Kinetics and mechanism of the Lewis acid-catalysed addition of alcohols to ketenes in diethyl ether solution

Abstract

A kinetic study is reported of the addition of alchols to dimethyl- and diphenyl-ketene in diethyl ether solution at 25 °C in the presence of HgCl₂, ZnCl₂, SbCl₃, and Buⁿ₂SnCl₂. Except for HgCl₂, which forms insoluble complexes with the reactants, the metal halides catalyse the addition to dimethylketene. The catalysed addition involves the reaction of metal halide–alcohol adducts with the ketene. In general, the stronger the adduct as a Brønsted acid, the faster is the addition of the alcohol to the ketene, although C–O, as well as C–H, bond formation is probably significant kinetically. A cyclic transition state is suggested. For diphenylketene, strongly acidic adducts transfer alcohol to the ketene so slowly that inhibition of alcoholysis is observed. Inhibition arises because the spontaneous alcoholysis is reduced as free alcohol is removed as unreactive adduct. However, weak-metal halide acids probably lead to catalysis. It is concluded (a) that the different behaviour of the two ketenes in these reactions is in keeping with their behaviour observed when they react with carboxylic acids in ether, and (b) that metal halide-catalysed alcoholysis of ketenes is generally similar to that observed with aryl isocyanates, except that tin-based catalysts are relatively more effective with isocyanates.