Mutarotation of glucose derivatives in solutions of surfactants in organic solvents: co-operativity and bimodal catalytic behaviour

Abstract

The mutarotation of glucose, 2,3,4,6-tetra-O-methylglucose, 3-O-hexyl-, 3-O-dodecyl-, 4,6-O-butyl-idene, 4,6-O-hexylidene, and 4,6-O-decylidene-glucose has been studied kinetically in aqueous solution and in the following surfactant–solvent systems: AOT–heptane, AOT–CHCl₃, CPC–CHCl₃, CTAC–CHCl₃, CPS–CHCl₃ and C₁₆E₆–tetradecane. Below a low critical surfactant concentration, mutarotation is undetectably slow, but above it the rate increases, usually in a sigmoidal fashion reaching a maximum value at concentrations above ca. 40 mmol l^–1. Maximum rates are usually less than those observed in water except for AOT-containing systems which often, but not always, give higher rates. The dependence of rate on surfactant concentration does not in general fit the pseudophase model of micellar catalysis, but can be treated using Piszkiewicz's co-operativity model. This indicates in a number of cases bimodal catalytic behaviour, a non-co-operative mode at concentrations just above the critical level, and a co-operative mode giving more efficient catalysis at higher concentrations. For AOT–heptane the bimodal pattern is reversed and evidence is presented that the co-operative effects observed at low surfactant concentrations probably represent catalysis in premicellar aggregates. N.m.r. spectroscopic studies (δ, T₁) of the protons of water solubilised in the surfactant–solvent systems are reported but do not show helpful correlations with catalytic efficiency in the systems studied. A better guide to catalystic efficiency is provided by solvatochromic measurements using N-hexadecylpyridinium iodides incorporated into the catalytic aggregates to report on the polarity in the interior. A possible extension to this approach is discussed.