n+2. Examples are methane(CH₄), propane(C₃H₈) and butane(C₄H₁₀).
2. Cycloalkanes or cyclic alkanes : They have the general formula C_nH_2n. Examples are cyclopropane(C₃H₆) and cyclobutane(C₄H₈).

 

2. Alkenes: These are hydrocarbons that contain at least one carbon-carbon double bond. For example, ethene, but-2-ene, but-1-ene.
3. Alkynes: These hydrocarbons contain at least one carbon-carbon triple bond. For example, ethyne, propyne.
4. Arenes: These are hydrocarbons that contain at least one special type of hexagonal ring of carbon atoms with three double bonds in their alternate positions. The ring is called aromatic or benzene ring.

For example, benzene, toluene, o-xylene. They also contain more than one benzene rings. For example, naphthalene (2 rings) and anthracene (3 rings).

Hydrocarbons can also be classified into:

1. Saturated hydrocarbons: Those that contain carbon-carbon single bonds e.g. alkanes(C-C).