Mechanism, kinetics and selectivity of a Williamson ether synthesis: elucidation under different reaction conditions

Abstract

The best route to uncover the mechanism of chemical reactions remains a topic of intense debate. In this work, we deploy a three-faceted approach that combines experimental probing, detailed kinetic modelling and quantum-mechanical calculations for the study of the mechanism and regioselectivity of a Williamson ether synthesis, which is of interest because of its simplicity and its broad scope in laboratory and industrial synthesis. The choice of solvent is found to have a large impact on the experimental regioselectivity, with ratios of O-alkylated to C-alkylated product at 298 K of 97 : 3 in acetonitrile and of 72 : 28 in methanol. Through experiments and kinetic modelling, we identify reaction networks that differ significantly from solvent to solvent, providing insights into the factors (proton-exchange, solvolysis and product degradation) that impact on regioselectivity and the relative rates of O-alkylation and C-alkylation. The kinetic models yield detailed information on reaction rates and energy barriers and on the existence of an additional double alkylation pathway. We carry out quantum mechanical calculations and elucidate the transition states for the two main alkylation pathways. The quantum-mechanical calculations highlight structural differences between the transition states found for the two alkylation pathways and provide information on the effect of the solvent on the stabilisation/destabilisation of various structures and hence on reaction selectivity. The three-faceted approach provides complementary information into the elementary steps of the reaction mechanism.