Ionization Constants of Substituted Benzoic Acids in Ethanol-Water

The thermodynamic ionization constants of meta and para substituted allyl- and propenylbenzoic acids were determined potentiometrically in aqueous ethanol of varying ethanol concentration. The pK values increase with increasing ethanol content. The relative acid strength does not vary with change in solvent concentration. The ΔpK/Δ percent ETOH increases with ethanol content of the medium until a maximum is reached around 40 weight-percent ethanol. A plot of pK + log [H₂O] versus the reciprocal of the dielectric constant of the solvent gives a nearly linear relationship to about 44 weight-percent ethanol. From pK values found in the literature a similar linear relationship exists for other benzoic acids. With the exception of the p-propenylbenzoic acid the σ-substituent constants of the Hammett equation do not change greatly with ethanol concentration. This behavior may be indicative that the solvation shell surrounding the p-propcnyl-acid differs from that of other benzoic acids.