Denitrosation of N-methyl-N-nitrosotoluene-p-sulphonamide in acid solution

Abstract

N-Methyl-N-nitrosotoluene-p-sulphonamide (MNTS) undergoes quantitative denitrosation in dilute aqueous acid solution, to give N-methyltoluene-p-sulphonamide (MTS) and nitrous acid. The reaction is first order both in MNTS and acid, and zero order in added chloride, bromide, and thiourea. The rate of reaction is unaffected by the addition of nitrous acid traps such as sulphamic acid, and is also unchanged by the addition of MTS. From rate measurements in D₄SO₄–D₂O, the kinetic isotope effect k_H : k_D was found to be 1.5. General acid catalysis was observed for dichloroacetic acid reactions. The observations are best fitted to a mechanism in which the proton transfer from the solvent to the reactant MNTS is rate limiting. Loss of NO⁺ from the conjugate acid occurs either unimolecularly or by reaction with a nucleophile (such as bromide ion) : it is not possible to distinguish between these alternatives. The results are very similar to those obtained for the denitrosation pathway of N-nitroso-amides, and contrast with those found for the denitrosation of N-nitroso-amines, where the protonation is fast, and nucleophilic catalysis is observed. The dominating effect in the present work appears to be the strong electron-withdrawing effect of the SO₂ group.