Effect of cetyltrimethylammonium micelles with bromide, chloride, and hydroxide counterions on the rates of decomposition of para-substituted aryl-2,2,2-trichloroethanols in aqueous NaOH

The effect of cetyltrimethylammonium micelles with bromide, chloride, and hydroxide counterions, CTAX, X = Br, Cl, OH, on the decomposition of aryl-2,2,2-trichloroethanols was determined. This reaction follows an (Elcb)R type mechanism, i.e., rapid reversible deprotonation of the alcohol by OH^- followed by rate determining loss of trichloromethyl carbanion to give the benzaldehyde product. In the presence of surfactants, despite the 3-5-fold decrease in the rate constant for the unimolecular decomposition of the alkoxide ion in the micellar phase, k_m, the dependence of k_m on substituents effects (values of ρ of -0.49, -0.46, and -0.45 were obtained from the corresponding Hammett plots for CTABr, CTACl, and CTAOH) is similar to that observed in water (ρ = -0.5). The observed inhibition can be rationalized in terms of charge stabilization of the ground state, i.e., quaternary ammonium ions interact more strongly with alkoxy anions than with trichloromethyl carbanions.