Dissociation, tautomerism and electroreduction of xanthene and sulfonephthalein dyes inN,N‐dimethylformamide and other solvents

Protolytic equilibria in DMF of 10 dyes, fluorescein and its halogen derivatives, rhodamine B and N , N ‐diethylrhodol, and of four sulfonephthaleins were studied by visible spectroscopy. The ratios of the stepwise dissociation constants of the substances in DMF, and the changes of these constants on transferring from water to DMF and other solvents, were explained taking into account the nature of functional groups and of the state of tautomeric equilibria. From the correlations between the Gibbs energies of tautomerization of the neutral species (quinonoid ⇌ colorless lactone) and the normalized Reichardt's parameter, ETN , the ‘limiting’ values of tautomerization constants of fluorescein and eosin at ETN → 0 (non‐polar aprotic media) were evaluated. The conversion of the neutral form of sulfonephthaleins into colorless sultones in DMF is demonstrated. The peculiarities of the reduction of the substances on a dropping mercury electrode in non‐aqueous media were revealed, namely the differentiating action of DMF in the first and second stages of electroreduction of the R 2− anions.