Quinone-hydroquinone reaction

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... 

Quinone-Hydroquinone Exchange Reactions. I. Non-Exchange in Duro-... 

The processes of oxidation of cyclohexadiene, 1,2-substituted ethenes, and aliphatic amines are decelerated by quinones, hydroquinones, and quinone imines by a similar mechanism. The values of stoichiometric inhibition coefficients / and the rate constants k for the corresponding reactions involving peroxyl radicals (H02 and >C(0H)00 ) are presented in Table 16.3. The/coefficients in these reactions are relatively high, varying from 8 to 70. Evidently, the irreversible consumption of quinone in these systems is due to the addition of peroxyl radicals to the double bond of quinone and alkyl radicals to the carbonyl group of quinone. 

More generally, double bonds between two carbons or one carbon and a heteroatom, possibly conjugated with other unsaturated moieties in the molecule, are eligible for two-electron/two-proton reactions according to Scheme 2.20. Carbonyl compounds are typical examples of such two-electron/two-proton hydrogenation reactions. In the case of quinones, the reaction that converts the quinone into the corresponding hydroquinone is reversible. With other carbonyl compounds, the protonation of the initial ketyl anion radical compete with its dimerization, as discussed later. 

Note 1 Reversible redox reaction can take place in a polymer main-chain, as in the case of polyaniline and quinone/hydroquinone polymers, or on side-groups, as in the case of a polymer carrying ferrocene side-groups. 

There are indications that another type of catalysis is present in the reaction between hydroquinone and silver ions in alkaline solution. The increase of rate with increasing hydroquinone concentration is greater than direct proportionality. This situation is similar to that observed in the oxygen oxidation of durohydroquinone (tetramethylhydroquinone) (James and Weissberger, 16) where the quinone formed in the reaction catalyzes subsequent oxidation. A direct check on quinone catalysis of the hydroquinone-silver ion reaction was not made, since quinone is unstable in alkaline solution, particularly in the presence of sulfite which reacts with it. Experiments were made, however, on the reaction between durohydroquinone and silver ion. This reaction shows the same dependence of rate upon the square root of the silver ion concentration as the hydroquinone reaction does. Addition of duroquinone to the reaction mixture produces a definite acceleration, as shown in Table II. 

Kuhn and colleagues (797) investigated the kinetics of the heterogeneously catalyzed benzene oxidation at Pb/Pb02 electrodes in sulfuric acid. This reaction was worked out on a semitechnical scale for quinone/ hydroquinone production (792) ... 

A system has been proposed which involves quinone/hydroquinone interconversions. A number of such couples are insoluble in acid solution and can therefore be used as active solid masses when made conductive by the addition of graphite. A system which has been studied experimentally is based on anthraquinone, Q, and tetrachloro-p-benzoquinone (chloranil), Qr, and their reduced or hydroforms. The overall cell reactions are then... 

Substances undergoing redox reactions (such as quinone-hydroquinone, sulphide-disulphide, metal complexes, redox couples) may serve as electron carriers and allow the coupling of oxidation-reduction processes across membranes (see, for instance, [6.44-6.46]) to cation or anion transport. 

This chapter intends to discuss the fundamental role played by carbons, taking particularly into account their nanotexture and surface functionality. The general properties of supercapacitors are reviewed, and the correlation between the double-layer capacitance and the nanoporous texture of carbons is shown. The contribution of pseudocapacitance through pseudofaradaic charge transfer reactions is introduced and developed for carbons with heteroatoms involved in functionalities able to participate to redox couples, e.g., the quinone/hydroquinone pair. Especially, we present carbons obtained by direct carbonization (without any further activation) of appropriate polymeric precursors containing a high amount of heteroatoms. 

The AE value between peaks (see Chapter 8) is about 0.1 V, indicating a fast electron exchange between polypyrrol and quinone-hydroquinone in solution. However, a decrease in velocity is shown in the steady-state reaction visible in the Tafel lines of Fig. 11.14. 

