Hydrolysis of some N-alkylmaleimides
The kinetics of the hydrolysis of maleimide (MI), N-methylmaleimide (MMI), N-ethylamleimide (EMI), and N-hydroxymethylmaleimide (HMMI) were studied spectrophotometrically in pH-controlled buffer solution over the temperature range 10–50 °C. The rate of hydrolysis is proportional to both the concentration of N-alkylmaleimide and that of hydroxide ion in the pH region between 7 and 9, and is independent of pH below pH 4. The catalytic rate constants for alkaline hydrolysis increase in the order: HMMI > MI > MMI > EMI. The entropies of activation for the hydrolysis at pH 7–9 are large and negative ranging from –132 to –162 J mol–1 K–1 and both the enthalpies and the entropies of activation increase in the order: EMI > MMI > MI > HMMI. The effect of substituent on nitrogen on the rate of hydrolysis is explained satisfactorily by Taft's ρ*σ* relationship, with a value of ρ* of 0.05 at 30 °C. The experimental results can be explained by a bimolecular mechanism, in which nucleophilic attack by hydroxide ion on N-alkylmaleimide is the rate-determining step. The hydrolysis products are N-alkyl-maleamic acids.