Kinetics of aliphatic nitrone formation on the addition of N-alkyl-hydroxylamines to aliphatic aldehydes in aqueous solution

Abstract

The formation of aliphatic nitrones from N-cyclohexylhydroxylamine and aldehydes has been followed spectro-photometrically in aqueous buffer solution of pH 1·5–9·4. The reaction is second-order at constant pH, being dependent on the free hydroxylamine concentration and the free aldehyde concentration. The second-order rate constant shows a linear dependence on [H⁺] below pH 7, but is independent of it above pH 8. From the values of the reaction constant, ρ*, the activation parameters ΔS^‡ and E_a, and the known mechanism of several carbonyl reactions, a reaction scheme is proposed. Intermediate diol formation is a specific-acid-catalysed reaction and is rate-determining in acidic media, but the dehydration of the intermediate diol is rate-determining in alkali.