Kinetic sonication effects in aqueous acetonitrile solutions. Reaction rate levelling by ultrasound

The kinetics of the pH-independent hydrolysis of 4-methoxyphenyl dichloroacetate were investigated with and without ultrasonic irradiation in acetonitrile-water binary mixtures containing 0.008 to 35 wt.% of acetonitrile and the kinetic sonication effects (kson/knon) were calculated. Molecular dynamics (MD) simulations of the structure of the solutions were performed with ethyl acetate as the model ester. The ester is preferentially solvated by acetonitrile. The excess of acetonitrile over water in the solvation shell grows fast with an increase in the co-solvent content in the bulk solution. In parallel, the formation of a second solvation shell rich in acetonitrile takes place. Significant kinetic sonication effects for the hydrolysis were explained with facile destruction of the diffuse second solvation shell followed by a rearrangement of the remaining solvent layer under sonication. The rate levelling effect of ultrasound was discussed. In an aqueous-organic binary solvent, independent of the solvent composition, the ultrasonic irradiation evokes changes in the reaction medium which result in an almost identical solvation state of the reagent thus leading to the reaction rate levelling.

Kinetic sonication effects in light of molecular dynamics simulation of the reaction medium

Molecular dynamics (MD) simulation of the structure of ethyl acetate solutions in two water-ethanol mixtures was performed at 280 and 330K. The MD simulations revealed that ethyl acetate was preferentially solvated by ethanol, water being mainly located in the next solvation layer. With increasing temperature ethanol was gradually replaced by water in the first solvation shell. These findings explain the decrease in the rate of ester hydrolysis with increasing molar ratio of ethanol in the solution as the reaction rate was linearly dependent on the relative ethanol content in the first solvation shell of the ester. Predominance of ethanol results in decreased polarity and water activity in the shell and accordingly in a decreased reaction rate. Based on the results of the MD simulations, the principal conclusion of this work is that ultrasound enhances the kinetic energy (the effective temperature) of species in the solution and, in this way, evokes shifts in the solvation equilibria thus affecting the reaction rate. It appears that ultrasound does not completely break down the solvent shells or clusters in the solution as previously believed. Phenomena of thermo-solvatochromism and reaction rate levelling by ultrasound in binary solvents are described.

Role of water in determining organic reactivity in aqueous binary solvents

Kinetic data for organic reactions in various binary water-organic solvent mixtures were collected and quantitatively analysed in terms of linear-free-energy relationships by using tert-butyl chloride (2-chloro-2-methylpropane) solvolysis as the reference system. Linear similarity plots for these kinetic data were determined for solvent systems ranging from pure water mixtures up to considerable amount of cosolvent, and 161 similarity coefficients were calculated from slopes of these plots. The existence of these linear plots demonstrated that the solvent effects are of some common nature in all analysed reaction mixtures independent of the reaction type and the cosolvent used. Therefore it was concluded that the observed effects could be connected to the specific solvating properties of water, which govern reactivity even in significant dilution of water by an organic cosolvent. This conclusion was supported by the linear interrelationship between the slopes of similarity plots of different reactions, and hydrophobicity parameters log P of the reacting compounds. The relative solvent effects observed in binary water-organic solvent mixtures were for the first time directly related to the structure of reacting compounds.