Nucleophilic Substitution Reactions of 2,4‐Dinitrophenyl X‐Substituted‐Benzenesulfonates and Y‐Substituted‐Phenyl 4‐Nitrobenzenesulfonates with Azide Ion: Regioselectivity and Reaction Mechanism

Abstract

The second‐order rate constants for reactions of 2,4‐dinitrophenyl X‐substituted‐benzenesulfonates (4a–4f) and Y‐substituted‐phenyl‐4‐nitrobenzenesulfonates (5a–5f) with ion have been measured spectrophotometrically. The reactions of 4a–4f proceed through SO and CO bond fission pathways competitively. Fraction of the SO bond fission decreases rapidly as the substituent X in the benzenesulfonyl moiety changes from an electron‐withdrawing group to an electron‐donating group. The Hammett plots for reactions of 4a–4f are linear with ρX = 1.87 and 0.56 for the SO and CO bond fission, respectively. The fact that the substituent X is further away from the reaction site of the CO bond fission than that of the SO bond fission is one reason for the smaller ρX value. The nature of the reaction mechanism (i.e., a stepwise mechanism in which expulsion of the leaving group occurs after the rate‐determining step) is also responsible for the smaller ρX value obtained from the CO bond fission. The Brønsted‐type plot for the reactions of 5a–5f is linear with βlg = −0.63, which is typical for reactions reported previously to proceed through a concerted mechanism. Effects of substituents X and Y on regioselectivity and reaction mechanism are discussed in detail.