The Electrochemistry of Quinones in Aprotic Solvents
Quinones are electroactive organic molecules that are used by biological systems as electron shuttles, are used by humans as dyes, pharmaceuticals, reactants, and catalysts, and are being investigated for use in energy transduction and organic electronics, all of which have to do with their electrochemistry. Studies of them have contributed to understanding not only their applications, but also certain aspects of physical organic chemistry in general such as the intersection between hydrogen bonding and electron transfer, the square scheme theoretical concept, and proton coupled electron transfer (PCET).
Quinones are extremely reactive in their fully reduced form to the point that they will undergo SN2 reactions with supposedly stable solvents such as dichloromethane in the absence of water to H-bond with and stabilize the quinone dianions. This stabilization is both kinetic and thermodynamic in nature—meaning that it comes both from water molecules drawing electron density away from the quinones through their hydrogen bonds, thus making the SN2 reaction less favorable, and from the water molecules themselves getting in between the quinone dianion and its electrophilic target, thus making the reaction slower due to sterics.
Cyclic voltammograms (CV's) of quinones never undergo a set of two ideal redox waves because the presence of oxygenated, protonated functional groups on the surface of the analytical electrodes used to study them causes them to form hydrogen bonds and undergo proton transfer at the surface even if there is nothing in solution that can be a hydrogen bonding or proton transfer guest. The only apparent way to get rid of these groups is a long process of ablation with high temperature hydrogen atoms. The presence of water in solution appears to enhance the non-ideality.
In order to accurately describe the quinone-phenol system, which is the subject of a great deal of the active research in this area, it is necessary to consider hydrogen bonding, electron transfer, and proton transfer reactions, not just two of the three. This is easy to do using the “wedge” scheme, which is a theoretical model to organize mechanisms that require three types of reactions done over and over again. Wedge schemes are shown to give good qualitative approximations of the CV’s of duroquinone and 2-naphthol, with the best including proton transfer from a naphthol-naphthol hydrogen bonded complex that forms slowly in solution.