Acylation. Part XXXVII. The catalysed benzoylation of ethylene oxides by benzoyl halides in aprotic solvents

Abstract

The benzoylation of ethylene oxide by benzoyl chloride in aprotic solvents occurs slowly at 70°, but is catalysed by acids and bases. At ethylene oxide concentrations [graphic omitted]1M the only significant product is 2-chloroethyl benzoate. The hydrogen chloride catalysed reaction is zero order in oxide and obeys the rate equation –d[ethylene oxide]/dt=k₂[catalyst][benzoyl chloride]. The hydrogen chloride and excess of oxide lead rapidly to an equivalent quantity of 2-chloroethanol, the latter being acylated by the benzoyl chloride in the slow step. Added 2-chloroethanol also acts as a catalyst and leads to the same rate equation. Catalysis by tetraethylammonium chloride also involves a process zero order in ethylene oxide and a rate equation analogous to that found for catalysis by hydrogen chloride. The reaction orders, the effects of substituent changes in the reactants, and the known reactivities of alkoxide ions suggest a mechanism of benzoylation involving a rapid, initial association between ethylene oxide and the ammonium chloride to give a polar complex, which then attacks the benzoyl chloride in a slow step. Values of ΔS^‡ and E_a have been determined. The kinetic from of the pyridine catalysed reaction has also been studied briefly.