Acid–base equilibria of substituted benzoic acids. Part II. Thermodynamic parameters for dissociation of t-butyl- and trimethylsilyl-substituted acids. A re-evaluation of the results of the Chatt and Williams experiment

Abstract

The dissociation constants for m- and p-t-butyl- and trimethylsilyl-benzoic acids have been determined by spectrophotometry. The thermodynamic quantities ΔG°, ΔH, and ΔS° are calculated at a number of selected reference temperatures within the range 15–40°. The results show the order of acid strength (K_a, ΔG°) above 25° to be : m-trimethylsilyl > p-trimethylsilyl > m-t-butyl > benzoic > p-t-butyl; however, these substituted acids are shown to have very much higher ΔH and ΔS° values than benzoic acid itself, or the halogeno-, nitro-, cyano-, and hydroxy-derivatives previously studied by this method with the p-trimethylsilyl acid exhibiting at once the largest ΔH and ΔS° values over the entire range of the temperatures studied for any of the acids. The new information indicates that conclusions drawn from free-energy measurements alone are not valid.