MONONITROFLUORESCEINS IN AQUEOUS MEDIA: ACID-BASE EQUILIBRIA, TAUTOMERISM, AND HYDROLYSIS OF DIACETATES

This article reports equilibrium and kinetic data for three fluorescein dyes containing the nitro group in the residue of the phthalic acid, 3'-, 4'-, and 5'-nitrofluorescein, in water and 50 mass % aqueous ethanol. These compounds, particularly the recently synthesized 3'-nitrofluorescein, were less considered polyprotic acids than many other fluorescein dyes. The dyes behave as diprotic acids, which can also be protonated: H3R+⇌ H2R ⇌ HR–⇌ R2–. The dissociation constants in water and 50 mass % aqueous ethanol were obtained in buffer solutions of diluted HCl using the spectrophotometric method accompanied by a potentiometric determination of pH at an ionic strength of 0.05 M and 25.0 ± 1 °C. The dye concentrations in the working solutions were, as a rule, within the range of (5–10)×10–6 M. The thermodynamic values of the dissociation constants were calculated using the Debye–Hückel equation, the second approach, for ionic activity coefficients. The absorption spectra in the visible region of the individual molecular and ionic forms were determined for the above-mentioned compounds and the ester of 3'-nitrofluorescein. For the last, the dissociation constants were determined in aqueous ethanol. Whereas in water the fraction of the zwitterionic tautomer of nitro­fluoresceins increases as compared with the parent compound, in aqueous ethanol, the colorless lactone predominates. The microscopic dissociation constants were determined and their values are in line with the influence of the electrophilic nitro group. The alkaline hydro­lysis of the nitrofluorescein diacetates was investigated in the two aforementioned solvents. The obtained results allowed us to single out the rate constants of the two-step kinetics of the hydrolysis of the diacetyl derivatives of nitrofluoresceins.