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

摘要

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 N-3(-) ion have been measured spectrophotometrically. The reactions of 4a-4f proceed through SO and CO bond fission pathways competitively. Fraction of the SO 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 rho(X)=1.87 and 0.56 for the SO and CO bond fission, respectively. The fact that the substituent X is further away from the reaction site of the CO bond fission than that of the SO bond fission is one reason for the smaller (X) 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 (X) value obtained from the CO bond fission. The BrOnsted-type plot for the reactions of 5a-5f is linear with (lg)=-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.