Reactions of Phosphinates. The Acid-Catalyzed and Acid-Inhibited Hydrolysis of p-Nitrophenyl Diphenylphosphinate

Abstract

The rates of hydrolysis of p-nitrophenyl diphenylphosphinate have been studied in acidic dioxane-water (40:60, v/v). Oxygen-18 studies show that hydrolysis occurs by cleavage of the P-O bond. The dependence of rate on acid concentration passes through a maximum at ~ 1.5 M HClO4. The observed data give a w value (Bunnett) of 12 although ∆S* is only - 27 eu. Measurement of the basicity of dimethylphosphinic acid (pKa = -4.0) and methyl diphenylphosphinate (pKa = -4.8) has shown that phosphinates have very different protonation behavior compared to Hammett bases, and it is clear that very little of the substrate is protonated at the acidity giving a maximum rate of hydrolysis. The observed maximum rate at ~1.5 M acid then is not due to extensive protonation of substrate as is true for carboxylic amides nor is it due to solvation effects alone as the Bunnett hypothesis would suggest. Rather, it is primarily due to large changes in activity coefficients with increased acidity, although solvation of the transition state seems to be of some importance in the cause of this phenomenon. © 1966, American Chemical Society. All rights reserved.