Comparison of the reactivity of nine nitrous acid scavengers

Abstract

Rate constants have been measured for the reaction of nitrous acid in aqueous solution at 25 °C with urea, 2,4-dinitroaniline, hydroxylamine, sulphanilic acid, hydrazoic acid, sulphanilamide, hydrazine, sulphamic acid, and 4-nitroaniline, as a function of acidity up to ca. 1.5M. The acid–rate profiles, which are all consistent with mechanisms involving electrophilic nitrosation and various protonation equilibria, give the order of reactivity at each acidity of these potential nitrous acid scavengers. The order of reactivity varies with the acidity because of the different acidity dependences, e.g. the ‘best’ scavenger at 0.05M-acid is 4-nitroaniline followed by sulphamic acid, whilst at 1.3M-acid the most reactive is the hydrazinium ion followed by hydrazoic acid. The effect of added halide ion, thiocyanate ion, and thiourea was also examined. Nitrosation of urea, 2,4-dinitroaniline, hydrazinium ion, and sulphamic acid was not catalysed by any added nucleophile, probably because of the importance, for these substrates containing electron-withdrawing groups, of the reversibility of the initial nitrosation. For the remaining five substrates normal nucleophilic catalysis was observed; analysis of the kinetic data enables the second-order rate constant for attack by the nitrosyl halide, etc. to be evaluated. The reactivity sequence NOCl > NOBr > NOSCN > NO[graphic omitted]C(NH₂)₂ was found in each case.