Mechanism of electrophilic substitution at a saturated carbon atom. Part X. Anion-catalysed acidolysis of α-ethoxycarbonylbenzylmercuric chloride

Abstract

The demercuration of α-ethoxycarbonylbenzylmercuric chloride in ‘70%’ aqueous dioxan is a chloride-ion-catalysed acidolysis. The reaction requires one hydrogen ion and two chloride ions stoicheiometrically, and two hydrogen ions and two chloride ions kinetically. The reaction is also auto-anticatalytic. Its products are the trichloromercurate ion and ethyl phenylacetate, and its rate is inversely proportional to the concentration of the former. Although two proton transfers are significant for the kinetics, hardly any kinetic isotope effect appears when the water in the solvent is replaced by deuterium oxide.

These observations are consistent with the assumption that the acidolysis is a two-anion catalysed, bimolecular, electrophilic protodemercuration with rearrangement, S_E2′–2X^–. One previous example of the mechanism is known. The rearrangement arises because the immediate product of this mechanism is the enolic form of ethyl phenylacetate. This is formed in a pre-equilibrium, and its subsequent ketonisation so readjusts the overall thermodynamics that the demercuration goes to completion.