Reactivity in w/o microemulsions. Activation parameters for solvolysis in AOT/isooctane/water systems

Abstract

A kinetic study was carried out on the solvolysis of different benzoyl halides in microemulsions of AOT/isooctane/water: 4-methoxybenzoyl chloride (4-CH₃O), 3-methoxybenzoyl chloride (3-CH₃O), benzoyl fluoride (BzF) and 4-trifluoromethylbenzoyl chloride (4-CF₃). The application of the formalism of the pseudophase allows us to obtain the true rate constants for the solvolysis in the interface, k_i, which decreases 350 times on going from W = 50 to W = 2 (W = [H₂O]/[AOT]) for 4-CH₃O; 35 times for 3-CH₃O and 3 times for BzF. In the solvolysis of 4-CF₃k_i decreases 3.5 times on going from W = 2 to W = 50. This behavior has been interpreted as a consequence of a decrease of the electrophilic character of water when W decreases, whereas the nucleophilic character increases. As a consequence the rate constant for the dissociative processes decreases as W decreases, due to a reduction in the water’s ability to solvate the leaving group, and the rate of the associative processes increases due to the greater nucleophilicity of the water. The obtained results are compatible with the activation parameters determined in the microemulsion: for the solvolysis of BzF and 4-CF₃, ΔS^≠ takes values which are close to ΔS^≠ ≈ −180 J mol⁻¹ K⁻¹ and ΔS^≠ ≈ −150 J mol⁻¹ K⁻¹ respectively. These values are lower than those observed in pure water and compatible with a greater degree of structuring of the interfacial water than that of bulk water. For the solvolysis of 4-CH₃O it is observed that ΔS^≠ decreases from ΔS^≠ ≈ −40 J mol⁻¹ K⁻¹ for W = 50 to ΔS^≠ ≈ −150 J mol⁻¹ K⁻¹ for W = 2. This behavior is consistent with a change from a dissociative mechanism to an associative one which also is shown when studying a Hammett correlation using Taft σ values.