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, km, the dependence of km 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.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry