A mechanistic study of the reactions of formaldehyde with aniline in the presence of sulfite

Abstract

¹H NMR results are reported for the reactions of hydroxymethanesulfonate, 1, with aniline and its derivatives to produce anilinomethanesulfonates, 3. The mechanism of the reaction involves dissociation of 1 to produce formaldehyde, as a steady state intermediate, which reacts with aniline to give a carbinolamine; subsequent dehydration and reaction with sulfite produces the product. The kinetics of individual steps have been investigated. The release of sulfite from 1, measured by reaction with iodine, is shown to involve the ionised, dianionic form when pH > 3. Rate constants, k₂, and equilibrium constants, K₂, are reported for carbinolamine formation; the values of k₂, but not K₂, are affected significantly by substituents in the aniline. It is deduced that the rate-limiting step in the overall formation of products 3 changes in the pH range 6–8 from carbinolamine formation to carbinolamine dehydration.