What are hydrocarbons? Discuss their classification.
These are organic compounds which contain only two elements viz carbon and hydrogen.
Classification. These have been classified into two main classes:
(a) Open chain hydrocarbons or acyclic hydrocarbons
(b) Closed chain or cyclic hydrocarbons.
(a) Open chain hydrocarbons: These hydrocarbons contain an open chain of carbon atoms in their molecules which may be either straight chains or branched chains in nature.
These are also called aliphatic hydrocarbons and burn with a non-smoky flame. These have been further divided into two classes:
(a) Saturated hydrocarbons: These are open chain hydrocarbons which contain single covalent bonds between either carbon and carbon atoms (C – C) or carbon and hydrogen atoms (C – H). These are represented by the general formula CnH2n+2 where n is the number of carbon atoms. e.g.
(b) Unsaturated hydrocarbons: These are open chain hydrocarbons which contain carbon-carbon multiple bonds in their molecules. These are further classified as:
(i) Alkenes: These are unsaturated hydrocarbons which contain a double bond between two carbon atoms (>C= C<) in their molecules. These are represented by the general formula 
where n is the number of carbon atoms. e.g.
(ii) Alkynes: These are unsaturated hydrocarbons which contain a triple bond between two carbon atoms 
in their molecules.
These are represented by the general formula 
where n is the number of carbon atoms. e.g. 
II. Closed chain or cyclic hydrocarbons. These hydrocarbons contain closed chains or ring of carbon atoms in their molecules. They are subdivided into two classes:
(i) Alicyclic hydrocarbons: These are hydrocarbons which contain rings of three or more carbon atoms in their molecules. They resemble aliphatic hydrocarbons in most of their characteristics. For example
All hydrocarbons containing a benzene type ring in their molecules are known as aromatic hydrocarbons or arenes.
Thus organic compounds containing one or more fused or isolated benzene rings and their functionalized derivatives are called aromatic compounds. For example,
