Mechanistic studies in strong acids. VIII. Hydrolysis mechanisms for some thiobenzoic acids and esters in aqueous sulfuric acid, determined using the excess acidity method

Article abstract of DOI:10.1139/v82-438

The excess acidity method has been applied to hydrolysis rate data, obtained as a function of medium composition, for four thiobenzoic acids, thioacetic acid, eight ethyl thiolbenzoates, and eight ethyl thionbenzoates in aqueous sulfuric acid.The mechanistic behaviour thus revealed has both similarities to and differences from that of a typical ester like ethyl benzoate, which gives benzoic acid by an A-2 reaction involving two water molecules in weak acid, and by A-1 acylium ion formation in strong acid.The thioacids follow this behaviour, except that the A-2process involves three water molecules, and that the mechanistic changeover occurs in 60percent rather than 80percent acid.The A-2 process for the ethyl thiolbenzoates is slow; the major hydrolysis mechanism is acylium ion formation, not in an A-1 reaction but by a concerted A-S_E2 process involving both proton transfer to sulfur and carbon-sulfur bond breaking.The major proton transfer agent is the undissociated sulfuric acid molecule.The thionbenzoate esters, in contrast, undergo very fast A-2 hydrolysis; so fast, in fact, that the initial protonation of sulfur is the rate-determining step in acids more dilute than about 62percent w/w.It appears that proton transfer to sulfur is a comparatively slow process.