Surfactant Effects on Decarboxylation of Alkoxynitrobenzisoxazole-3-carboxylate Ions. Acceleration by Premicelles

Abstract

Decarboxylation of 6-nitro-5-alkoxybenzisoxazole-3-carboxylate ion (1,OMe and 1,OTD, alkoxy ) MeO, n-C14H29O, respectively) is accelerated by cationic micelles of cetyltrialkylammonium bromide (CTABr, CTEABr, CTPABr, CTBABr, alkyl ) Me, Et, n-Pr, n-Bu respectively). The first-order rate constants kobs for reaction of 1,OMe increase monotonically with [surfactant] and become constant when the substrate is fully bound and kobs ) k′M. The values of kobs for 1,OTD increase sharply with increasing [surfactant] and reach well-defined maxima at [surfactant] at or below the cmc, before decreasing to values corresponding to kobs ) k′M. The magnitude of the rate maxima and the increase in k′M are in the sequence CTABr < CTEABr < CTPABr < CTBABr. The rate maxima are due to formation of premicellar complexes of substrate with one or a few surfactant monomers, but they "dissolve" in micelles at higher [surfactant]. A rationalization is provided for acceleration by very dilute surfactants.

Specific interactions of anions and pre-micelles in the alkaline fading of crystal violet carbocation

“…Table 4 Values [SDS] (mM) At low surfactant concentration there is a possibility of formation of pre-micellar aggregates formed between a reactant species and a small number (n ≈ 2-4) of surfactant monomers [15] and such aggregates are different from normal micelles. The decrease of reaction rate for the alkaline hydrolysis of crystal violet in presence of anionic surfactant at low concentration (much below CMC) indicates the possibility of formation of pre-micellar aggregates [17,37]. In absence of detergent, the hydrolysis of CV + occurs through the nucleophilic attack by hydroxyl ion in the rate determining step leading to the formation of colorless CV-OH.…”

“…However, the retarding effect may be explained owing to the adsorption of one reactant on the polar micellar surface and repulsion of the other by the same [15,16]. It may be worth to mention that in a number of instances, inhibiting/accelerating effects on reaction rates by different surfactants have also been observed [17][18][19][20] even at the pre-micellar region (surfactant concentrations much below CMC). The influence of surfactants in the pre-micellar region may arise due to the aggregation of the monomeric surfactant molecules with substrate to form pre-micelles [15].…”

The dependence of micellar rate effects upon reaction mechanism

“…Rates of many reactions of hydrophobic compounds, or polyvalent counterions, are affected by surfactant at concentrations below the cmc in water. These rate effects are understandable in terms of reactant-induced micellization, although in some reactions there are rate extrema in very dilute surfactant due to acceleration by premicelles [7,[13][14][15]. The association constant of the substrate can also be written in terms of the concentration of micelles, and the equilibrium constants then differ by the value of the micellar aggregation number which is generally ca.…”

Amphiphilic nanostructures in aqueous solutions of triethyleneglycol monododecyl ether

“…This fact supports the notion that premicellization of low molecular mass surfactants might be a universal bulk solution phenomenon, depending mostly on the number density of the amphiphilic molecules. In the last years a pool of other experimental [14][15][16][17][18][19][20][21][22][23][24]28] and theoretical [25][26][27] data has emerged, that substantiates this concept.…”