Reverse Micellar Aggregates: Effect on Ketone Reduction. 1. Substrate Role

The reduction of three aromatic ketones, acetophenone (AF), 4-methoxyacetophenone (MAF), and 3-chloroacetophenone (CAF), by NaBH4 was followed by UV-vis spectroscopy in reverse micellar systems of water/AOT/isooctane at 25.0 °C (AOT is sodium 1,4-bis-2-ethylhexylsulfosuccinate). The first-order rate constants, kobs, increase with the concentration of surfactant due to the substrate incorporation at the reverse micelle interface, where the reaction occurs. For all the ketones the reactivity is lower at the micellar interface than in water, probably reflecting the low affinity of the anionic interface for BH4 -. Kinetic profiles upon water addition show maxima in kobs at W0 ca. 5 probably reflecting a strong interaction between water and the ionic headgroup of AOT; at W0 < 5 by increasing W0 BH4 - is repelled from the anionic interface once the water pool forms. The order of reactivity was CAF>> AF > MAF. Application of a kinetic model based on the pseudophase formalism, which considers distribution of the ketones between the continuous medium and the interface, and assumes that reaction take place only at the interface, gives values of the rate constants at the interface of the reverse micellar system. At W0 = 5, we conclude that NaBH4 is wholly at the interface, and at W0 = 10 and 15, where there are free water molecules, the partitioning between the interface and the water pool has to be considered. The results were used to estimate the ketone and borohydride distribution constants between the different pseudophases as well as the second-order reaction rate constant at the micellar interface.