Solvent effects on ester linkage of 4-nitrophenyl acetate in aqueous and ethanol solutions with imidazole and hydroxide ion as nucleophiles

مقال فى مجلة
A, AL-TERARY Seham ; BASAIF Salim ; EL-AWADY Abbas . 2008
نوع عمل المنشور
مقال علمي
Research Article
Indian journal of chemistry- Section A Inorganic, Physical, Theoretical and Analytical Chemistry
رقم الانشاء
رقم المجلد
Vol. 47
تاريخ المؤتمر
ملخص المنشورات


The reaction of 4-nitrophenyl acetate in aqueous and ethanol solutions with imidazole as a nucleophile has been monitored spectrophotometrically. The second order rate constants of these reactions are 10 times higher in water than in alcoholic solutions. This is attributed to better solvation of the initial state and less solvation of the excited state in the alcohol medium. The entropy of activation in water is more negative indicating the greater structuredness of the excited state in water. In addition to the above, the base hydrolysis reaction of the ester using OH⁻ as a nucleophile in buffered aqueous solutions has been followed spectrophotometrically as a function of pH. The observed pseudo first order rate constant obeys the relationship kobs=ko+kOH [OH⁻] where ko represents the water reactions and the buffer dependent rate constant, and kOH is the rate constant for the OH- catalyzed (specific base) reaction. For both cases, a mechanism involving a tetrahedral intermediate and in which the nucleophile attacks at the electrophillic carbon of the ester C=O is proposed.