SOLVATION AND STERIC EFFECTS ON ELECTROPHILIC REACTIVITY OF ETHYLENIC COMPOUNDS .2. KINETIC CRITERIA FOR NUCLEOPHILIC ASSISTANCE AND RETURN IN BROMINATION

Three kinetic criteria, m(Br), R(k(aqEtOH)/k(AcOH))Y, and KSIE (kinetic solvent isotope effect), are used to estimate the magnitude of the electrophilic and nucleophilic involvement of protic solvents and the occurrence of return in the bromination of alkenes. m(Br) are obtained from m(Br)Y(Br) correlations using only water, methanol, ethanol, and their aqueous mixtures, whose nucleophilicities are almost identical. R are the deviations from the Winstein-Grunwald plots of the less nucleophilic acetic acid and occasionally of trifluoroethanol. KSIE in MeOD or EtOD are compared with the factors-phi-Br- corresponding to their maximum values for fully developed bromide ions. The values of these criteria are measured for methylideneadamantane and allylbenzene, which are brominated via Ad(E)C1 and Ad(E)C2-intermediate mechanisms, respectively. The first mechanism, involving only electrophilic solvent assistance to the rate-limiting ionization, is characterized by an m(Br) value of 1.1, associated with R close to unity and high KSIE (1.3). For the second, in which the solvent also assists nucleophilically, m(Br) (0.8) is significantly smaller than unity and R (8.0) is markedly greater; the KSIE is still high. The reaction of adamantylidenealkanes, Ad = CHt-Bu, Ad = CMe2, and Ad = CMei-Pr, is also investigated; R are close to unity but m(Br) are only approximately 0.8 and significantly smaller KSIE (1.13 for Ad = CHt-Bu) are found. This set of values is consistent with reversible formation of highly congested bromonium ions in protic solvents. The results are compared with those obtained in halogenated solvents and in acetic acid, where return has been observed. The different magnitudes of return in protic and nonprotic media are discussed in terms of substituent dependence of solvent involvment.