Direct nitrosation of aniline derivatives and other nucleophilic species by N-nitrosodiphenylamine

Abstract

In the presence of a relatively high concentration of added sodium azide. N-nitrosodiphenylamine undergoes denitrosation in aqueous acid solution to give diphenylamine irreversibly. Reaction can be effected by Cl^–, Br^–, SCN^–, I^–, SC(NH₂)₂, or H₂O, and the reactivity can be reasonably well correlated with the Pearson nucleophilicity parameter n, except for I^– and SC(NH₂)₂. where it is thought a steric effect may operate. N-Nitrosodiphenylamine is more reactive than N-methyl-N-nitrosoaniline by a factor of ca. 10². Denitrosation is acid-catalysed and shows a solvent isotope effect (at low concentration of added nucleophile), (k₀)_D2O: (k₀)_H2O, of 2.0. These facts are consistent with a mechanism involving rate-determining attack by the nucleophile on the protonated form of the nitroso-amine. At high [Br^–] the rate constant is no longer dependent on [Br^–] and the observed solvent isotope effect changes to (k₀)_D2O : (k₀)_H2O= 0.8, suggesting that under these conditions proton transfer to the nitroso-amine becomes rate-limiting. A direct reaction occurs between N-nitrosodiphenylamine (again in aqueous acid solution) and aniline (and its ring-substituted derivatives), but not with the primary amines n-butylamine and cyclohexylamine. The acidity dependence, together with the substituent effects, can be interpreted in terms of a reaction between the protonated form of the nitroso-amine and the anilinium ion, initial attack occurring at the ring rather than at the amino-nitrogen atom, as has been suggested for diazotisation of aniline derivatives in 3M-perchloric acid. The solvent-promoted denitrosation is acid catalysed with k₀∝h₀^1.0. Generally the rate of denitrosation is reduced by the addition of diphenylamine, and from an analysis of the variation of the rate constant with [diphenylamine] in the presence of various nitrite traps, it is possible to confirm the overall reactivity sequence of some nitrite traps towards a free nitrosating agent as HN₃ > NH₂SO₃H > NH₃OH.