Effect of Ultrasound on the Base-Catalyzed Hydrolysis of 4-Nitrophenyl Acetate in Aqueous Ethanol

Abstract

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.

Ultrasonic on-line spectrophotometric system for reaction kinetic measurements

“…However, several problems such as difficult temperature control and well-known erosion of equipment materials caused by cavitation arise in kinetic measurements. For instance, metal particles dispersing from the sonication horn and contaminating the reaction mixture may interfere with various kinetic measurements or even catalyse the reactions under examination [7].…”

“…The sonolytic degradation of 4-nitrophenol (4-NP) was also monitored in an identical manner because during the ester hydrolysis reaction under sonication this degradation can lead to defacement of the kinetic results [7]. Both 4-NPCA and 4-NP were purchased from commercial sources and were used without further purification.…”

“…Application of quantitative methods, including kinetic measurements and correlation analysis, to the study of homogeneous ionic reactions under sonication in aqueous and aqueous-organic binary solvents has brought to light specific solute-solvent interactions and hydrophobic effects that are not manifested in conven-* Corresponding author, sander.piiskop@ut.ee tional kinetic investigations [7][8][9][10]. It has been concluded that the sonochemical effects are related to the perturbation of the molecular structure of the solvent and the destruction of hydrophobic solute-solvent interactions.…”

“…It has been concluded that the sonochemical effects are related to the perturbation of the molecular structure of the solvent and the destruction of hydrophobic solute-solvent interactions. In this regard, ultrasonic kinetic measurements as a method are a useful tool for physico-chemical investigations to obtain information not easily accessible by any other method [5,[7][8][9].…”

“…Nevertheless, cavitation causes erosion of various materials, damaging electrodes and reducing their lifetime. Metal traces from the immersed titanium sonication horn or other materials may also cause difficulties, contaminating the reaction mixture [7].…”