Micellar catalysed elimination reactions of p-substituted phenethyl bromides and related compounds in alkaline solutions

Abstract

The rates of dehydrobromination of para-substituted β-phenethyl bromides (3) were determined in alkaline solutions under micellar (k_ψ) and non-micellar (k₀) conditions by using hexadecyltrimethylammonium bromide (1) and di-methyl-(2-hydroxyethyl)octadecylammonium chloride (2) as the micellar surfactants. Large micellar effects (k_ψ/k₀) were observed. Good Hammett relationships were obtained for both k_ψ and k₀ with σ_p^– constants. Fairly large isotope effects (k_H/k_D= 8–9) were also observed for both k_ψ and k₀. The results suggest that the transition states are more carbanion-like under the micellar conditions compared with the non-micellar conditions. However, H–D exchange of the starting bromides with solvent water, which would indicate carbanion formation prior to elimination, was not detected.