Reactions of N-heteroaromatic bases with nitrous acid. Part 7. Kinetics of the nitrosation of secondary and of the diazotisation of primary β-aminopyridines

Abstract

The nitrosation of 3-methylaminopyridine and 3-methylaminopyridine 1-oxide and the diazotisation of 3-amino-, 3-amino-2-chloro-, and 3-amino-6-methoxy-pyridine in 0.002–0.50M-perchloric acid are first-order in both the amine and nitrous acid. The rate coefficients of these reactions increase with an increase in the concentration of perchloric acid and of 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 3-methylaminopyridine and of the diazotisation of 3-aminopyridine show only a rectilinear dependence on the [H⁺] of the medium. The nitrosation of 3-methylaminopyridine and the diazotisation of 3-amino- and 3-amino-6-methoxy-pyridine proceed mainly by the interaction of the nitrous acidium ion with the monoprotonated form of these amines whilst the nitrosation of 3-methylaminopyridine 1-oxide and the diazotisation of 3-amino-2-chloropyridine proceed by the simultaneous interaction of the nitrous acidium ion with the protonated and the free form of both amines. The nitrosation and diazotisation of the free β-aminopyridines involve an initial interaction between the nitrous acidium ion and the heteroaromatic nucleus whilst the nitrosation and diazotisation of the monoprotonated β-aminopyridines proceed by direct interaction between the nitrous acidium ion and the amino-group. These results are contrary to those of the nitrosation and diazotisation of the free and the protonated aromatic amines. Furthermore the nitrous acidium ion seems to show a distinct discrimination in its reaction with the free β-aminopyridines as is evident from a rectilinear relationship between the rate coefficients of their nitrosation and diazotisation and their K_a1 values. The similarity between the nitrosation and the diazosation results shows that the formation of the nitrosamine is the rate-determining stage of the diazotisation of the β-aminopyridines in the acid range examined. pK_a Values are recorded.