Haloalkanes react with cyanide ions to form nitriles, compounds containing the -C≡N group. The chain gains one carbon atom.Reagents and conditions: sodium cyanide or potassium cyanide in ethanol, heated under reflux. Ethanol is the solvent; if water were used, hydroxide ions would also substitute and some alcohol would form.EquationCH3CH2Br + CN− → CH3CH2CN + Br− Mechanism: nucleophilic substitution
• The C-Br bond is polar; the carbon is δ+.
• The cyanide ion is the nucleophile. A lone pair on its carbon atom is attracted to the δ+ carbon, forming a new C-C bond.
• At the same time the C-Br bond breaks heterolytically, and the bromide ion leaves.Naming nitriles
The carbon in the C≡N group is counted in the chain. CH3CH2CN has three carbons, so it is propanenitrile. Bromoethane (two carbons) therefore gives propanenitrile (three carbons); 1-bromopropane gives butanenitrile.Cyanide salts are highly toxic, so the reaction is done in a fume cupboard with great care.Worked example: what is the maximum mass of pentanenitrile that can form from 13.7 g of 1-bromobutane (Mr 136.9)?
n(1-bromobutane) = 13.7 ÷ 136.9 = 0.100 mol. Pentanenitrile, CH3CH2CH2CH2CN (C5H9N), has Mr = 60.0 + 9.0 + 14.0 = 83.0.mass = 0.100 × 83.0 = 8.30 g
• The C-Br bond is polar; the carbon is δ+.
• The cyanide ion is the nucleophile. A lone pair on its carbon atom is attracted to the δ+ carbon, forming a new C-C bond.
• At the same time the C-Br bond breaks heterolytically, and the bromide ion leaves.Naming nitriles
The carbon in the C≡N group is counted in the chain. CH3CH2CN has three carbons, so it is propanenitrile. Bromoethane (two carbons) therefore gives propanenitrile (three carbons); 1-bromopropane gives butanenitrile.Cyanide salts are highly toxic, so the reaction is done in a fume cupboard with great care.Worked example: what is the maximum mass of pentanenitrile that can form from 13.7 g of 1-bromobutane (Mr 136.9)?
n(1-bromobutane) = 13.7 ÷ 136.9 = 0.100 mol. Pentanenitrile, CH3CH2CH2CH2CN (C5H9N), has Mr = 60.0 + 9.0 + 14.0 = 83.0.