Classification of organic compounds

There are millions of organic compounds and this makes it physically impossible to study each individual compound. To facilitate their study, organic compounds are classified into various groups and sub-groups.

They may be broadly classified into the following classes:

• Open chain or acyclic compounds.

• Closed chain or cyclic (or ring) 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. The name aliphatic is derived from the Greek word aleiphar meaning fats, as the earlier known compounds of this class were either obtained from animal or vegetable fats, or had fat like character.

Straight chain (or non-branched) compounds

CH_3-CH_2-CH_2-CH₃ CH_3-CH_2-CH_2-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:

• Homocyclic compounds, or carbocyclic compounds

• 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 east 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 generally 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 monocyclic, 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 'inc', 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:

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:

• Open chain or acyclic (simple alkanes not having any closed chains). They have the general formula C_nH_2n+2. Examples are methane(CH₄), propane(C₃H₈) and butane(C₄H₁₀).

• Cycloalkanes or cyclic alkanes (having a closed chain or rings in their molecules). They have the general formula C_nH_2n. Examples are cyclopropane(C₃H₆) and cyclobutane(C₄H₈).

Alkenes

These are hydrocarbons that contain at least one carbon-carbon double bond. For example, ethene, but-2-ene, but-1-ene.

Alkynes

These hydrocarbons contain at least one carbon-carbon triple bond. For example, ethyne, propyne.

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:

• Saturated hydrocarbons

Those that contain carbon-carbon single bonds e.g. alkanes(C-C).

• Unsaturated hydrocarbons

Those that contain carbon-carbon double or triple bonds e.g. alkenes(C=C), alkynes(C=C).