Anomalous salt effects on a micellar‐mediated reaction of bromide ion

Reaction of bromide ion with α‐picryl‐p‐bromoacetophenone (1) is speeded by aqueous cationic micelles of cetyltrimethylammonium bromide (CTABr; C₁₆H₃₃NMe₃Br) and dodecyltrimethylammonium bromide (DoTABr; C₁₂H₂₅NMe₃Br) and rate constants reach limiting values when 1 is fully bound to micelles of CTABr. Limiting values are not reached in DoTABr, but the data can be fitted to a simple model for the distribution of reactants between water and micelles. Estimated second‐order rate constants at the micellar surface are similar to values in water, but this model cannot explain the observed rate enhancements on addition of NaBr to CTABr. Inert anions such as nitrate, mesylate, n‐butanesulfonate, phenylmethanesulfonate and camphor‐10‐sulfonate inhibit reaction in CTABr by competing with Br⁻ at the micellar surface. Other n‐alkanesulfonate ions (RSO 3−, R = n‐C₅H₁₁, n‐C₆H₁₃, n‐C₇H₁₅, n‐C₈H₁₇) and arenesulfonate ions (benzene‐, toluene‐, naphthalene‐1‐ and naphthalene‐2‐sulfonate) behave anomalously. These ions expel Br⁻ from the micelle, as shown electrochemically, but there are maxima in plots of rate constant against mole fraction of Br⁻. These rate extrema are apparently due to perturbation of the micellar surface structure that overcomes the inhibition due to competition with Br⁻. These results show that the simple pseudo‐phase, ion‐exchange model can be applied only in dilute electrolyte and in the absence of hydrophobic anions.