DISSOCIATION-CONSTANTS OF BRONSTED ACIDS IN D2O AND H2O - STUDIES ON POLYAZA AND POLYOXA-POLYAZA MACROCYCLES AND A GENERAL CORRELATION

Using data on a series of polyaza and polyoxa-polyaza macrocyclic compounds, a satisfactory linear correlation was established between the dissociation constants of their acid forms obtained in deuterium oxide (pK(D)) and in water (pK(D)). The isotopic effect, DELTA-pK = pK(D) - pK(H), increases with increasing pK, being larger for the weaker acids. The pK(D) vs. pK(H) correlation may be used to determine pK(D) or pK(H) values when one of the constants is known; the statistical errors of the estimates are not significatively higher than those of the experimental determinations. By including data available in the literature for a series of different acids (mineral and carboxylic acids, phenols, protonated amines, amino acids, etc.; a total of 138 pairs of data for 70 different compounds), a good general pK(D) vs. pK(H) correlation was also obtained (correlation coefficient 0.998), showing that the dependence of the isotopic effect on the nature of the acid is not as important as previous studies, based on limited and not always satisfactory DELTA-pK vs. pK(H) correlations, suggested.