Alkaline hydrolysis of substituted phenyl N-phenylcarbamates. Structure–reactivity relationships consistent with an E1cB mechanism

Abstract

The rates of hydrolysis of substituted phenyl esters of N-phenylcarbamic acid have been measured over an alkaline pH-range at 25 °C, 0·1M ionic strength. The rate constant for unsubstituted phenyl ester was proportional to hydroxide ion concentration up to pH 12. The bimolecular rate constant for alkaline hydrolysis of the esters had a Hammett sensitivity of 2·86 and σ^– values were needed to fit the 4-nitro-, 4-cyano-, 4-acetyl, and 4-formyl substituents indicating resonance interaction with the transition state of the rate-determining step. The results are interpreted according to an E1cB mechanism for hydrolysis involving phenyl isocyanate intermediate. Bimolecular attack of hydroxide ion on neutral ester, bimolecular attack of water on anionic ester, and attack of hydroxide on the nitrogen proton concerted with departure of phenolate anion are excluded as possible mechanisms.