Effect of a phenyl substituent on the acidity and rate of ionisation of disulphonyl-activated carbon acids

Abstract

Replacement of one of the acidic hydrogen atoms in bisethylsulphonylmethane by a phenyl group results in a slight increase in the acidity of the other proton (0.38 pK units) but reduces the rate of ionisation 15-fold. In a cyclic disulphone in which the bulky sulphonyl groups are ‘tied-back’, a phenyl substituent has a much larger acid-strengthening effect (1.63 pK units) but nevertheless the ionisation rate is only slightly increased. An isoamyl substituent introduced into bisphenylsulphonylmethane also results in a lower rate than predicted from the pK value. By comparison with the effects of a phenyl substituent on the pK values and rates of ionisation of less sterically crowded monosulphones the unusual effect of the phenyl substituent in the disulphones is explained in terms of steric factors.