Hammond postulate solvent effects

The Hammond postulate says that any factor stabilizing the intermediate carbocation should increase the rate of an S l reaction. Solvation of the carbocation—the interaction of the ion with solvent molecules—has just such an effect. Solvent molecules orient around the carbocation so that the electron-rich ends of the solvent dipoles face the positive charge (Figure 11.14), thereby lowering the energy of the ion and favoring its formation. 

In conclusion, bromination is a particularly attractive reaction for studying the origin of reactivity-selectivity effects in detail, since it is now well established that substituent and solvent effects arise not only from changes in the stability of the cationic intermediate but also from transition-state shifts, in agreement with the Bema Hapothle, i.e. RSP, Hammond postulate and Marcus effects. 

Thus, because this reaction would be even more exothermic, it would be predicted based on the Hammond Postulate [107] that there would be less charge transfer onto the benzylic carbon in the transition state. The fact that p values in THF are much larger than in hydrocarbon solution can also be explained by the effect of solvent on the counterion solvation of the counterion would decrease the interaction of the counterion with the carbanion which would result in more delocalization of charge. Or in terms of the Winstein spectrum of ionic species [3, 108], solvation of the counterion and an increase in the dielectric constant of the medium would shift the structure towards a more ionic species. 

---