Answer. All known organic compounds have been broadly divided into two categories depending upon their carbon skeleton. These are:
(i) Open chain or acyclic compounds.
(ii) Closed chain or cyclic compounds.
(i) Open chain or acyclic compounds.
Acyclic compounds contain open chains of carbon atoms in their molecules. These chains may be either straight or branched. For example,
(a) Straight chain compounds are those in which carbon atom links with other carbon atoms or other atoms through a single, double or triple bonds forming a straight chain, such as
I I I I I I
―C-C-C-C-C-C―
I I I I I I H3C-CH2-CH2-CH3
Straight chain Straight chain (n-Butane)
(b) Branched chain compounds are those in which there is a branch along a straight chain, such as:
H3O-CH-CH3
CH3
Branched chain Branched chain (isobutane)
Open chain compounds are also called aliphatic compounds.
(ii) Closed chain or Cyclic compounds
Closed chain or cyclic compounds contain one or more closed chains, i.e., the carbon atoms at the end of the chain are not free. They are further divided into two classes:
(a) Homocyclic or carbocyclic compounds.
(b) Heterocyclic compounds.
(a) Homocyclic or Carbocyclic compounds.
Homocyclic or carbocyclic compounds contain rings which are made up of only one kind of atoms, i.e., carbon atoms. These are further divided into two classes:
• Aromatic compounds
• Alicyclic compounds
Aromatic compounds: These organic compounds contain at least one benzene ring in their molecule. A benzene ring is made up of six carbon atoms with three alternating double bonds. They are called aromatic because of aroma. Aroma means smell. Examples,
Benzene Napthehatene
They are also called benzenoid compounds.
Alicyclic or non-benzenoid compounds: Carbocyclic compounds which do not have benzene ring in their molecules are called alicyclic or non-benzenoid compounds. For example
Cydobutan Cyclohexane
Heterocyclic compounds
Cyclic compounds that contain one or more atoms other than that of carbon atoms in their rings are called heterocyclic compounds.
Thiophene Pyridine
The above classification may be summarized as follows: