A DFT study on the mechanism of the sulfonic acid + alcohol esterification reaction

Four alternative mechanisms for the benzenesulfonic acid + methanol esterification reaction have been studied at the B3LYP/aug-cc-pVTZ level. The participation of a pentacoordinate sulfur intermediate (in either neutral or protonated form) can be disregarded according to energy considerations. Instead, results show a low activation barrier for the S(N)1 pathway (through a sulfonylium cation intermediate) and a moderate barrier for the S(N)2 path (involving protonated methanol as an alkylating reagent).