Effect of substituents at the 5-position on the first and second dissociation constants of isophthalic acid

The dissociation constants of benzoic acid and 20 of its meta- or para-substituted derivatives and of isophthalic acid and ten of its 5-substituted derivatives have been measured in 50 wt% aqueous methanol. The Hammett ρ value for benzoic acid is 1.28; for isophthalic acid the ρ values are 1.21 (pK1) and 1.20 (pK2). The σmeta values for hydroxy and acetoxy are –0.01 and +0.29, respectively, in this system. Values for σmeta for CO2H and CO2– are calculated to be +0.28 and –0.20, respectively; however there are indications that these values are not completely structure-independent.