Question

Alkanes constitute a homologous series of straight chain saturated hydrocarbons. Write down the structural formulae of the first five homologues of this series. Write their molecular formulae and deduce the general formula of such homologous series.

Answer 1

The first five homologues are generated by adding one – CH₂ – at a time, starting with the first homologue, methane (CH₄).

By counting carbon and hydrogen atoms in the five homologues, we get their molecular formulae as CH₄ , C₂ H₆ , C₃ H₈ , C₄ H₁₀ and C₅ H₁₂ . Comparing these molecular formulae and assigning the number of carbon atoms as ‘n’, the following general formula is deduced: C_n H_{2n + 2}