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

Stojković Goran (Institut za hemiju, Prirodno-matematički fakultet, Univerzitet 'Sv. Kiril i Metodij', Skopje)
Popovski Emil (Institut za hemiju, Prirodno-matematički fakultet, Univerzitet 'Sv. Kiril i Metodij', Skopje)

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 200-350nm 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 Ø were calculated using the excess acidity method and the Bunnett-Olsen method. The pK BH + values obtained with the H A function (defined by the average m* - values) are in satisfactory agreement with those calculated with the previously mentioned methods. The pK BH + values were correlated with structure using the Hammett (ρ=-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). .

Keywords: benzamides, protonation, spectrophotometry, Hammett sigma constants