Electrophilic Bromination of Micelle-Associated Alkenes as a Probe of Micelle Structure

Article abstract of DOI:10.1021/ja00273a035

The products and kinetics have been examined for the bromination in aqueous micelles of a series of unsaturated fatty acids, esters, and alcohols.A limited kinetic study of mercuration in SDS micelles is also reported.Bromination products are the following: CTAB micelles, 100percent dibromide; CTAC, principally α-bromochloride, with some bromohydrin and trace dibromide; SDS, a 2:1 mixture of bromohydrin and a bromosulfate ester derived from reaction with the headgroup of the micelle.These products are formed stereospecifically anti,and there is no significant dependence on the original position or geometry of the double bond.The bromination kinetic studies, carried out in SDS, Brij-35, and CTAB micelles, reveal considerably slower rates as compared to those in water and, in contrast to homogeneous media, sensitivities to the double bond position, its geometry and the presence of other unsuturated chains, and an enhanced terminal alkene reactivity.A dependency on micelle charge type and added salts is also observed.Both products and kinetics support a mechanism where the bromonium ion intermediate is formed and trapped in a relatively polar region of the micelle, intimately associated with the headgroup and its counterions.A model, the segment protrusion model, is proposed in which the double bond enters the "reactive" region by protrusion of a chain segment from the relatively nonpolar hydrocarbon region.The rate of reaction is determined by the absolute rate in this region, which is large, offset by the probability of the double bond being found there, which is small.The fraction of double bonds in a reactive state is determined by two factors: the probability that the chain segment containing the double bond will reach an interfacial location and the free energy cost of solubilizing a chain segment in a polar environment.This model is examined in the context of other recent experimentally and theoretically based models of micelles.