Hydroquinones electrochemical reoxidation

The first combined HPLC-electrochemical measurements of vitamin K used the reductive mode, but this technique suffered from interference from the reduction of oxygen. A redox method was later developed that eliminated this interference, and provided a 10-fold increase in sensitivity over photometric detection and an improved selectivity. The coulometric detector employed in the redox mode is equipped with a dual-electrode cell in which phylloquinone is first reduced upstream at the generator electrode and the hydroquinone is reoxidized downstream at the detector electrode. 

The yield of hydroquinone is 85 to 90% based on aniline. The process is mainly a batch process where significant amounts of soHds must be handled (manganese dioxide as well as metal iron finely divided). However, the principal drawback of this process resides in the massive coproduction of mineral products such as manganese sulfate, ammonium sulfate, or iron oxides which are environmentally not friendly. Even though purified manganese sulfate is used in the agricultural field, few solutions have been developed to dispose of this unsuitable coproduct. Such methods include MnSO reoxidation to MnO (1), or MnSO electrochemical reduction to metal manganese (2). None of these methods has found appHcations on an industrial scale. In addition, since 1980, few innovative studies have been pubUshed on this process (3). 

In one procedure, as mentioned above, Mn02 was employed as the oxidant to reoxidize the hydroquinone to benzoquinone. In another study it was demonstrated that the hydroquinone can be recycled electrochemically by anodic oxidation [61]. The reaction is performed in acetic acid with LiC104 as electrolyte with catalytic amounts of both Pd(OAc)2 and p-benzoquinone in a membrane-separated cell. 

The role of centre 2, for which the potentiometric reduction potential, at — 320 mV, is too low to be involved directly in electron transport between hydroquinone and fumarate, is not clear [164,234,235]. In these electrochemical experiments we have so far detected neither its electrode response nor any change in the steady-state biocatalytic current as the potential is lowered beyond — 320 mV. Cammack and co-workers have proposed [234, 235] that this centre might be active in one-electron reduction and reoxidation of the more reducing FAD radical species that must be generated, albeit transiently, during turnover. Their mechanism of reduction of oxidized FAD in SDH is termed the dual-pathway model since the two single electrons flowing consecutively out of FAD do so by different routes. The lower potential pathway uses centre 2. An equivalent mechanism, with the direction of electron flow reversed, is applicable for FR. 

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