Quaternary Ammonium Salts

A quaternary ammonium ion is a positive ion in which a nitrogen atom is bonded to four alkyl (or aryl) groups, R₄N⁺. Together with a negative ion, such as a halide, it forms a quaternary ammonium salt, for example tetraethylammonium bromide, (CH₃CH₂)₄N⁺Br⁻.These salts are the end point of the reaction between ammonia and a haloalkane. The lone pair on nitrogen makes ammonia a nucleophile, and every amine formed still has a lone pair, so substitution carries on: primary amine → secondary amine → tertiary amine → quaternary ammonium salt. The last step is a tertiary amine attacking another haloalkane molecule:(CH₃CH₂)₃N + CH₃CH₂Br → (CH₃CH₂)₄N⁺Br⁻The mechanism is nucleophilic substitution, as in the earlier steps. Once the nitrogen has formed four bonds it has no lone pair left, so the quaternary ion cannot act as a nucleophile and the substitution stops.To make a quaternary ammonium salt the haloalkane is used in excess. (To make a primary amine instead, ammonia is used in excess.) Starting from ammonia, four haloalkane molecules are needed per nitrogen atom, one for each new N-C bond.Worked example. How many molecules of bromomethane react with one molecule of dimethylamine, (CH₃)₂NH, to make tetramethylammonium bromide? Dimethylamine already has two N-C bonds and needs four, so 2 more bromomethane molecules are needed.Quaternary ammonium salts are ionic, so they are crystalline solids that dissolve in water.