Electrochemical behaviour of anthraquinone dyes in non-aqueous solvent solution. Part II. Controlled potential electrolysis of alizarin

Abstract

This article is the second part of our already published research results (Caram, Banera, Martínez Suárez, Mirífico, 2017). The present work aims to characterise/identify reaction intermediates and final products of the cathodic electrolysis of alizarin (AH2) in solution of DMF and ACN with TBAP and NaClO4 as supporting electrolytes to corroborate the reaction mechanism previously published by us. Intermediates and reaction products are accumulated by controlled potential electrolysis (CPE) of AH2 performed at the potentials of the first, second, and third charge transfers. Progress of the electrolysis is followed by CV. The techniques employed for the characterisation/identification of the involved chemical species are CV, UV–Vis, FTIR, 1H and 13C NMR, ESR, elemental analysis, and quantum chemical computations based on density functional theory (DFT). Some aspects of the mechanism are corroborated, some details are modified, and new insights are obtained. Intermediates are detected by CV, and the structure of some of them (AH2● ─, AH3●, and AH─) is confirmed by ESR and by the identity of the isolated reaction products (1-hydroxy-2-methoxyanthracene-9,10-dione (AH-CH3) by adding an excess of CH3I at the end of the CPE in DMF/TBAP and NaClO4 as the electrolyte systems, and sodium 1-hydroxy-9,10-dioxo-9,10-dihydroanthracene-2-olate (AH-Na) in ACN/NaClO4). Ion pair formation between AH2● ─ semiquinone radical anion with Na+ is significant. The homogeneous dissociation equilibrium AH3─ ⇄AH22– + H+ does not occur, and AH2• – + AH2 ⇄ AH – + AH3• is a slow process more shifted to the right in DMF/TBAP electrolyte medium. AH-CH3 and AH-Na are clear evidence of the self-protonation reaction of AH2• –. Ca. 20 % of the initial moles of AH2 are converted into AH-CH3 in DMF/TBAP and NaClO4 as the electrolyte systems. CPE of AH2 in ACN/NaClO4 results in reaction products that are insoluble and are coating the electrode, ca. 80 % of the initial moles of AH2 are recuperated as pure AH-Na. AH-CH3 and AH-Na contain the carbonyl-quinone groups in their structures. The applied potential does not modify these results. AH-CH3 and AH-Na are electrochemically (CV) characterised. The acidity of the quinone dye, the homogeneous protonation/deprotonation equilibriums, the intramolecular hydrogen bonding, and the easy air oxidation and protonation of some of the intermediates have led to the obtained results.