A Linear Free Energy-Enthalpy-Entropy Relationship for the Ionization of Benzoic Acids Sir:It can be shown1 that a linear relationship between the free energies of two reaction series can be expected only if each reaction series shows one of the following types of behavior: (a) °is constant throughout the series, (b) AS°is constant, or (c) AH°is linearly related to AS°. Of the many linear free-energy relationships now known, the Hammett equation is undoubtedly the most widely used, yet, as many reviewers have noted (e.g., ref 1-3), its defining reaction series, the ionization of benzoic acids, does not appear to belong to any one of these categories. This, as Shorter2 3has commented, makes the empirical success of the Hammett equation something of a mystery and it also, to some extent, reduces our confidence in the use of enthalpies and entropies as quantitative indexes of substituent effects on chemical reactivity.Recently Larson and Hepler4 have drawn attention to large discrepancies existing between sets of measurements of enthalpies of ionization of benzoic acids and also to the antiquity of some of the most frequently quoted measurements.combine to make accurate calorimetric measurements difficult. Furthermore, the very small AH°2i value means that the pK& values show very little variation with temperature, making van't Hoff-type determinations of
Thermodynamic acidity
constants have been measured over the temperature range 5-50� for aqueous
solutions of sodium 4?-dimethylaminoazobenzene- 4-sulphonate (methyl orange)
and sodium 4?-dimethylaminoazobenzene-2- sulphonate (ortho-methyl orange). From
these data values of the standard enthalpy, entropy, and heat capacity changes
have been calculated for these compounds. These results are discussed in
conjunction with previous spectrophotometric and other data with reference to
the nature of the equilibrium systems involved in these protonation reactions.
It is concluded that existing evidence does not allow an unequivocal assignment
of the sites of protonation of these and related molecules.
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