Pseudophase model for reactivity in reverse micelles. The case of the water-promoted reaction between two lipophilic reagents, bromine and 1-octene, in AOT microemulsions

Kinetic investigation of 1-octene bromination in AOT-isooctane-water microemulsions (13 less than or equal to w = [H2O]/[AOT] less than or equal to 24 and 6 less than or equal to z = [IO]/[AOT] less than or equal to 57) shows that the reaction is first-order in alkene and first-order in bromine, as usually found in protic media. Although both reagents are mainly located in the isooctane phase (K-tr, transfer coefficients from isooctane to water, are 1.5 x 10(-5) and 8.8 x 10(-3) for alkene and bromine, respectively), bromination occurs in an aqueous microenvironment, as illustrated by the high sensitivity of the bromination rate to the water content of the microemulsion. A kinetic pseudophase model describes the rate constant dependence on microemulsion composition satisfactorily by assuming competition between reactions at the interface and in the aqueous phase. Reasonable values for the coefficients of reagent partition between the interface and the two microphases and for the local bromination rate constants are obtained from the kinetic equations derived from the model. In particular, spectroscopically observed AOT-bromine complexation is in agreement with the high bromine concentration at the interface (K-2, bromine partition coefficient from isooctane to interface, = 6.8). The water-phase bromination rate constant, k(w) = 1 x 10(8) M(-1) s(-1), is in the same range as that measured in bulk water. The lower limit for the interfacial rate constant, k(i), is 10(3) M(-1) s(-1), a value close to that observed in poorly aqueous methanal (MeOH/H2O = 95/5 v/v). These data are compared with those recently obtained in the same microemulsions for solvolysis, a reaction which, Like bromination, is water-promoted but supposed to take place at the interface only. The results are discussed in terms of the chemical properties of the water molecules encased in the microemulsion droplets.