REVERSIBLE HYDRATION OF PYRUVIC ACID .I. EQUILIBRIUM STUDIES

The position of equilibrium for the reversible hydration of pyruvic acid was determined using an nmr technique. Ratios of the hydrated to unhydrated forms in aqueous solutions were determined as a function both of temperature and pH by comparing the relative areas of signals derived from the hydrated and unhydrated forms. These appeared at 2.58 and 3.44 ppm, respectively, upfield from the nmr signal of water which was used both as the lock and internal reference. The temperature studies which were carried out at pH 0.35 allowed the evaluation of the thermodynamic parameters for the hydration of pyruvic acid from a plot of log K vs. 1/T: ΔG° = -0.24 kcal mol-1, ΔH° = -7.8 kcal mol-1, and ΔS°= -26.8 eu. The variation of the fraction of hydration, χ, with pH clearly showed that although the hydration of pyruvic acid is favorable (χ0.0° = 0.82, χ25.0° = 0.61) the corresponding hydration of pyruvate anionis slight (χ0.0° = 0.17, χ25.0° = 0.06). This dependency of the extent of hydration upon the relative concentration of pyruvic acid and pyruvate anion is quantitatively illustrated by plots of the fraction of hydration vs. pH at 0.0° and 25.0°. The resulting sigmoid curves showing maxima at low pH reveal points of inflection at pH 2.2 which correspond to the pKa of pyruvic acid.