Solvolysis of Benzoyl Halides in AOT/Isooctane/Water Microemulsions. Influence of the Leaving Group

Abstract

A kinetic study was carried out on the solvolysis of benzoyl bromide, BzBr, benzoyl chloride, BzCl, and benzoyl fluoride, BzF, in microemulsions of AOT/isooctane/water at 25 °C, where AOT is sodium bis(2-ethylhexyl)sulfosuccinate. In all cases, the pseudo-first-order rate constant, k obs, increases together with the [AOT] as a consequence of incorporating the substrates into the interface of the microemulsion. The application of a kinetic model based on the formalism of the pseudophase, which considers the distribution of the reagents between the continuous medium and the interface and that the reaction is taking place only at the interface, has enabled us to obtain the intrinsic values of the rate constant at the interface of the microemulsion for the three benzoyl halides, . The values of vary together with the water content of the microemulsion. The study of the variation of with W (W = [H2O]/[AOT]), the correlations with the E T polarity parameter, and the effect of the leaving group have allowed us to conclude that a mechanistic displacement has occurred along with the variation in W. For high water contents, the reaction occurs by an associative and dissociative path simultaneously, the relative weights of which depend on the leaving group, with the dissociative character increasing as the salient group of the benzoyl halide improves. As W diminishes, the associative path is favored as a consequence of an increase in the nucleophilicity of the water, and the dissociative path is disfavored due to the reduction in the capacity of the interfacial water to solvate the leaving group, X-. Consequently, a displacement of the reaction mechanism takes place from a dissociative extreme to an associative one as the water content of the microemulsion is reduced, which causes the sensitivity of the reaction to the nature of the leaving group to vary with W.

Cleavage of Carbofuran and Carbofuran-Derivatives in Micellar Aggregates

“…The hydration of the anionic head groups of the surfactant increases the electronic density on the hydrogen atoms in the water molecules, with the consequent rupture of the hydrogen bonds of the bulk water, increasing its nucleophilicity and consequently its reactivity. This behaviour has been previously reported by our research group for reactions of benzoyl halides [56]. Contrary to HCF and CF, an inhibitory effect for the basic hydrolysis of KCF in AOT-based microemulsions was observed.…”

Microemulsions as microreactors in physical organic chemistry

“…When comparing rates of solvolysis of benzoyl fluoride, chloride, and bromide in AOT-based microemulsions [28], the ratio k Br /k F decreases 40 times when W decreases from W = 50 to W = 2. Such decrease of the ratio k Br /k F agrees with a more efficient associative solvolytic mechanism on decreasing the water content of the microemulsion.…”

Pseudophase Model in Microemulsions

“…The pseudophase model was first extended by our research group in order to quantitatively analyse the kinetic behaviour of nitrosation reactions in microemulsions based on AOT [43,44]. Afterwards, this extended model, with minor corrections, has been satisfactorily tested on microemulsions covering all possible cases [45][46][47][48][49][50][51][52][53][54][55] such as: (i) different chemical reactions (ionic or non-ionic), (ii) reactants distributed throughout the different hydrophobic domains or (iii) with different reaction loci.…”