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

Article abstract of DOI:10.1021/ja00144a002

The enantiomer-specific oxygen exchange rate constants of 1-phenyl-1-ethanol (1) and 1-phenyl-1-butanol (2) as a function of the change in configuration at the chiral center in micellar sodium dodecyl sulfate (SDS) at 64.5 ± 0.5 °C have been determined and found to differ from their values in nonmicellar media. Addition of 0.1 M SDS to the aqueous reaction media reduces the overall rate of racemization and selectively alters the enantiomer-specific oxygen exchange reactions with the solvent for both alcohols. Although the rate constant for oxygen exchange of the C1 hydroxyl with the solvent with inversion of configuration, k_EI, decreases to the same extent as that for the overall racemization, that for exchange with retention, k_E, increases markedly 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) × 10² for 2 and (1.3 ± 0.1) × 10² for 1. A 60% reduction of ¹H NMR longitudinal relaxation times, T₁, for the methyl protons of both 1-phenylalkanols by 0.1 M SDS is consistent with the alcohols being in a more constrained environment in micellar media than in unorganized media. Although the kinetic results suggest that micellar SDS strongly perturbs the solvation sphere of the intermediate carbocations, the identity of the ¹H NMR chemical shifts of 1 and 2 in water and in 0.1 M SDS indicates, however, that the polarity of this microenvironment is similar to that of water.