Acceptor properties of metal halides. Part VII. Tin, zinc, and antimony halides as catalysts for the racemisation of α-methylbenzyl chloride in diethyl ether

Abstract

The catalytic efficiencies of stannic chloride, stannic bromide, zinc chloride, zinc bromide, trichloro(phenyl)tin, and antimony trichloride in the racemisation of α-methylbenzyl chloride in ether have been compared. For all the catalysts the reaction is first order in the alkyl halide, but the order in the catalyst concentration depends upon the system. With stannic and zinc halides the order is a mixture of first and second, with trichloro(phenyl)tin a mixture of first and third, and with antimony trichloride fourth. The reaction mechanism is discussed. The first-order terms in catalyst concentration are interpreted as representing paths in which the transition state is solvated by ether molecules, and the higher order terms in catalyst concentration as representing paths in which the transition state is solvated by metal halide–ether molecules. The first-order catalytic rate constants are in the sequence SnCl₄ > SnBr₄ > ZnBr₂ > ZnCl₂ > PhSnCl₃ > SbCl₃. The enthalpy of activation and entropy of activation have been determined for both paths for the stannic halides and the zinc halides. The values generally support the suggested mechanism. The effect of water on the stannic chloride and on the zinc chloride catalysis has been studied. The racemisation rate is decreased: the reduction corresponds to the formation of 1SnCl₄,2H₂O and of 1ZnCl₂,1H₂O adducts. Addition of hydrogen chloride leads to an increase in the rate of the stannic chloride and of the antimony trichloride catalysed reactions. This increase probably results from transition state solvation by hydrogen chloride–ether molecules. Molecular weight determinations show that zinc bromide, zinc iodide, and stannic bromide are monomeric in ether, and that zinc chloride is monomeric in acetone.