Kinetic Study on Alkaline Hydrolysis of Y‐substituted Phenyl X‐substituted Benzenesulfonates: Effects of Changing Nucleophile from Azide to Hydroxide Ion on Reactivity and Transition‐State Structure

Abstract

Second‐order rate constants () for alkaline hydrolysis of 2,4‐dinitrophenyl X‐substituted benzenesulfonates (1a–1f) and Y‐substituted phenyl 4‐nitrobezenesulfonates (2a–2g) have been measured spectrophotometrically. Comparison of with the values reported previously for the corresponding reactions with has revealed that OH  is only 103‐fold more reactive than , although the former is 11 pK a units more basic than the latter. The Yukawa–Tsuno plot for the reactions of 1a–1f results in an excellent linear correlation with ρ X = 2.09 and r = 0.41. The Brønsted‐type plot for the reactions of 2a–2g is linear with β lg = −0.51, which is typical for reactions reported to proceed through a concerted mechanism. The Yukawa–Tsuno plot for the reactions of 2a–2g exhibits excellent linearity with ρ Y = 1.85 and r = 0.25, indicating that a partial negative charge develops on the O atom of the leaving group in the transition state. Thus, the alkaline hydrolysis of 1a–1f and 2a–2g has been concluded to proceed through a concerted mechanism. Comparison of the ρ X and β lg values for the reactions with ions suggests that the reactions with hydroxide ion proceed through a tighter transition‐state structure than those with azide ion.