Reactions within association complexes: acid catalysed hydrolysis of substituted benzaldehydeacetals in the presence of detergents

Abstract

First order rate constants for decomposition of substituted benzaldehyde dimethyl acetals at constant pH in solutions of sodium dodecyl sulfate (SDS) and Aerosol AOT (AOT) maintained at constant [Na⁺] are linear in surfactant concentration. When the only Na⁺ is that derived from the added SDS the kinetics exhibit a saturating rate law in SDS concentration. The present study shows that the curvature is not due to complexation of substrate but to ion exchange whereby increasing [Na⁺] expels H⁺ from the Stern region where reaction of the substrate occurs. Increasing [Na⁺] at constant [SDS] inhibits the SDS-catalysed acid hydrolysis of 4-isopropylbenzaldehyde dimethyl acetal and the dissociation constant of the Na⁺–SDS micelle complex is 0.0281 M which agrees with values calculated from previous data using different reactions. The complex to SDS of a neutral analogue of the acetals exhibits a dissociation constant (K_i) which is not significantly dependent on the overall concentration of Na⁺ ions in the range 0.07 to 0.42 M. Increasing concentrations of cetyltrimethylammonium bromide (CTAB; cetyl = hexadecyl) inhibit the acid catalysed hydrolysis; equilibrium constants (K_i) for the dissociation of the CTAB-complexed acetals fit a Hansch plot and possess no contribution from Hammett's sigma.