This paper is devoted to the phenolsulphonephthalein nitro derivative 3,3 0 -dinitrophenolsulphonephthalein, also called nitrophenol violet (NPV). The neutral molecular form, H 2 R, was isolated as a sultonic tautomer, and an X-ray crystal structure analysis was carried out. UV-vis absorption spectra in methanol, DMSO, acetonitrile, and water at different pH values were ascribed to the molecular and anionic (H 2 R, HR À , and R 2À ) species. Whereas the pK a values of this acid-base indicator (HR À ¡ R 2À + H + ) in water and DMSO are close to those of 3,3 0 ,5,5 0 -tetrabromophenolsulphonephthalein (or bromophenol blue), replacing the four Br atoms with two NO 2 groups results in a pronounced tendency to carbinol formation. In weakly acidic aqueous media, the HR À anion slowly converts into the colourless carbinol H 2 ROH À . The latter is transformed to the orange carbocation only in concentrated (70-94 wt%) sulphuric acid. The formation of H 2 ROH À is atypical for the common sulphonephthalein indicators and should be ascribed to the enhanced positive charge density on the nodal carbon atom. The reaction mechanism and kinetic equation explaining this pH-dependent process are proposed, in addition to a kinetic study of the common process R 2À + HO À ? ROH 3À in the alkaline region. The numerical characterisation of the protolytic processes obtained for NPV is also helpful in gaining a better understanding of the properties of previously studied 3,3 0 ,5,5 0 -tetranitrophenolsulphonephthalein, which is much less accessible for a quantitative description.

Among the vast series of phenolsulfonephthalein dyes, the nitro derivatives and especially 3,3 0 ,5,5 0tetranitrophenolsulfonephthalein (nitrophenol crimson) remain practically unexplored, whereas the halogen and alkyl derivatives have been studied comprehensively. This striking difference is probably due to the enormous influence of the four NO 2 groups on the properties of the dye. As a result, the protolytic behaviour is unlike even that of tetrabromo phenolsulfonephthalein, and the recognised scheme of acid-base and tautomeric equilibrium of the sulfonephthaleins is unable to explain it. The molecular form H 2 R was isolated as a sultonic tautomer, and an X-ray crystal structure analysis was carried out. Our studies of the UV-vis absorption spectra in water, methanol, dimethyl sulfoxide, acetonitrile, acetone, and dichloromethane, as well as in aqueous micellar solutions of surfactants, allowed us to evaluate the true molar absorptivity of the dianion R 2À , and to elucidate the enormous tendency to form yellow trianionic carbinol ROH 3À , even in the presence of traces of H 2 O. Nuclear magnetic resonance and electrospray data confirm the proposed scheme of ionisation and tautomerism of nitrophenol crimson.

No abstract

No abstract

The micellar effect of surfactants of various types on the rate of the reaction between methyl violet and hydroxide ion is studied. The absorption spectra show that the cation of methyl violet is bound by micelles of all types at proper concentrations of surfactants. The observed rate constant in micellar systems containing nonionic Brij‐35, zwitterionic 3‐(dimethyldodecylammonio)‐propanesulfonate, cationic cetyltrimethylammonium bromide and hydroxide surfactants is higher, whereas in solutions of the anionic surfactant sodium dodecylsulfate is lower than that one in the surfactant‐free system. Piszkiewicz's, Berezin's, and pseudophase ion‐exchange models of the kinetic micellar effect are used for the treatment of the dependences of the above‐mentioned constants on the surfactant concentration. The values of the corresponding kinetic parameters are compared and discussed. The influence of nonionic, zwitterionic, and anionic micelles on the reaction rate is discussed on the basis of medium and concentration kinetic effects. The character of the cationic micelles effect is somewhat paradoxical. Although the observed pseudo–first‐order reaction rate constant substantially increases in the presence of such micelles, the second order‐rate constant in these micelles is lower than the corresponding value in surfactant‐free aqueous solution. As a possible explanation, the decrease in the reactivity of the HO– ions is proposed, owing to their electrostatic association with the cationic headgroups (“diverting effect”).