Bromination solvation effects

Discrete and continuum models for the solvent involvement have been employed to steady equilibrium and non-equilibrium solvation effects on bromination of ethylene. Two mechanisms were identified that lead to transition states of different symmetry. One mechanism operates in the gas phase and non-polar solvents. The second one, that leads to the typical C2V transition state, holds in medium-to-very polar solvents. In water, the solvent molecules participate actively and non-equilibration solvations effects proved to be substantial and larger than those previously reported for the >SN2 reaction.23... 

Steric and solvation effects on the reactivity of ethylenic compounds are also important in bromination reactions145. Exocyclic a,/ -unsaturated ketones are brominated highly stereose-lectively146. 

The conclusion from this work is that stable surface alkyls are not observed, even at temperatures as low as 123 K. Furthermore coadsorbed dimethyl ether does not perturb the reaction pattern. At this temperature cleavage of the carbon-bromine bond is facile and the process is largely insensitive to solvation effects, but such bond-cleavage does not lead to the formation of a stable magnesium-carbon bond. 

In these solvents at sufficiently low Br2 concentration (< 10-3 m) the kinetics are first order both in the olefin and in Br2 and the main solvent effect consists of an electrophilic solvation of the departing Br ion. A nucleophilic assistance by hydroxylic solvents has also been recognized recently (ref. 26) (Scheme 10). So far, return during the olefin bromination in methanol had been admitted only for alkylideneadamantanes, and was ascribed to steric inhibition to nucleophilic attack at carbons of the bromonium ion (ref. 26). 

Fig. 9 Comparison of polar and steric effects of alkyl groups on bromination rates of linear ( ), branched (O) and adamantyl (A) alkenes in acetic acid and in methanol (Ruasse and Zhang, 1984 Ruasse et al., 1990). Polar effects are identical in both solvents [full line, eq. (24)], but steric effects differ. Deviations of branched alkenes are attributed to steric inhibition of nucleophilic solvation by methanol.

It has been shown that in the case of bromine addition to 1-pentene in solvents of different polarity, the overall rate constant varies by a factor of 10 ( ) [81]. This dramatic solvent effect has been taken - together with other findings - as strong evidence for the so-called AdnCl-mechanism, which involves considerable charge separation in the activation step. It has also been demonstrated that protic solvents enhance this addition by a specific electrophilic solvation of the anionic part of the activated complex... 

---