Reactivity in reverse micelles: acid-catalysed hydrolysis of para-substituted benzaldehyde diethyl acetals

Abstract

Acid-catalysed hydrolysis for a series of para-substituted benzaldehyde diethyl acetals in the presence of reverse micelles prepared from water, heptane, and bis-(2-ethylhexyl) sodium sulphosuccinate (AOT) have been studied. The measured rate constants, k_obs, are not proportional to the catalyst concentration [H₃O⁺]; each substituted benzaldehyde diethyl acetal yields a plot of k_obsversus[H₃O⁺] which exhibits a sharp break. Above and below this break, the variation is roughly linear; this seems to indicate two domains of proton activity. In the two proton concentration ranges, we have determined the Hammett ρ constant of the reaction (ρ=–2.46 at weak proton concentrations; ρ=–2.59 at high proton concentrations). Comparison of our results with ρ-values determined in homogenous media leads to the conclusion that the reaction occurs in the very polar environment of the interface where the positively charged transition state of the reaction is stabilized by the sulphonate head-groups of the surfactant.