We recently reported the effects of ultrasound on acid-catalyzed hydrolysis of alkyl esters in EtOH - H₂O mixtures.¹ We speculated that the observed sonochemical acceleration might be related to changes in the molecular structure of the binary solvent, i.e. to a perturbation of hydrophobic solute-solvent interactions.

To gain a better insight in the matter, we extended our kinetic investigation to a mechanistically complementary reaction, the base-catalyzed hydrolysis of 4-nitrophenyl acetate (4-NPA).² The kinetics was studied in H₂O-EtOH mixtures under ultrasound (US) and without it. The effect of US was investigated over the 0-50 wt % range of EtOH concentration and over the 7.5-9 pH range.

A specific feature of our work consisted in the comparative use of two different immersion horns a quartz (21.1 kHz) and a Ti horn (22 kHz). With the quartz probe the well-known problems arising from the erosion of titanium horns did not arise, and it was unnecessary to filter the samples for spectrophotometric measurements. These were carried out with and without sonication successively on the same reaction system.

Experiments with the immersed Ti horn (in H₂O at pH 7.5) revealed a catalytic action of materials formed from dispersed Ti metal and degradation products of 4-NPA. In fact, sonication experiments with a Ti horn on alkyl esters did not exhibit any catalytic phenomena.¹

Experiments with a quartz horn were carried out in H₂O at pH 7.5, 8.0 and 9.0, as well as in aqueous EtOH over the range of 0.8-50 wt% at pH 8.0 and 9.0. Remarkably, no catalytic effect was observed. In H₂O or in the presence of about 1 wt% EtOH the sonication effect did not exceed 10%. With a further increase of the EtOH content the ultrasonic acceleration increased rapidly, reaching a maximum at X_EtOH » 0.08; thereafter it dwindled gradually to became almost negligible at X_EtOH » 0.25.²

Surprisingly, the plots of sonochemical acceleration (k_son/k_non) vs. X_EtOH for 4-NPA were found to be mirror images of that previously obtained for acid-catalyzed hydrolysis of ethyl acetate¹. Their extremes lie at the same EtOH mole fractions.

The present results led us to relate ultrasonic acceleration of ester hydrolysis in H₂O and in EtOH-H₂O mixtures to a perturbation of solute-solvent interactions by power US. In fact the sonication effect varied inversely with the strength of substrate interactions with the solvent structure. This conclusion, that apparently applies to both acid-catalyzed and base-catalyzed hydrolysis, can be helpful in rationalizing the influence of solvent-solute interactions on these reactions.

[1] Tuulmets A., Salmar S., Hagu H. J. Phys. Chem., B., 107, (2003): 12891-12896

[2] Salmar S., Cravotto G., Tuulmets A., Hagu H. J. Phys. Chem., B., (2006) DOI: 10.1021/jp057405w

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1. Ultrasound-assisted mild and efficient one-pot synthesis of room-temperature ionic liquids
2. Ultrasonic revelation of hydrophobic interactions.