Specific salt effects on the hydrolysis reaction rate of tropolone tosylate in binary MeCN–H2O media containing n-Bu4NOH

Abstract

The hydrolysis reaction rates of tropolone tosylate have been investigated in 20–90% (v/v) MeCN–H2O binary media containing n-Bu4NOH at 50°C. Increasing concentration of n-Bu4NOH causes an increase in the hydrolysis rate. Two diverse effects have been observed with increasing content of MeCN, i.e., a decrease in the rate and the successive increase for up to the 60% (v/v) MeCN and further increasing MeCN proportions. At a constant n-Bu4NOH concentration, the added salts (0.0–1.0moldm−3) have caused various influences on the “pseudo” first order rate constant [log (k/s−1)] of the substrate. Among alkali metal and alkaline earth metal perchlorates, NaClO4 and Ba(ClO4)2 cause the obvious rate acceleration especially in the media of higher MeCN proportions. However, LiClO4 and Ca(ClO4)2 cause the rate deceleration, whereas Mg(ClO4)2 has resulted in a total suppression of the hydrolysis reaction. All the salts of NaN3, Et4NCl, and Et4NBr appear to increase the log (k/s−1) value in the order of Et4NCl–Et4NBr<NaN3. The Arrhenius plots of the hydrolysis in the 50% (v/v) MeCN–H2O binary media containing 2.0mmoldm−3n-Bu4NOH in the presence of various kinds of salts have been evaluated. All the salt systems have given a good linearity from 35 to 60°C with the activation energy (Ea) of 83.7, 79.3, 81.9, 79.1, 87.1, and 79.1kJmol−1 for no salt, 0.50moldm−3 LiClO4, NaClO4, Ba(ClO4)2, Et4NBr, and NaN3, respectively. These large values suggest that the hydrolysis reactions in the presence of various salts are controlled by the normal temperature dependent and non-catalytic mechanism with the ions from the added salt. The emerged scenarios have been discussed in terms of changes in the structure of bulk water and/or activities of H2O and OH− in the presence of both the added organic solvent and salts, and also in terms of the nucleophilicities of anions or the coordination abilities of the metal ions in the “modified” media.