In this section, unlike the previous one, we deal with less heavily doped, semiconductor diamond. Quantitative studies of reaction kinetics have been performed in Fe(CN)63 -/4, quinone/hydroquinone (recall that this is an inner-sphere reaction), and Ce3+/4+ systems [94, 104, 110]. Potentiodynamic curves recorded in solutions containing only one (either reduced or oxidized) component of a redox system are shown on Figs. 22a and b the dependences of anodic and cathodic current peak po-... 

Typical values of transfer coefficients a and ji thus obtained are listed in Table 4 for single crystal and polycrystalline thin-film electrodes [69] and for a HTHP diamond single crystal [77], We see for Ce3+/ 41 system (as well as for Fe(CN)63 /4 and quinone/hydroquinone systems [104]), that, on the whole, the transfer coefficients are small and their sum is less than 1. We recall that an ideal semiconductor electrode must demonstrate a rectification effect in particular, a reaction proceeding via the valence band has transfer coefficients a = 0, / =l a + / = 1 [6], Actually, the ideal behavior is rarely the case even with single crystal semiconductor materials fabricated by advanced technologies. Departure from the ideal semiconductor behavior is likely because the interfacial potential drop is located in part in the Helmholtz layer (due e.g. to a high density of surface states), or because the surface states participate in the reaction. As a result, the transfer coefficients a and ji take values intermediate between those characteristic of a semiconductor (0 or 1) and a metal ( 0.5). 

In Section 2 we showed that the properties of amorphous carbon vary over a wide range. Graphite-like thin films are similar to thoroughly studied carbonaceous materials (glassy carbon and alike) in their electrode behavior. Redox reactions proceed in a quasi-reversible mode on these films [75], On the contrary, no oxidation or reduction current peaks were observed on diamondlike carbon electrodes in Ce3+/ 41, Fe(CN)63 4. and quinone/hydroquinone redox systems the measured current did not exceed the background current (see below, Section 6.5). We conventionally took the rather wide-gap DLC as a model material of the intercrystallite boundaries in the polycrystalline diamond. Note that the intercrystallite boundaries cannot consist of the conducting graphite-like carbon because undoped polycrystalline diamond films possess excellent dielectric characteristics. 

Thiophenols and thiophenol derivatives chemisorbed on well-defined electrode surfaces have also been studied by HREELS. The cyclic voltammetric peaks for the quinone/hydroquinone redox reaction of the 2,5-dihydroxythiophenol immobilized on the Pt surface was much broader than for the unadsorbed species the broadening vanished when a methylene group was placed between the—SH group and the phenyl ring. These results indicated strong substrate mediated adsorbate-adsorbate interactions. Such... 

Two cases are distinguishable according to Fig. 3. The conventional one (a) holds for all molecular systems, which will be treated in Section 2, e.g., quinones or disulfides. The redox partner reacts in the dissolved state this means at low concentrations for electrodes of the second kind. The reactant is positioned in the outer Helmholtz plane, or even in the inner Helmholtz plane, as shown for adsorbed molecules. One example is the quinone/hydroquinone redox reaction according to Eq. (13) ... 

Prominent examples are the redox pairs o- or p-quinones/hydroquinones, the corresponding quinoneimines, the diimines and the azobenzenes and disulfides [68]. V. Stackelberg [69] has pointed out that the exclusive formation or cleavage of 0-H, N-H, S-H, or S-S bonds is a necessary precondition for reversible organic redox partners. This can be clearly recognized in the case of the quinone/hydroquinone redox reaction (cf. Eq. (13)). Only O -H bonds are formed or cleaved. In contrast, in the case of the acetone/isopropanol redox system, 0-H and C-H bonds participate. 

More properly, the above remark refers to the initial step of this reaction. Studies performed using platinum and Sn02 electrodes indicate that the quinone/hydroquinone redox reaction involves two distinct, consecutive charge transfer steps. Also the hydroquinone oxidation at the Ti02 photoanode follows presumably a two-step mechanism. 

Bibenzyls can be obtained in a remarkable reaction from toluene, p-xylene, or ethylbenzene in 60-100% yield [24], For this reaction, catalytic amounts of quinones are irradiated in the presence of the alkylbenzenes and the anode potential is held at that of the quinone-hydroquinone couple. The photo excited quinone abstracts a benzyl hydrogen to form a radical, which dimerizes. The resulting hydroquinone is reoxidized to the quinone. 

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