Mechanistic Study on the Electrochemical Reduction of 9,10-Anthraquinone in the Presence of Hydrogen-bond and Proton Donating Additives

Abstract

The electrochemical reduction of 9,10-anthraquinone (AQ) was investigated in CH3CN in both the absence and presence of the hydrogen-bond and proton donating additives, CH3OH, CH(CF3)2OH, phenol, 4-methoxyphenol, 4-cyanophenol, 2,4,6-trichlorophenol, and benzoic acid (BA). Three clearly different types of electrochemical behavior were observed with increasing concentrations of the additives, and were simulated to analyze the reaction mechanisms. Type I was observed for weakly interacting additives, such as CH3OH, characterized by positive shifts of the two well-separated reduction waves, corresponding to the formation of AQ • -and AQ 2-, with no loss of reversibility. The second wave shifted more strongly, and finally merged with the first. These behaviors are explained by the association of AQ 2-with the additives via strong hydrogen-bonding. Type II is attributed to a reduction mechanism involving quantitative formation of strong hydrogen-bonded complexes of AQ 2-with additives, such as CH(CF3)2OH, phenol and 4-methoxyphenol, showing a reversible or quasireversible two-electron reduction wave with increasing concentrations of the additives. The behavior of Type III, observed in the presence of strongly interacting additives, such as 2,4,6-trichlorophenol and BA, is characterized by a voltammogram composed of the 2-electorn cathodic and the broad anodic waves without keeping reversibility, facilitated by proton transfer in the hydrogen-bonded complexes, AQ • --BA and AQ 2--BA. The effects of hydrogen-bonding and protonation on the electrochemistry of AQ have been systematically demonstrated in terms of the potentials and reaction pathways of the various species, which appear in quinone-hydroquinone systems.

Measuring the Relative Hydrogen‐Bonding Strengths of Alcohols in Aprotic Organic Solvents

“…This is a consequence of the increase in electron density at the oxygen atoms after each reduction reaction permitting stronger hydrogen bonds to be formed between the DH compounds and acceptor Q 2À as compared to QC À and Q molecules. With that knowledge, the electrochemistry of quinones has been 1) used as an assessment of their own hydrogen-bond-acceptor ability [13] and 2) applied as measures of the hydrogen-bond-donor ability of alcohols, [13a-e, 14] polyalcohols, [13d,e] phenols, [14] and amides [15] in aprotic organic solvents, as well as for quantifying trace moisture. [13f, 16] Further studies have shown that the magnitudes of the potential shifts are dependent on the electronic and steric effects of the various quinone forms (Q, QC À and Q 2À ), the specific DH compounds, as well as the type of solvent [16] and supporting electrolyte [16b, 17] employed.…”

“…At present, almost all information about the hydrogen-bonding ability of DH compounds obtained by using the electrochemistry of quinones has been derived with the single global equilibrium model [13][14][15][16] or the model involving equilibria in n and m successive stages.…”

“…At present, almost all information about the hydrogen-bonding ability of DH compounds obtained by using the electrochemistry of quinones has been derived with the single global equilibrium model [13][14][15][16] or the model involving equilibria in n and m successive stages. [13d] The popularity of both models lies mainly in their ability to provide the number of DH molecules that are participating in the various quinone-DH hydrogenbonded complexes together with their associated global or equilibrium constants.…”

Redox chemistry of coenzyme Q—a short overview of the voltammetric features

“…Several recent comprehensive studies related to the voltammetric features performed with water-soluble quinones and coenzyme Qs in buffered and non-buffered aqueous media probably gave best explanations about the disputed redox behavior of these systems [41][42][43][44]. In fact, many authors claimed that the redox chemistry of coenzyme Qs and quinones from water solutions is always linked to a mechanism in which free radical intermediates are involved [see 2, 9, and 33, for example].…”

“…Keeping in mind all the possible redox transformations in which CoQs are undergoing in both aprotic or aqueous media [2,9,[31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47], several relevant theoretical models can be applied to describe their voltammetric behavior. In the last few years, several groups worked on the development of mathematical models of coupled redox reactions under conditions of cyclic and square-wave voltammetry.…”