The kinetic isotope effect in the pyrolysis of esters and the nature of the transition state

Abstract

The rates of gas-phase pyrolysis of 1-phenylethyl acetate and its 2,2,2-trideuterio-isomer have been measured by a very accurate static method at 656·1 K and a value for k_H:k_D of 2·15 has been obtained. The same value is obtained from pyrolysis of 1,2-diphenylethyl acetate and its 2,2-dideuteio-isomer at 632·0 K, consequently contributions of secondary isotope effects to the primary value are insignificant; confirmation was provided by kinetic studies of the pyrolysis of erythro- and threo-2-deuterio-1,2-diphenylethyl acetates. The first detailed portrayal of the transition state for ester pyrolysis is given which is consistent with all of the known features, particularly the previously irreconcilable magnitudes of the isotope effects and Hammett ρ-factors.

The pyrolysis of 1-phenylethyl acetate and its 1-deuterio-isomer at 656·1 K proceeds with a kinetic isotope effect k_H:k_D of 1·025, attributed to hybridisation changes at the 1-carbon atom in the transition state.