Nitrosation by alkyl nitrites. Part 2. Kinetics of reactions in aqueous acid solution with isopropyl and t-butyl nitrites

Abstract

The rates of reaction of isopropyl nitrite with sulphamic acid, hydrazoic acid, thioglycolic acid, cysteine, N-methylaniline, and thiourea are markedly reduced by the addition of propan-2-ol. A detailed kinetic analysis of the results is as expected for a mechanisms where hydrolysis of isopropyl nitrite occurs rapidly (and reversibly) and the released nitrous acid effects nitrosation. The analysis yields values for the equilibrium constant for isopropyl nitrite hydrolysis and also for the rate constants for nitrous acid nitrosation (for a range of substrates), which are in good agreement with the literature values obtained by direct measurement. For t-butyl nitrite the extent of hydrolysis is so great (and the reaction is so rapid) that the kinetics are identical with those obtained using nitrous acid. Under most of the experimental conditions employed the rate-limiting step is the reaction of H₂NO₂⁺ with the substrate, but at high [substrate] for the more reactive species, the rate-limiting step changes to that of hydrolysis of RONO. This is particularly true for t-butyl nitrite, where it is easier for the nitrosation of the substrate by nitrous acid to complete with the re-nitrosation of the alcohol, since the rate of the latter reaction is much smaller for a tertiary alcohol than it is for a secondary alcohol. Further experimental measurements on the ROH + HNO₂→ RONO system, for both isopropyl and t-butyl nitrites, confirms these mechanistic ideas.