ENTROPIES OF DISSOCIATION OF SOME MODERATELY STRONG ACIDS

The dissociation constants of seven moderately strong (0.5 < pKa < 1.4) uncharged acids have been measured as a function of temperature. The acids were chosen to serve as models for the interpretation of ΔS‡ values derived from rates. A simplified method for the determination of these dissociation constants is shown to give results of more than sufficient accuracy when applied to those few moderately strong acids for which clas-sically determined values exist. The near-zero values of ΔS° measured for the ionization of certain of the acids are discussed from the viewpoints of their implications concerning changes in solvent structure accompanying the activation process in solvolytic reactions and concerning the general use of equilibrium ΔS° values as models for interpreting ΔS‡ values. The parameters found for dissociation at 25 ° are (given in order for each acid are pKa ΔH° in kcal mole-1, and ΔS° in cal deg -1mole-1): Cl2FCCO2H, 0.46, -1.4, -7; F3CCO2H, 0.50, 0.4, -1; Cl3CCO2H, 0.515, 0.28, -1.4; [formula-omitted], 0.998, 0.41, -3.2; (CO2H)2, 1.30, -0.8, -9; F2CHCO2H, 1.34, -2.0, -13; Cl-2CHCO2H, 1.36, -1.9, -12. © 1969, American Chemical Society. All rights reserved.