Degradation in aqueous solution of O-aryl N-arylthioncarbamates to aryloxide ions and isothiocyanates. A study of Leffler's assumption

Abstract

Rate constants for the title reaction have been measured in both forward and reverse directions leading to rates and equilibrium constants for all the steps in equation (i). The effects of varying the structures of Ar Ar–O–CS–NHAr′⇆ Ar–O–CN–Ar′⇆ Ar–O^–+ Ar′NCS (i) and Ar′ on the equilibrium and rate constants were measured; identical Leffler–Grunwald indices (α=β_F/β_EQ= d log k/d log K) for the addition step are observed for variation in Ar and Ar′ although individual β values are markedly different. Identical α parameters arising from different substituent interaction pathways to a single bond are required if the reaction conforms to the Leffler assumption; on this basis the α parameter is considered a good measure of transition-state structure relative to reactant- and product-states for the addition reaction. ‘Effective charges’ on the phenolic oxygen in the thioncarbamate and its conjugate base indicate considerable C⁺–S character in the formal thioncarbamoyl bond.