Nucleophilic substitution reactions of benzyl benzenesulphonates (YC6H,CH20S02C6H,Z) with anilines (XC,H,NH,) have been studied in a series of methanol-acetonitrile mixtures. A more electron-donating substituent in the nucleophile ( X = p -M e O ) and a more electron-withdrawing substituent in the leaving group (Z = rn-NO,) led t o an increase in the rate and t o a later transition state with a longer substrate-leaving group bond and a shorter nucleophile-substrate bond. A n electron-withdrawing substituent in the substrate (Y = p-CI) was also found t o favour a later transition state. A n increase in the methanol content of the solvent increased reactivity but decreased selectivity in accordance with the reactivity-selectivity principle; however, the results of variations in the nucleophile, the leaving group, and the substrate in general violated the reactivity-selectivity principle. A n M O model based on energy decomposition analysis for predicting S,2 transition-state structure has been shown t o apply to the results of this work.The nucleophilic substitution of benzyl benzenesulphonates with anilines [equation (i)] in methanol-acetonitrile mixtures serves as a convenient model reaction for study of comprehensive effects of substituents (X, Y, and Z) as well as of the effect of solvent. In our previous paper,' we reported that reaction (i) proceeds oia a dissociative SN2 mechanism with less than 50% bond formation and extensive bond breaking at the transition state (TS), bond formation being favoured by a more polar (higher MeCN content) solvent. We also pointed out that the resonance contribution of the substituent Y to TS stabilization was substantial. In this work, we extend our studies on reaction (i) to investigate variations in TS structure caused by simultaneous changes in X, Y, and Z in a series of MeOH-MeCN solvent mixtures, with special reference to the effects of substituents on the degrees of bond-making and -breaking in the TS. Predictions of TS variation based on an energy decomposition model ' have been used in interpretation of the results.
Results and DiscussionEffect of Su@tituents.-Second-order rate constants for the reaction (i) (Y = H, p-Cl; Z = p-Me, H, p-C1, or rn-NO,; X = p-MeO, p-Me, H, p-CI, or rn-NO,) in MeOH-MeCN mixtures are summarized in Table 1. The rate is seen to increase with a more electron-donating substituent in the nucleophile (X = p-MeO) and with a more electron-withdrawing substituent in the leaving group (Z = m-NO,). Table 1 also reveals that the effect of substituent on the rate is stronger in the leaving group than in the nucleophile, indicating that carbon-leaving group (C,-L) bond-breaking is relatively more important than nucleophilecarbon (N-C,) bond formation at the transition state (TS).Variations of Hammett px values [obtained by changing substituents (X) in the nucleophile] with substituents in the substrate (Y) and in the leaving group (Z) are shown in Table 2 for various solvent mixtures. The px values range from -0.8 to -1.4, indicating that substantial...
