Reaction with KCN

Halogenoalkanes react with potassium cyanide, KCN, by nucleophilic substitution to form nitriles, compounds containing the -C≡N group.CH₃CH₂Br + CN⁻ → CH₃CH₂CN + Br⁻The nucleophile is the cyanide ion, :CN⁻. Its lone pair on carbon attacks the δ+ carbon of the C-Br bond, and the bromide ion leaves.Conditions: KCN dissolved in ethanol (with a little water), heated under reflux. Water must be limited, or hydroxide ions formed in solution would compete and give some alcohol.Naming nitriles: the carbon of the C≡N group is counted as part of the chain. So CH₃CH₂CN, with three carbons, is propanenitrile.Why this reaction is useful: a new carbon-carbon bond forms, so the carbon chain grows by one carbon atom. This is one of the few simple ways to lengthen a carbon chain in synthesis. The nitrile can then be converted into other compounds, for example hydrolysed to a carboxylic acid or reduced to an amine.Worked example: 1-bromobutane, CH₃CH₂CH₂CH₂Br, has four carbons. After substitution by CN⁻, the product CH₃CH₂CH₂CH₂CN has five carbons, so it is pentanenitrile.Safety: KCN is extremely toxic. It must not be mixed with acid, which would release poisonous hydrogen cyanide gas.