Nitrosation of ammonia

Abstract

The kinetics of the nitrosation of the ammonia in acid solution show a first-order dependence on [NH₄⁺] and [HNO₂], and a zero-order dependence upon [H⁺]. Reaction is very strongly catalysed by both Br^– and SCN^–. These facts are consistent with a mechanism in which BrNO or ONSCN reacts with the free base form of ammonia in the rate-limiting step, to give initially a N-nitroso ion which breaks down rapidly to give nitrogen. Kinetic analysis yields values of 2.6 × 10⁷ and 0.11 × 10⁷ l mol^–1 s^–1 for the second-order rate constants for attack of BrNO and ONSCN respectively. Similarly values of 2.3 × 10⁷ and 1.7 × 10⁷ l mol^–1 s^–1 were obtained for the corresponding reactions of the more basic dimethylamine. These results for BrNO, and others in the literature strongly suggest that for these and other very basic aliphatic amines, the reaction is encounter-controlled, although the rate constants are ∼10² smaller than the calculated value. The more limited results available for the thiocyanate catalysed reactions show that ONSCN is always less reactive than BrNO, but as with the most basic amines such as dimethylamine, it too probably reacts at the encounter rate. Differences between aliphatic and aromatic amine reactions are discussed.