Ionization of aqueous dimethylbenzoic acids: Conductance and thermodynamics

Precise conductance measurements are reported for four xylic acids, 2,3-2,5-2,6 and 3,5-dimethylbenzoics. Limiting molar conductances Λ_O and pK _a obtained from a Fuoss type analysis, are reported for each of these acids at five degree intervals covering the range from 0° to 100°C. The Λ_O values for each acid are described by a polynomial in the Celsius temperature. The pK _a were smoothed as a function of the Kelvin temperature T with an equation of the form: $$pK_{\text{a}} = A{\text{ }} + {\text{ }}B/T{\text{ }} + {\text{ }}C{\text{ }}logT{\text{ }} + {\text{ }}DT$$ where the term linear in T was required only for the 2,3-acid. Standard enthalpy, entropy, and heat capacity changes were calculated by suitable differentiation of this equation. Walden products were calculated for the four anions at each of the temperatures and are compared with earlier data for the toluate and the benzoate ions. Those acids with ortho groups undergo a large decrease in enthalpy on ionization and are substantially more acidic than benzoic acid. These effects are especially large for 2,6-dimethylbenzoic acid. A methyl group in the meta position lowers acidity slightly both in m-toluic and in 2,5-dimethylbenzoic acid. However, 2,3-dimethylbenzoic acid is more acidic than o-toluic.