Alkanes (Paraffins)

Alkanes are saturated hydrocarbons having the general formula C_nH_2n+2. The simplest alkane, methane (CH₄), is the principal constituent of natural gas. Methane, ethane, propane, and butane are gaseous hydrocarbons at ambient temperatures and atmospheric pressure. They are usually found associated with crude oils in a dissolved state.  Normal alkanes (n-alkanes, n-paraffins) are straight-chain hydrocarbons having no branches. Branched alkanes are saturated hydrocarbons with an alkyl substituent or a side branch from the main chain. Abranched alkane with the same number of carbons and hydrogens as an n-alkane is called an isomer. For example, butane (C₄H₁₀) has two isomers, n-butane and 2-methyl propane (isobutane). As the molecular weight of the hydrocarbon increases, the number of isomers also increases. Pentane (C₅C₁₂)  has three isomers; hexane (C₆H₁₄) has five. The following shows the An isoparaffin is an isomer having a methyl group branching from carbon number 2 of the main chain.

A naphtha fraction  (obtained as a light liquid stream from crude fractionation) with a narrow boiling range may contain a limited but still large number of isomers.