Reactions of N-heteroaromatic bases with nitrous acid. Part 6. Kinetics of the nitrosation of 2- and 4-methylaminopyridine and their 1-oxide derivatives

Abstract

The nitrosation of 2- and 4-methylaminopyridine and their 1-oxide derivatives in 0.002–5.00M-perchloric acid is of first order in both the amine and nitrous acid. The respective nitrosamines formed are easily denitrosated under the experimental conditions. The rate coefficients of the nitrosation increase with an increase in the concentration of perchloric acid and sodium perchlorate. In perchloric acid solutions whose ionic strength is maintained constant by the addition of sodium perchlorate the rate coefficients of the nitrosation of 2- and 4-methylaminopyridine only show a rectilinear dependence on the h₀ parameter of the medium. The nitrosation of 2- and 4-methylaminopyridine proceeds mainly by the interaction of the nitrous acidium ion with the protonated form of these amines whilst the nitrosation of 2- and 4-methylaminopyridine 1-oxide proceeds by the simultaneous interaction of the nitrous acidium ion with the protonated and the free form of both amines. The nitrous acidium ion seems to show a distinct discrimination in its reaction with the free form of the amines as evidenced by a rectilinear relationship between the rate coefficients of their notrosation and their K_a values. The protonated amine 1-oxides react faster than the protonated amines when the hydroxy-group is in the para-position with respect to the amino-group and therefore not involved in hydrogen bonding with it. The nitrosation of the free and the protonated amines involves an initial interaction between the nitrosating agent and the heteroaromatic nucleus. The present results show that the formation of the respective N-nitroso-derivative is the rate-determining stage of the diazotisation of the N-heteroaromatic amines over the whole of the acid range examined. pK_a Values are recorded.