Hydroquinone oxidations

Fig. 19. Proposed mechanism of hydroquinone oxidation by the C3dochrome bci complex (see text). Open circles indicate oxidized metal centers filled circles indicate reduced metsd centers.

Hydroquinone oxidation to quinone occurs via two linked one-electron transfer stages. The first step (typically metal catalysed) yields the semiquinone radical intermediate, which is resonance stabilised. The second step involves electron transfer to molecular oxygen to generate superoxide and quinone [70]. This reaction mechanism is common to all hydroquinones, catechols, resorcinols, and so forth. 

Phenols are more easily oxidized than alcohols, and a large number of inorganic oxidizing agents have been used for this purpose. The phenol oxidations that are of the most use to the organic chemist are those involving derivatives of 1,2-benzenediol (pyrocate-chol) and 1,4-benzenediol (hydroquinone). Oxidation of compounds of this type with silver oxide or with chromic acid yields conjugated dicarbonyl compounds called quinones. 

Propenaldehyde. Aldehyde group. Typically available as a 92% active, liquid. It is flammable, volatile, lachrymatory, and a strong irritant. Acrolein is a protein and enzyme poison. It is stabilized against polymerization by the addition of hydroquinone (oxidation results in polymerization, rendering acrolein inactive as a microbiocide). Although only small dose levels are required (1.5 to 3.0 ppm) to achieve threshold toxicity, rapid microbial resistance can occur. This product is seldom used today as the effort is often not worth the gain. 

Fig. 19. Dependence of (1) hydroquinone oxidation and (2) quinone reduction peak current on the square root of potential scan rate. Solution of 0.01 M hydroquinone (or quinone) +0.5 M H2SO4. Polycrystalline electrode (data of Yu. V. Pleskov and Yu. E. Evstefeeva).

In addition to quinone reduction and hydroquinone oxidation, electrode reactions of many organic compounds are also inner-sphere. In these charge transfer is accompanied by profound transformation of the organic molecules. Some reactions are complicated by reactant and/or product adsorption. Anodic oxidation of chlorpro-mazine [54], ascorbic acid [127], anthraquinone-2,6-disulfonate [128], amines [129], phenol, and isopropanol [130] have been investigated. The latter reaction can be used for purification of wastewater. The cyclic voltammogram for cathodic reduction of fullerene Cm in acetonitrile solution exhibits 5 current peaks corresponding to different redox steps [131]. 

A related piece of work by Russian workers showed that anisole was oxidized to a mixture of o- and p-methoxyphenol by MT0/H202 in MeCN.51 Oxidation of benzene to phenol was also observed, though this process was far slower. As with the hydroquinone oxidations, this likewise can be... 

The Q-cycle mechanism requires the presence of two separate quinone binding sites that are in contact with different sides of the membrane the hydroquinone oxidation (Q or Qp) site at the positive P side of the membrane and the quinone reduction (Q or Qn) site at the negative N side of the membrane. These sites have first been characterized by their different inhibitor binding properties [2] (see below) the existence of two distinct quinone binding sites was confirmed by the X-ray structure of the bc complex [3-6]. 

In addition to the three classes of organic compounds, zinc ions reversibly inhibit the bc complex. binds at the Qo site and prevents the release of protons which are formed during hydroquinone oxidation [8]. Binding of Zn " is not competitive to the binding of other Qo site inhibitors. 

Link, T. A., 1997, The role of the iRieskei iron sulfur protein in the hydroquinone oxidation (Qp-) site of the cytochrome hcp complex the iproton-gated affinity change mechanism, FEBS Eett. 412 257n264. 

Link, T. A., Haase, U., Brandt, U., and von Jagow, G., 1993, What information do inhibitors provide about the structure of the hydroquinone oxidation site of ubihydroquinone cytochrome c oxidoreductase J. Bioenerg. Biomembr. 25 221n232. 

Fig. 6. Kinetic curves of hydroquinone oxidation 1 H202, 2 Fe3+. Reaction conditions [Fe3+] - 0.25%, M - 50, temperature -60 °C...

It is interesting to note that the system containing hydroquinone does not initiate homopolymerization of aqueous AN under the conditions identical to those of grafting. Dolgoplosk attributed this effect to inhibition by hydroquinone oxidation products and suggested that inhibition can be suppressed by a fourth component reducing these products. The initiation of graft polymerization by this system appears to arise from the fact that chain termination in water-swollen cellulose is difficult. 

