Unusual Rate-limiting Proton Transfer in the Acid-catalysed Reactions of N-Nitroso Compounds

Article abstract of DOI:10.1039/P29840001803

For both N-methyl-N-nitrosoaniline and N-nitrosodiphenylamine, the catalytic effect of halide ion, thiocyanate ion, and thiourea in the denitrosation reaction in acid solution disappears at high nucleophile concentrations.This suggests a change to an earlier rate-limiting step.The data, including the variation of rate constant with acidity, are quantitatively in accord with a reaction mechanism involving protonation of the nitosamine followed by nucleophilic attack, either stage being rate limiting, depending on the reactivity and concentration of the nucleophile.The kinetic solvent isotope effect k_H2O/k_D2O decreases from 1.59 at 0.43M-thiourea to 0.71 at 0.015M-thiourea, which is consistent with a change from general acid catalysis at high to specific hydrogen ion catalysis at low .Substituent effects at the amino-nitrogen atom and also in the 4-position of a phenyl substituent are very amall, the largest effect being a three-fold reduction in rate constant effected by a 4-NO2 group.These results are in accord with the known dipolar nature of nitrosamines.There is a close analogy between this system and the acid-catalysed cleavage reaction of carbamate derivatives.A similar mechanism is proposed involving a pre-association of the reactants forming a hydrogen-bonded intermediate.

Full text of DOI:10.1039/P29840001803

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