Compounds containing only carbon and hydrogen are known as hydrocarbons. Petroleum and coal are the two important natural sources of hydrocarbons. Petroleum is a major source of aliphatic hydrocarbons while coal is a source of aromatic hydrocarbons. Hydrocarbons, broadly, may be of three types.
1. Aliphatic or Open chain : In this type the various carbon atoms are linked together to form an open chain. These may again be of two types.
(i) Saturated hydrocarbons are those in which all the four valencies of carbon atom(s) are satisfied by four atoms or groups, i.e. the carbon valencies are fully saturated. These compounds contain only single covalent C-C and C-H type of bonds. Since they are relatively inert towards most of the chemical reagents under ordinary conditions, they are called paraffins (Latin: parum-little; affinis- affinity). The IUPAC name for these compounds is alkanes.
(ii) Unsaturated hydrocarbons are those in which two carbon atoms are linked together by double (in case of olefins or alkenes) or triple (in case of acetylenes or alkynes) bond, i.e., they do not have the maximum possible number of hydrogen atoms.
2. Alicyclic hydrocarbons : Those cyclic hydrocarbons which resemble aliphatic hydrocarbons in several respects are known as alicyclic. These are also called as cyclic aliphatic hydrocarbons, cycloalkanes, cycloparaffins or polymethylenes, e.g. cyclohexane.
3. Aromatic : Those hydrocarbons which have at least one six membered carbocyclic ring with altemate single and double bonds (benzene ring) are called aromatic. Most of these have fragrant odour and possess distinctive properties from those of the aliphatic compounds. These are also called as arenes.
