General acid inhibition and the absence of deuterium isotope exchange in base catalysed hydrolysis of 2,4-dinitrophenyl arylmethanesulphonate esters in aqueous solution

Abstract

The hydrolysis of 2,4-dinitrophenyl arylmethanesulphonates in aqueous buffers exhibits general base catalysis at low buffer concentration. The rate constant–buffer concentration plots become non-linear at high concentrations and this is interpreted as due to a change in rate-limiting step from proton transfer to the breakdown of the sulphene–2,4-dinitrophenolate ion encounter complex. Experiments with added solutes and kinetic arguments indicate that the curvature in the buffer concentration plots is not due to specific solute effects. The absence of deuterium exchange at high buffer concentration is consistent with an internal return mechanism which is more efficient than exchange processes; a water molecule is proposed to act as a relay for the transfer of proton from carbon acid to base.