title: Tautomeric and Microscopic Protonation Equilibria of Anthranilic Acid and Its Derivatives.
abstract: The acid-base chemistry of three zwitterionic compounds, namely anthranilic (2-aminobenzoic acid),N-methylanthranilic andN-phenylanthranilic acid has been characterized in terms of the macroconstantsKa1,Ka2, the isoelectric point pHI, the tautomerization constantKzand microconstantsk11,k12,k21,k22. The potentiometric titration method was used to determine the macrodissociation constants. Due to the very poor water solubility ofN-phenylanthranilic acid the dissociation constants pKa1and pKa2were determined in MDM-water mixtures [MDM is a co-solvent mixture, consisting of equal volumes of methanol (MeOH), dioxane and acetonitrile (MeCN)]. The Yasuda-Shedlovsky extrapolation procedure has been used to obtain the values of pKa1and pKa2in aqueous solutions. The pKa1and pKa2values obtained by this method are 2.86 ± 0.01 and 4.69 ± 0.03, respectively. The tautomerization constantKzdescribing the equilibrium between unionized form ⇌ zwitterionic form was evaluated by theKzmethod based on UV-VIS spectrometry. The method uses spectral differences between the zwitterionic form (found at isoelectric pH in aqueous solution) and the unionized form (formed in an organic solvent of low dielectric constant). The highest value of theKzconstant has been observed in the case ofN-methylantranilic acid (log10Kz= 1.31 ± 0.04). The values of log10Kzfor anthranilic andN-phenylanthranilic acids are similar and have values of 0.93 ± 0.03 and 0.90 ± 0.05, respectively. The results indicate that the tested compounds, in aqueous solution around the isoelectric point pHI, occur mainly in the zwitterionic form. Moreover, the influence of the type of substituent and pH of the aqueous phase on the equilibrium were analyzed with regard to the formation and the coexistence of different forms of the acids in the examined systems.
authors: Department of Inorganic and Analytical Chemistry, Faculty of Chemistry, Rzeszow University of Technology, 6 Powstańców Warszawy Ave., 35-959 Rzeszow, Poland.
authors: Lidia
authors: L
authors: Zapała
authors: Department of Inorganic and Analytical Chemistry, Faculty of Chemistry, Rzeszow University of Technology, 6 Powstańców Warszawy Ave., 35-959 Rzeszow, Poland.
authors: Elżbieta
authors: E
authors: Woźnicka
authors: Department of Inorganic and Analytical Chemistry, Faculty of Chemistry, Rzeszow University of Technology, 6 Powstańców Warszawy Ave., 35-959 Rzeszow, Poland.
authors: Jan
authors: J
authors: Kalembkiewicz
keywords: Anthranilic acid; Dissociation constant; Microconstants; N-methylanthranilic acid; N-phenylanthranilic acid; Tautomerization constant