| Density functional theory and ab initio molecular orbital calculations have been carried out to investigate the molecular structures and properties of all 19 chlorophenol congeners. The results have been delineated systematically in terms of the position and the total number of chlorine substitutions. A simple practical method for predicting the acidity constants (as p Ka's) of chlorophenols is proposed based on density functional theory calculations of a series of hydrogen-bonded complexes of chlorophenols with a single probe molecule, either water or ammonia. Relevant structural parameters and molecular properties of these complexes, primarily involving the acidic hydroxyl group, correlate linearly with the acidity of chlorophenols. The acidity constants of two representative chlorophenols, 2,4,6-trichlorophenol and pentachlorophenol, have been measured by reversed-phase liquid chromatography using methanol-water mixtures as mobile phases. The aqueous pK a values are found to be 6.29 for 2,4,6-trichlorophenol and 4.94 for pentachlorophenol by linear extrapolation of the mixed solvent system and are comparable to published values. |
