ABSTRACT
The kinetics of KCN-catalyzed benzoin condensation of benzaldehyde in water and ethanol-water binary mixtures was investigated both under ultrasound at 22 kHz and without sonication. Thermodynamic activation parameters were calculated from kinetic data obtained at 35, 50, and 65 degrees C. Evidence that ultrasound can retard reactions is reported and hence a direct proof that sonochemical processes occur in the bulk solution. Former results and literature data for ester hydrolyses and tert-butyl chloride solvolysis are involved in the discussion. A quantitative relationship between sonication effects and the hydrophobicity of reagents is presented for the first time. Ultrasound affects hydrophobic interactions with the solvent, which are not manifested in conventional kinetics. When it suppresses the stabilization of the encounter complexes between reagents, sonication hinders the reaction but accelerates it when it perturbs the hydrophobic stabilization of the ground state of a reagent.
Subject(s)
Benzoin/chemistry , Ethanol/chemistry , Benzaldehydes/chemistry , Catalysis , Chemical Phenomena , Chemistry, Physical , Hydrolysis , Kinetics , Potassium Cyanide/chemistry , Temperature , Thermodynamics , Ultrasonics , WaterABSTRACT
Kinetics of the benzoin condensation of benzaldehyde in presence of KCN as the catalyst in water and in ethanol-water binary solutions were investigated without sonication and under ultrasound at 22 kHz. A statistically significant 20% decrease of the rate was observed in water. The retardation effect of ultrasound gradually decreases up to 45 wt% ethanol content. We report an evidence of ultrasonic retardation of reactions and thereby a direct evidence for sonochemical processes in the bulk solution. Ultrasound can disturb solvation of the species in the solution. If breaking down the stabilization of the encounter complexes between the reagents, sonication hinders the reaction while perturbation of the solvent-stabilization of the reagents accelerates the reaction.
Subject(s)
Benzoin/chemistry , Hydrophobic and Hydrophilic Interactions , Ultrasonics , SolutionsABSTRACT
The kinetics of base-catalyzed hydrolysis of 4-nitrophenyl acetate was studied in water and water-ethanol mixtures under ultrasound (21.1 kHz) using a probe equipped with a quartz horn and, by comparison, without sonication. The ethanol concentration was varied from 0 to 50 wt % and pH from 7.5 to 9. The increase of reaction rate was inversely proportional to the strength of substrate interactions with the solvent structure. It was found that the effect of ultrasound could be related to the perturbation of solute-solvent interactions. This conclusion seems to apply to both acid-catalyzed and base-catalyzed hydrolysis reactions. Parallel experiments with a titanium horn revealed the catalytic action of undetermined chemical species formed from dispersed titanium metal.
