Protolytic Equilibria in Organized Solutions: Ionization and Tautomerism of Fluorescein Dyes and Related Indicators in Cetyltrimethylammonium Chloride Micellar Solutions at High Ionic Strength of the Bulk Phase

Abstract

Ionic equilibrium of 22 hydroxyxanthenes, including halogen and nitro derivatives of fluorescein, and their thio- and aza analogues, were studied spectrophotometrically in micellar solutions of cetyltrimethylammonium chloride at ionic strength of the bulk phase 4.0 M KCl. This micellar pseudophase is characterized by the electrostatic surface potential of +(15–16) mV and the ETN value of 0.623. In the case of dyes bearing the COOH group, colorless lactone is the predominant tautomer of the molecular form H2R. A new classification of fluoresceins is developed. The dyes were divided into four groups based on the nature of tautomerism of the anions. In the case of the fluorescein type, the monoanions HR− exist predominantly as “carboxylate” tautomers, with ionized carboxylic and non–ionized hydroxylic group. For the dyes of the eosin type, the situation is opposite, while for the intervening type of compounds, the concentrations of the two tautomers are comparable. Dyes capable of forming lactone anions HR− were classified as the fourth type. For some of them, even the dianion R2− exists as a lactone. The relationship between the stepwise ionization constants, Ka1/Ka2, varies from 1.3 to 1.07 × 105 and is determined by the state of tautomeric equilibrium of molecules and ions.

Protolytic Equilibrium of Tetra- And Pentanitrofluoresceins in a Binary Solvent Acetonitrile – Dimethyl Sulfoxide (Mass Ratio 96 : 4)

“…The thermodynamic value Much more diluted solutions of picric acid were used in the case of 2,4,5,7-tetranitrofluorescein (Figure 2a). Here, the neutral form H 2 R is almost colorless due to predominance of the lactonic tautomer, as in other solvents [1,2,4,6]. The spectrum of the dianion R 2of the dye is obtained in DBU solution (Figure 2).…”

“…Nitro derivatives of fluores cein, however, are much less explored. During past two decades, some publications devoted to acid ionization and tautomerism of nitrofluoresceins appeared from this laboratory [1][2][3][4][5][6].In this paper, we continue our research in this direction.The compounds studied are as follows: 2,4,5,7-tetranitrofluorescein, 2,4,5,7,4'-pentanitrofluorescein, 2,4,5,7,5'-pentanitrofluorescein, and 2,4,5,7-tetranitrofluorescein methyl ester. Except the first one [4,6], they were synthesized and identified by us; the details will be published elsewhere.Scheme 1 -Molecular structure of 2,4,5,7-tetranitrofluorescein and its methyl ester; the pentanitrofluoresceins bear the NO 2 group in 4' or 5' positions.chemistry and contiguous fields of science.…”

“…Nitro derivatives of fluorescein, however, are much less explored. During past two decades, some publications devoted to acid ionization and tautomerism of nitrofluoresceins appeared from this laboratory [1][2][3][4][5][6]. In this paper, we continue our research in this direction.The compounds studied are as follows: 2,4,5,7-tetranitrofluorescein, 2,4,5,7,4'pentanitrofluorescein, 2,4,5,7,5'-pentanitrofluorescein, and 2,4,5,7-tetranitrofluorescein methyl ester.…”

“…Nitro derivatives of fluores cein, however, are much less explored. During past two decades, some publications devoted to acid ionization and tautomerism of nitrofluoresceins appeared from this laboratory [1][2][3][4][5][6].…”

Solvation Effects in Organic Chemistry: A Short Historical Overview

“…Incorporation of reactant molecules into aqueous micelles or other organized assemblies can lead to considerable alterations of rates and mechanisms of organic reactions, also called micellar catalysis , which has been comprehensively reviewed . A very recent well-studied example of the effects of organized solutions on ionization and tautomerism of fluorescein dyes can be found in ref . The molecular–microscopic interface polarity of micelles has been determined by solvatochromic probes such as the betaine dye B30 and can be correlated with variations in the micellar aggregation number .…”