Tautomerization equilibria for phosphorous acid and its ethyl esters, free energies of formation of phosphorous and phosphonic acids and their ethyl esters, and pKa values for ionization of the P—H bond in phosphonic acid and phosphonic esters

From data in the literature the free energies of formation in aqueous solution of triethyl phosphite and diethyl phosphonate can be calculated as −138.4 ± 1.7 and −165.1 ± 2.0 kcal mol −1 , respectively. From these values, by application of free energy relations which we have published, the free energies of formation of the corresponding hydroxy compounds can be calculated and thence the equilibrium constants for tautomerization, which are 10 7.2 , 10 8.7 , and 10 10.3 in favor of the tetracoordinate phosphonate tautomer for P(OEt) 2 OH, P(OEt)(OH) 2 , and P(OH) 3 , respectively. Using estimated pK a values for the tricoordinate phosphite species the tautomerization equilibria for the anions could also be calculated, as could the pK a values from the P—H bonds: 13, 26, and 38 for H—PO(OEt) 2 , H—PO 2 (OEt) − , and H—PO 3 2− , respectively.