The effect of changing the substituent in the sulphonyl group in base catalysed hydrolysis of 2,4-dinitrophenyl arylmethanesulphonate esters in aqueous solution

Abstract

The base catalysed elimination of 2,4-dinitrophenyl arylmethanesulphonates to yield sulphene has been studied as a function of the aryl group. The Brønsted selectivity (β)versus the pK_a of the corresponding substituted phenol is –0.85 and –0.50, respectively, for pyridine and hydroxide ion. The change in effective charge on the sulphonate ester during sulphene formation indicates that build-up of charge on the α-carbon atom is not far advanced in the transition state; the change in β with change in strength of the base is consistent with an unsymmetrical transition-state where C–H bond cleavage is in advance of S–O fission. Assuming conservation of effective charge there is no significant change in charge on the sulphone moiety at the transition state during the elimination. The charge distribution in the transition state for the E1cB_r mechanism indicates large changes at the sulphone, α-carbon, and leaving oxygen from the anionic ground state. Data drawn from the literature yields a charge distribution during attack of hydroxide on aryl phenyl sulphonates which provides further evidence for a concerted nucleophilic displacement process.