A mechanistic study on alkaline hydrolysis of Y-substituted phenyl benzenesulfonates

Abstract

Second-order rate constants (kOH -) have been measured spectrophotometrically for reactions of Y-substituted phenyl benzenesulfonates (1a-h) with OH- in H2O containing 20 mol % DMSO at 25.0 ± 0.1 °C. The Brønsted-type plot is linear with βlg = -0.55 including the points for the reactions of 2,4-dinitrophenyl benzenesulfonate (1a) and 4-chloro-2-nitrophenyl benzenesulfonate (1c), indicating that the ortho-nitro group on the leaving aryloxide does not exert steric hindrance in the current reactions. The Hammett plot correlated with σ° constants exhibits highly scattered points, while the Hammett correlation with σ- constants results in a slightly better correlation but still many points deviate from the linearity. In contrast, the Yukawa-Tsuno plot shows an excellent linear correlation with r = 0.52, implying that leaving-group departure occurs at the RDS either in a stepwise mechanism or in a concerted pathway. However, the stepwise mechanism in which the leaving group departs in the RDS is excluded since the incoming OH- is much more basic and a poorer nucleofuge than the leaving aryloxide. Thus, the alkaline hydrolysis of 1a-h has been concluded to proceed through a concerted mechanism.