Reactions within association complexes: The reaction of imidazole with substituted phenyl acetates in the presence of detergents in aqueous solution

The bimolecular rate constants for reaction of imidazole with phenyl acetat es complexed with sodium dodecyl sulfate (SDS) or cetyltrimethylammonium br omide (CTAB) micelles obey Bronsted equations with beta(1g) similar to that of the reaction in aqueous solution. The dissociation constants of ester ( Ks) and the hypothetical dissociation constant (K-TS) Of the transition sta te of the micelle complexes obey Hansch equations with similar sensitivitie s (p) to pi(-0.66 and -0.589 for K-S and -0.735 and -0.495 for K-TS, respec tively). The slopes also indicate that the microsolvation environments asso ciated with the transition state and the complexed ester have aqueous chara cter. The relative values of K-TS and K-S indicate that the transition stat e of the reaction of imidazole with ester is more weakly complexed to both micelles than is the reactant ester. Log K-TS values are linear functions o f log K-S for reactions with both CTAB and SDS; the slopes are, respectivel y, -0.893 and -1.19 consistent with a slightly more "water-like" medium for the transition state than for the site of binding of ester with CTAB-micel le and slightly less for the SDS-micelle. The results for ester and transit ion state are consistent with the location of the phenyl residue in a hydro phobic region that possesses water molecules. It is concluded that the acet yl group in the complexed transition state is located in an aqueous part of the Stern region, whereas the phenyl residue is in a part of the Stern reg ion that possesses alkane components. The derived kinetic and complexation parameters in these experiments refer to micelles with Stern regions that h ave been maintained at constant ionic compositions.