Results on catalytic activity often are only fundamental, if properties of the porphyrin are retained through covalent binding to polymers. Peroxidative activity with (20) and hydroquinone oxidation with (2i) was reported. 

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. 

Jeng, C. Y., Langer, S. H. Hydroquinone oxidation for the detection of catalytic activity in liquid chromatographic columns, J. Chromatogr. Sci., 1989, 27, 549-552. 

Domenech, A., Garcia, H., Marquet, J., Bourdelande, J.L., and Herance, J.R. 2006b. Modelling electrocatalysis of hydroquinone oxidation by nicotinamide dinucleotide coenzyme encapsulated within SBA-15 and MCM-41 mesoporous aluminosilicates. Electrochimica Acta 51, 4S97-490S. 

The ligands for the two iron atoms are located in the loops between P-4/P-5 [Cys-139 and His-141] and P-6/P-7 [Cys-158 and His-161], near the tip of the fold in Fig. 4 (C) [note that the residue numbers in the mitochondrial R-[2Fe 2S] differ from those in the Nostoc Rieske protein shown in Fig. 4 (B)]. Thus, Cys-139 and Cys-158 coordinate with one iron and His-141 and His-161 coordinate to the second iron. The tip location of the FeS cluster allows close contact with Cyt b, which together with the R-[2Fe 2S] cluster provides a pocket where hydroquinone oxidation can occur. 

As seen above, the two electrons produced by hydroquinone oxidation at the Q, binding site take a bifurcated electron-transfer pathway, a high-potential pathway leading to the Rieske ISP and Cyt Cj and a low-potential pathway to Cyt b. The initial crystallographic determination reported by Xiaeta/. with crystals of the Cyt bc complex of bovine-heart mitochondria provided sufficient electron density to allow identification of the [2Fe 2S] cluster of the Rieske ISP at a position 31 A from the heme iron... 

Oxidation of the hydroquinone at the Q site is rate-determining and has a relatively high activation barrier. However, activation energies of partial reactions that contribute to movement of the [2Fe 2S] domain have been measured and shown to be lower than the activation energy associated with the ratedetermining reaction of hydroquinone oxidation. It has also been shown that reactions that contribute to movement of the [2Fe 2S] domain are rapid compared to the rate-limiting step. 

Tony Crofts and Ed Berry and their colleagues have made further detailed studies on the role of the iron-sulfur protein and its mobility on the mechanism of hydroquinone oxidation hy the Cyt bc complex , the stmctural aspects of the movement of the [2Fe 2S] domain during hydroquinone oxidation, and the role of the different domains of the Qq binding site in the binding of inhibitors they have also prepared a comprehensive review entitled Structure and function of cyt be complexes for volume 69 of the Annual Review of Biochemistry (2000) . ... 

Vivo, 1955 Fukuzumi et al., 1975). Radical formation at pH 6 apparently depends on both the concentration of hydroquinone and the amount of oxide. This is in accord with the studies of Fukuzumi et al. (1975) and Ono et al. (1977) at pH 9, in which the formation of radicals obeys first-order kinetics with respect to both phenol concentration and the amount of Mn oxides. The presence of semiquinone radicals indicates that the reduction of hausmannite involves a one-electron transfer process. The radical concentration initially increases, but then decreases simultaneously with the consumption of dissolved O2 (Fig. 8-12). Once O2 is depleted, the concentration of free radical gradually increases again. The rapidly generated semiquinone anion radical is apparently slowly oxidized by dissolved O2 in solution. The radical becomes more abundant at relatively high concentrations of hydroquinone. Oxide suspensions containing high concentrations of hydroquinone have insufficient capacity to oxidize hydroquinone to quinone completely, resulting in the accumulation of the semiquinone radicals. 

However, the semiquinone does not act as a redox probe, because its concentration is zero as long as hydroquinone itself has not started the transfer. Then it is always produced in the close vicinity of the supercritical cluster already selected by the hydroquinone, and it reacts readily with the same cluster before diffusion. It is worth noting that the semiquinone essentially amplifies the catalytic transfer. The overall hydroquinone oxidation into the quinone produces twice as many silver atoms as the initial QH2 concentration (reactions... 

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