Micellar Effects upon the Reaction of Hydroxide Ion with N-Alkyl-2-bromopyridinium Ion

The reactivity of N-alkyl-2-bromopyridinium ions (alkyl = Me, Et, n-C₁₂H₂₅, n-C₁₄H₂₉, n-C₁₆H₃₃) toward OH⁻ is affected by cationic micelles of alkyltrimethylammonium chloride or bromide (alkyl = n-C₁₄H₂₉, n-C₁₆H₃₃) which inhibit reactions of the methyl and ethyl substrates and catalyze reactions of the more hydrophobic derivatives. These results are understandable, qualitatively, in terms of the distribution of both reactants between the aqueous and micellar pseudophases. The distribution of OH⁻ between aqueous and micellar pseudophase is governed by an ion-exchange equilibrium. The rate enhancements can be treated quantitatively, and second-order rate constants in the micellar pseudophases are essentially independent of substrate hydrophobicity and are slightly smaller than those in water. The rate differences are understandable in terms of the high electrolyte concentration at the micellar surface.