Reactions within association complexes: acid catalysed hydrolysis of substituted benzaldehyde acetals 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 ϩ ] are linear in surfactant concentration. When the only Na ϩ 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 ϩ ] expels H ϩ from the Stern region where reaction of the substrate occurs. Increasing [Na ϩ ] at constant [SDS] inhibits the SDS-catalysed acid hydrolysis of 4-isopropylbenzaldehyde dimethyl acetal and the dissociation constant of the Na ϩ -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 ϩ 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.

Improved Synthesis Strategy for N‐Methoxy‐1,3‐oxazinane Nucleic Acids (MOANAs)

“…The value obtained for the reaction constant, 1 = À 1.40 � 0.05, indicates considerable build-up of positive charge on going from initial state to rate-limiting transition state, consistent with a developing iminium ion at the benzylic carbon. In contrast to what has been reported for hydrolysis of benzaldehyde and acetophenone acetals, [34][35][36] the observed rate constants correlated well with the standard Hammett σ scale, suggesting that resonance stabilization of this carbocation by the aromatic ring plays only a minor role, if any. Presumably, such stabilization would be less significant with the more stable oximinium ion than with an oxocarbenium ion.…”

The Fluorescence Labelling of Primary Amines with Perylenetetracarboxdiimides

“…We started the synthesis with 4-cyanobenzaldehyde (2) and protected the aldehyde group as the dimethyl acetal 3 [14,15] because a free amino group would cause self-condensation. The cyano group was reduced [16] with lithium aluminium hydride to the amine 5.…”

Theoretical Study of Benzylic Oxidation and Effect of Para- Substituents by Using Hyperchem Program

“…We test that conclusion by investigating the stabilities of similar cations using a computational chemistry tool called an isodesmic equation, as shown in Figure 2. The rates of these reactions were measured experimentally by Rose and Williams [19].…”