Reduction of benzoquinones

The quinone-hydroquinone system represents a classic example of a fast, reversible redox system. This type of reversible redox reaction is characteristic of many inorganic systems, such as the interchange between oxidation states in transition metal ions, but it is relatively uncommon in organic chemistry. The reduction of benzoquinone to hydroquinone... 

The use of electrochemical transmittance spectroscopy in both the UV-visible and IR regions of the spectrum is elegantly shown by the work of Ranjith et al. (1990) who employed an OTTLE cell to study the reduction of benzoquinone, BQ. The authors were the first to report the UV-visible spectrum of BQ2- and to demonstrate the quantitative aspects of the technique by reporting extinction coefficients for the major bands of BQT and BQ2- in both the UV-visible and IR. 

Using Cyclic Voltammetry and Molecular Modeling to Determine Substituent Effects in the One-Electron Reduction of Benzoquinones 92... 

Equation 9 indicates the addition of benzoquinone to CoH to form a new complex which cannot react further with CoH. Equation 10 defines the role of excess alkali in effecting the catalytic reduction of benzoquinone. As shown in previous examples, the hydroxo complex may then undergo the reverse aging process, leading to hydrogen absorption. The over-all result is reduction of benzoquinone to hydroquinone when limited amounts of substrate are available, and to quinhydrone when excess substrate is available. Equation 11 is an attempt to explain the lowered amount of hydrogen absorption noted when cyanocobaltate(II) is prepared in the presence of excess benzoquinone. Displacement of reduced substrate from this binuclear complex by alkali is assumed, since quinone was catalyti-cally reduced when the above procedure was carried out in the presence of added alkali. 

An electrochemical reaction, the reduction of benzoquinone, is exemplarily described. An electrochemical micro structured reactor is divided into a cathode and anode chamber by a Nafion hollow-fiber tube. The anode chamber is equipped with a platinum electrode and the cathode chamber contains the analyte and carbon or zinc electrodes. Current density and flow rate are controlled to maximize current efficiency as determined by analysis of the formed hydroquinone by an electrochemical detector. Hydroquinone is extracted subsequently in a micro extractor from the resulting product stream [84],... 

Whereas hydroquinone (p-HOPhOH) in acetonitrile is oxidized via an irreversible two-electron process at +1.18 V versus SCE (Eq. 12.34 and Table 12.2), its dimethyl ether (p-MeOPhOMe) is significantly more resistant with a reversible one-electron oxidation at +1.30 V versus SCE (Figure 12.5).16 The initial oxidation of the latter is followed by a second irreversible one-electron oxidation ( + 1.81 V vs. SCE) that yields a product that is reduced at +0.59 V versus SCE [consistent with the reduction of benzoquinone in the presence of hydronium ions (Table 12.2)] ... 

Richard, C. (1994). Photocatalytic reduction of benzoquinone in aqueous ZnO or Ti02 suspensions. New Journal of Chemistry, 18(4), 443 445. 

The reduction of benzoquinone in NMA at a platinum wire electrode has been studied143). In unbuffered solutions the reduction gave two fast, one-electron waves of almost equal height ( 1 2 = —0.04 V and —0.38 V versus a normal hydrogen reference electrode). It was proposed that these represented the formation of the radical anion and of the hydroquinone dianion, respectively. In buffered solutions an irreversible two-electron reduction (probably to hydroquinone) was found. It thus appears that certain radical anions may be much more stable in NMA than in water. 

Interestingly, reduction of benzoquinone on a hydridic surface occurs only via the valence band, a result associated with the strong cathodic flat-band shift of the latter surface compared with the hydroxylated surface, as seen in Fig. 44(c). 

Reduction of benzoquinone Acetonitrile/0.6 M (C2Hs)4NC104/ platinum electrode 298 Howell and Wightman (1984) 0.39... 

An indirect amperometric immimosensor for the detection of HCG was described by Chetcuti et al. [97]. The assay consisted of an anti-HCG monoclonal antibody immobilized on a GCE and the use of a sandwich assay with HRP conjugated to a second anti-HCG monoclonal antibody. Electrochemical detection of the enzymatic reduction of benzoquinone to... 

Tip/substrate separations were established with positive feedback measurements on the one-electron reduction of benzoquinone and the one-electron oxidation of ferrocene. The former mediator was found to be preferable in... 

Figure 14.4.3 Experimental results for the reduction of benzoquinone (BQ) through a film of poly(vinylferrocene) on a Pt RDE. The reciprocal current [normalized for the concentration of BQ in solution 5.82, 3.84, and 1.96 mM (top to third curves)] vs.

The redox nature of palladium(ii) salts has been investigated, methods being described for the study of the rates of oxidation of olefins. The reduction of benzoquinone by 7r-allylpalladium chloride in aqueous hydrochloric acid solutions has also been reported. ... 

Cyclohexene-l-one, methyl vinyl ketone, phenylacetylene, diphenylacety-lene, benzaldehyde, perfluoro-l-heptene, and cyclohexene were found to not be effective dienophiles for the reaction. Only polymeric o-xylylene products were seen in the reaction mixture. In addition, no cycloadduct was produced by using / -benzoquinone as the dienophile. Only hydroquinone and a,a -diiodo-o-xylene were recovered. The hydroquinone presumably results from the reduction of benzoquinone by nickel. Since sodium iodide is known to react with dibromo-o-xylene to give diiodo-o-xylene [112], the diiodo-o-xylene could result from the reaction of unconsumed dibromo-o-xylene with lithium iodide which is present in the reaction flask. These results are analogous to those reported in a similar reaction by Scheffer [113], where the use of zinc metal and ultrasound gave only hydroquinone and a quantitative yield of the unreacted dibromo-o-xylene. 

Figure 1-6 The reduction of benzoquinone reduction (top) and the oxidation of Tempo (bottom).

Figure 1-7 Thermodynamic cycles for the reduction of benzoquinone and dhe oxidation of Tempo.

The reversibility of the Diels-Alder reaction has been put to good use in the protection of olefins. Ardis [69] used the butadiene adduct to protect the olefin in his synthesis of vinylidene cyanide. Alder [70] was able to monoepoxidise various benzoquinones as their cyclopentadiene adducts, and Chapman [71] used the same method to obtain half-reduction of benzoquinone. Regeneration of the olefin is achieved thermally and Alder [70] noted that this was easier with the fulvene adduct than with the cyclopentadiene adduct. Sauer [72] observed that the retro-Diels-Alder reaction was easier with adducts involving furan, fulvene, and anthracene as the diene component, and he made the anhydride (8) using this method. 

There is another effective method for the determination of phenol derivatives that makes use of enzymes. The enzyme tyrosinase oxidizes phenol derivatives to benzoquinone derivatives by consuming dissolved oxygen in the solution. Benzoquinone derivatives can be easily reduced electrochemically in the moderately negative potential range. By using an electrode with immobilized tyrosinase, the phenol derivatives can be detected from the response due to the electrochemical reduction of benzoquinone derivatives. 

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