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 (k<inf>OH</inf>-) 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 k<inf>OH</inf>- with the k<inf>N3</inf>- values reported previously for the corresponding reactions with N<inf>3</inf>- has revealed that OH- is only 103 -fold more reactive than N<inf>3</inf>-, although the former is 11 pK<inf>a</inf> units more basic than the latter. The Yukawa-Tsuno plot for the reactions of 1a-1f results in an excellent linear correlation with ρ<inf>X</inf> = 2.09 and r = 0.41. The Brønsted-type plot for the reactions of 2a-2g is linear with β<inf>1g</inf> = -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<inf>Y</inf> = 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<inf>X</inf> and β<inf>1g</inf> values for the reactions with OH- and N<inf>3</inf>- ions suggests that the reactions with hydroxide ion proceed through a tighter transition-state structure than those with azide ion. © 2015 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.