Isotope effects in nucleophilic substitution reactions XI. The effect of ion-pairing, substituents, and the solvent on S(N)2 transition states

The secondary alpha deuterium and primary leaving group nitrogen KIEs and H ammett rho values found for the free ion and ion-pair S(N)2 reactions betwe en benzyldimethylphenylammonium ion and sodium para-substituted thiophenoxi des in methanol at 20.000 degrees C show how (i) ion-pairing of the nucleop hile, (ii) a change in substituent in the nucleophile, and (iii) a change i n solvent alters the structure of a Type LI SN2 transition state. Ion-pairi ng shortens the weaker sulfur - alpha carbon (S-C-alpha) transition state b ond significantly but does not alter the stronger alpha carbon leaving grou p (C-alpha-N) transition state bond as the bond strength hypothesis predict s. However, the effect of ion pairing, i.e., the decrease in the S-C-alpha bond on ion-pairing, decreases as a more electron-withdrawing substituent i s added to the nucleophile, and the S-C-alpha bond actually increases when the nucleophile is the p-chlorothiophenoxide ion. The identical Hammett rho values of -0.85 and -0.84 for the free ion and ion-pair reactions, respect ively, may be observed because, on average, the S-C-alpha bonds are identic al in the free ion and ion-pair transition states. When a more electron-don ating substituent is added to the nucleophile, an earlier transition state is found in both the ion-pair and free ion reactions. However, the substitu ent effect is smaller in the ion-pair reactions, presumably because the cha nge in the negative charge on the sulfur atom with substituent is greater i n the free ion than in the ion-pair. The substituent effect on transition s tate structure suggested by the KIEs is not predicted by any of the theorie s that are used to predict substituent effects on S(N)2 reactions. Both the secondary alpha deuterium and primary leaving group nitrogen KIEs and the Hammett rho values indicate that the transition state is earlier when the s olvent is changed from DMF to methanol as the "solvation rule for S(N)2 rea ctions" predicts. This probably occurs because an earlier, more ionic, tran sition state is more highly solvated (more stable) in methanol.