Aliphatic compounds are open-chain or acylic compounds, and the name aliphatic arises from the fact that the first compounds of this class to be studied were the fatty acids.

Carbon form a large number of compounds with hydrogen only and these are known collectively as hydrocarbons. There are two groups of hydrocarbons : (i) saturated hydrocarbons, (ii) unsaturated hydrocarbons.

The alkanes or the paraffins are the saturated hydrocarbons. Many occur naturally, and the chief source of the alkanes is mineral oil or petroleum, which occurs in many parts of the world.

Structural formula

The simplest alkane is methane, and its molecular formula is CH4. Assuming the quadrivalency of carbons and the univalency of hydrogen, we find that there is only one structure possible for methane, viz.,
Alkanes, structure of alkanes

Study of the reactions of methane shows that all four hydrogen atoms are equivalent, e.g.,  methylene dichloride, CH2Cl2, prepared by totally different methods, is always the same. Thus (II) and (III) are different ways of writing the same structure. At first sight it may appear that these two structural formula are different. They are different. They are different if the molecule is two-dimensional, but, in saturated compounds the four valencies of carbon are arranged tetra-hedrally. 

Substitution reactions of alkanes (methane)

Chlorine has no action on methane in the dark. In bright sunlight the reaction is explosive, and hydrogen chloride and carbon are formed.

CH4 + 2Cl2 → C + 4HCl

In diffused sunlight no explosion occurs, but a series of reactions takes place whereby the four hydrogen atoms in methane are successively replaced by chlorine atoms :

CH4 + Cl2 → CH3Cl + HCl                  methyl chloride
CH3Cl + Cl2  →  CH2Cl2 + HCl          methylene dichloride
CH2Cl2  +  Cl2 →  CHCl3 + HCl         chloroform
CHCl3 + Cl2  →  CCl4 +HCl               carbon tetrachloride