Add time:07/25/2019         Source:sciencedirect.com

The hydrolysis reaction rates of p-nitrophenyl anthranilate (p-NPA) have been surveyed in aqueous buffer solutions mixed with acetonitrile (MeCN) containing various salts at 50 ± 0.1 °C. Increase in pH of the buffer solution from 8.50 to 10.0 results in an increase of the hydrolysis rate constant [log (k/s− 1)] in all solutions mixed with 0–75% (v/v) MeCN. The log (k/s− 1) values significantly decelerate as MeCN contents increase to about 50% (v/v). With further increment of MeCN contents, however, the reversal increases in log (k/s− 1) are observed. All the added salts significantly influence the hydrolysis rate in solutions containing the borate buffer of pH = 9.18. Alkali metal perchlorates (LiClO4 and NaClO4) cause the deceleration with increasing salt concentration. The presence of as low as 0.1 mol dm− 3 of NaN3 causes a three-fold acceleration, compared to the rate without the salt in no MeCN media. The rate acceleration by NaN3 becomes gradually weaker as the MeCN contents increase. Contrastingly, the rate acceleration by Et4NBr is enhanced with increasing MeCN contents. In 50% (v/v) MeCN solution, the acceleration in log (k/s− 1) caused by added salts is in the order of NaN3 > (n-Bu)4NBr ~ Et4NBr > Et4NCl. The Arrhenius plots in the 50% (v/v) MeCN media without salts and with 0.20 mol dm− 3 of LiClO4, NaN3 and Et4NBr salts give the good linearity of high activation energy values in the temperature range of 35 to 60 ± 0.1 °C, suggesting that the hydrolysis reactions are just of temperature dependence. The whole results have been discussed in terms of changes in the water structure and/or activities of H2O and OH− in the presence of both the added organic solvent and salts, and also in terms of the nucleophilicity of anions from the added salts in the “modified” media.

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