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 seconda??order rate constants for reactions of 2,4a??dinitrophenyl Xa??substituteda??benzenesulfonates (4aa??4f) and Ya??substituteda??phenyla??4a??nitrobenzenesulfonates (5aa??5f) with ion have been measured spectrophotometrically. The reactions of 4aa??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 electrona??withdrawing group to an electrona??donating group. The Hammett plots for reactions of 4aa??4f are linear with ??Xa??=a??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 ??X value. The nature of the reaction mechanism (i.e., a stepwise mechanism in which expulsion of the leaving group occurs after the ratea??determining step) is also responsible for the smaller ??X value obtained from the C?￡?O bond fission. The Br??nsteda??type plot for the reactions of 5aa??5f is linear with ?2lga??=a??a??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.