Thermodynamics of ionization of hypophosphorous and phosphorous acids. Substituent effects on second row oxy acids

Abstract

An NMR method has been used to determine the p K a values for H 3PO 2 and H 3PO 3 at 5, 25, 45, 65, and 85°C. The standard enthalpies and entropies of ionization of these acids are: for H 3PO 2 −12.69 kJ and −66.3 JK −1, for H 3PO 3 −14.86 kJ and −73.8 J K −1, and for H 3PO 4 −7.74 kJ and −67.1 J K −1. Because of its ability to participate in backbonding, substitution of OH for H leads to decreased acidity in the phosphorous oxy acid series.