Determination and structural correlation of pKBH+ for meta- and para- substituted benzamides in sulfuric acid solutions

The protonation of some meta and para substituted benzamides in sulfuric acid solutions was studied by UV spectroscopy in the 190–350 nm region. Principal component analysis was applied to separate the effect of protonation from the medium effect. The spectral region 200–350 nm was used for the calculation of the ionization ratio from coefficient of the first principal component, which explains about 95–98 % of the total variability. The dissociation constants as well as the solvation parameters m* and f were calculated using the excess acidity method and the Bunnett–Olsen method. The pKBH+ values obtained with the HA function (defined by the average m* -values) are in satisfactory agreement with those calculated with the previously mentioned methods. The pKBH+ values were correlated with structure using the Hammett (r = – 0.91) and Taft approach. It was found that the inductive effect is more relevant than the resonance one for both substituted benzamides (meta and para)