Kinetics and mechanism of base-catalysed degradations of substituted aryl-N-hydroxycarbamates, their N-methyl and N-phenyl analogues

Abstract

The kinetics and mechanism of the degradation reactions of substituted phenyl N-hydroxycarbamates and their N-methyl and N-phenyl analogues have been studied at pseudo-first-order reaction conditions in aqueous buffers and sodium hydroxide solutions at 20 °C and 60 °C and at I = 1 mol·l⁻¹. The dependence of log k_obs on pH for phenyl N-hydroxycarbamates at pH < 9 and pH > 13 is linear with the unit slope; at pH 10–12 log k_obs is pH independent. The Bronsted coefficient β_lg is about −1 (pH 7–13) and −1.53 (pH > 13) indicating that the degradation reaction of phenyl N-hydroxycarbamates follows an E1cB mechanism giving the corresponding phenol/phenolate and HO–NCO. The latter species undergoes further decomposition to give carbonate, nitrogen and ammonia as final products. In contrast to the phenyl N-hydroxycarbamates the N-methyl derivatives at pH 7–9 undergo degradation to the corresponding phenol/phenolate, carbonate and methylamine via a concerted mechanism (β_lg is about −0.75). The only exception is 4-nitrophenyl N-hydroxy-N-methylcarbamate in which the predominant break down pathway proceeds via the Smiles rearrangement to give sodium N-methyl-(4-nitrophenoxy)carbamate. At pH > 9 the reaction of N-hydroxy-N-methylcarbamates is kinetically complex: the dependence of absorbance on time is not exponential and it proceeds as a consecutive two-step reaction. N-Hydroxy-N-phenylcarbamate under the same conditions undergoes degradation to phenol, carbonate, aniline and azoxybenzene.