STUDIES OF HYDROGEN-BONDED COMPLEX FORMATION WITH P-FLUOROPHENOL .V. LINEAR FREE ENERGY RELATIONSHIPS WITH OH REFERENCE ACIDS

Linear free energy relationships have been established in the formation of hydrogen-bonded complexes of various OH reference acids with a wide variety of proton acceptors. The effects of temperature, solvent, and substituents have been examined. A unique base parameter, pKHB, has been defined which measures the relative strength of the acceptor in hydrogen-bonded complex formation with any suitable OH reference acid. pKHB values do not correlate with aqueous pKA values, except within series having a common functional center and variable electronic effects of substituents. pKHB values also are not applicable to reference acids involving internal hydrogen bonding and are presumably not applicable to systems in which there is substantial formation of the hydrogen-bonded ion pair (in mobile equilibrium with the hydrogen-bonded complex). Evidence is presented that the pKHB scale is applicable (at least qualitatively) to other relatively weak interactions between bases and a “shielded” center of positive charge. The highly dispersed family relationships between pKHB and corresponding pKA values are indicated to be useful in distinguishing the atomic center of complexing in polyfunctional bases. © 1969, American Chemical Society. All rights reserved.