Substituent Dependence of the Selectivity in Alkene Bromination through Bromocarbenium Ions

Article abstract of DOI:10.1021/ja00408a019

Substituent effects on the rates of bromination of alkenes G_αR_αC=CR_βR'_β, where G is a conjugatively electron-donating group, are consistent with a carbenium ion like transition state, whereas by the same criteria the transition state for nonconjugated alkenes is bromonium type.The reactivities of compounds with the same substituent G are analyzed in terms of the sensitivity to structural and solvent effects.The lowest sensitivity is attributed to the earliest transition state.The dependence of the reaction constant on G leads to a general equation including cross terms: log k = -7.7Σ(?_p⁺)_α - 13.7Σ(?_m⁺)_β - 7.0(?_p⁺)_Gα(?_p⁺)_Rα - 5.8Σ(?_p⁺)_αΣ(?_m⁺)_β + 1.64.The carbenium ion character, common to both bromination and hydration, results in highly dissymmetric α- and β-substituent effects.However, these two reactions respond differently to β-substituents: a β methyl increases the bromination rate but decreases the hydration rate.Similarities and differences in the transition state models are discussed; the kinetic data suggest that the transition state is earlier in bromination than in hydration.