MICELLE-INDUCED CHANGES IN THE SOLVATION OF CARBOCATIONS - EFFECT OF SODIUM DODECYL-SULFATE MICELLES ON THE ENANTIOMER-SPECIFIC OXYGEN-EXCHANGE REACTIONS OF 1-PHENYL-1-ETHANOL AND 1-PHENYL-1-BUTANOL

The enantiomer-specific oxygen exchange rate constants of 1-phenyl-1-e thanol (1) and 1-phenyl-1-butanol (2) as a function of the change in c onfiguration at the chiral center in micellar sodium dodecyl sulfate ( SDS) at 64.5 +/- 0.5 degrees C have been determined and found to diffe r from their values in nonmicellar media. Addition of 0.1 M SDS to the aqueous reaction media reduces the overall rate of racemization and s electively alters the enantiomer-specific oxygen exchange reactions wi th the solvent for both alcohols. Although the rate constant for oxyge n exchange of the Cl hydroxyl with the solvent with inversion of confi guration, k(EI), decreases to the same extent as that for the overall racemization, that for exchange with retention, k(E), increases marked ly in SDS media. For 1 in water, k(E)/k(rac) = 0.35 +/- 0.05; in 0.1 M SDS, k(E)/k(rac) = 0.63 +/- 0.01. For 2 in water, k(E)/k(rac) = 0.47 +/- 0.03; in 0.1 M SDS, k(E)/k(rac) = 1.67 +/- 0.04. The larger effect of SDS on the reactions of 2 reflects the larger association constant , K-assoc, between this alcohol and micellar SDS: K-assoc = (5.7 +/- 0 .7) x 10(2) far 2 and (1.3 +/- 0.1) x 10(2) for 1. A 60% reduction of in NMR longitudinal relaxation times, T-l, for the methyl protons of b oth 1-phenylalkanols by 0.1 M SDS is consistent with the alcohols bein g in a more constrained environment in micellar media than in unorgani zed media. Although the kinetic results suggest that micellar SDS stro ngly perturbs the solvation sphere of the intermediate carbocations, t he identity of the H-1 MMR chemical shifts of 1 and 2 in water and in 0.1 M SDS indicates, however, that the polarity of this microenvironme nt is similar to that of water.