Solvent-assisted transition state formation

Bromination is less stereoselective, and the reactions of cis- and trans-olefins tend to be stereoconvergent. The stereospecific formation of the mixed bromoadducts in protic media, such as methanol or acetic acid, could be interpreted in the light of the recent finding (Ruasse et al., 1991) that these solvents assist the formation of the ionic intermediate nucleophilically. If a solvent molecule is close to the cationic part of the transition state in the rate-limiting step, the intermediate can be trapped by this solvent molecule in a necessarily trans mode with respect to the first bromine, before the two components of the ion-pair diffuse away from each other (15). This would... 

When two reacting species form a transition state in which ions are being formed, the volume of activation is often negative. The formation of such a transition state is assisted by a solvent that has a high solubility parameter. The reaction... 

Evidence for a 5-coordinate intermediate is strong, and the transition state sometimes may even be 6-coordinate, with assistance from solvent. The highest energy transition state may be either during the formation of the intermediate or as the exiting ligand dissociates from the intermediate. 

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