Reactions within association complexes: Acid catalysed hydrolysis of substituted benzaldehyde acetals in the presence of detergents

Article abstract of DOI:10.1039/b203909p

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.