Quinone-hydroquinone formation

Quinone/Hydroquinone Formation. In studies on biodegradation of lignin models with ligninolytic cultures of P. chrysosporium (and other fungi), a number of quinones and hydroquinones were isolated. In other studies their formation has been implied from the isolation of their structural counterparts (29,30), where rapid fungal degradation of the quinones prevented their isolation. Various routes to these metabolic intermediates exist, which have been extensively reviewed (26,27). 

Quinone dyes, 9 503 Quinone ketals, anodic oxidation of hydroquinone ethers to, 21 264 Quinone methides, 2 209-211 Quinone Michael addition chemistry, 21 248-249, 250, 252 Quinone monoacetals, 21 251 Quinone monoimine (QMI), 19 246 Quinone oximes, formation of,... 

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. 

Alternatively, one may polarize the platinum disk to the quinone-hydroquinone equilibrium potential with the help of a current supplied by an auxiliary circuit. Then one may determine the required current 7, and the rate of consumption of quinone or the rate of formation of hydroquinone. At the quinone-hydroquinone equilibrium potential the electrochemical reduction of hydroquinone vanishes. Consequently, a finite rate of the formation of hydroquinone at the quinone-hydroquinone equilibrium potential equals the partial rate due to the nonelectrochemical mechanism according to Eqs. (VIII.lla)-(VIII.11c). 

Oxidation of hydroquinone and pyrocatechol in aqueous solution is accelerated by quinones [228,229], which seems to be due to the formation of semiquinone radicals and their fast reaction with oxygen. Duro-hydroquinone oxidation is autocatalytic [229], due to the accumulation of quinone and formation of semiquinone radicals [230—233],... 

The most interesting reactions of the natural quinone-hydroquinone pairs are (a) their reversible one-electron redox reactions, (b) the Michael reactions of quinones, and (c) the formation of polyquinones. Oxidation of tocopherol with FeClj leads to a cleavage of the enol ether and quinone formation (Scheme 7.2.2). 

Redox-active quinones/hydroquinones were proposed to assist in the reoxidation of Pd(0) in addition to Pd-ICC-NR Br systems [43]. Formation of HQ carbonate as a side product was not detected [27]. A similar system, Pd(OAc)2/ BQ/Co(acac)3/Bu NBr, in the presence of dmphen delivered a Pd TON of 700 [21]. The ratio BQ Pd affects the catalytic activity until it reaches 30 1, and the addition of dmphen just shifts the curve to higher TON [21]. BQ and anthraquinone were also used with copper ICCs [44]. Addition of BQ increased Pd TON two to four times in the systems Pd- Co - TBAB and Pd - Cu - TB AB [45]. 

While the chemical yield of hydrogen peroxide in the AO process is very high, the loss of quinone/hydroquinone via the formation of these by-products necessitates the regeneration of the reaction mediators, hydrogenation catalyst, and removal of organic by-products. Periodically, fresh anthraquinone and solvent are added to compensate for losses. 

Dehydrogenation of Hydrocarbons. The mechanism by which quinones effect dehydrogenation is believed to involve an initial rate-determining transfer of hydride ion from the hydrocarbon followed by a rapid proton transfer leading to hydroquinone formation. Dehydrogenation is therefore dependent upon the degree of stabilization of the incipient carbocation and is enhanced by the presence of functionality capable of stabilizing the transition state. As a consequence, unactivated... 

The absorption of light in the reaction center (RC) of photosynthetic bacteria induces electron transfer from the special bacteriochlorophyll pair (P) through a series of one-electron acceptors (bacteriopheophytin, and a primary quinone, Q ) to a two-electron acceptor quinone, Qg [1], In RCs from sphaeroides, both and Qg are ubiquinone-10. It is generally believed that the doubly reduced secondary quinone (hydroquinone dianion) will form quinol (hydroquinone) by taking up two protons before being released from the RC and replaced by another quinone from the quinone-pool. The rate of quinol formation can be limi ted by either of these processes the second electron transfer from Qb to Q/vQb the... 

However, Trivedi et al. [95] have argued that when there is a possibility of autocatalytic growth of polyaniline, the lower electrochemical potential does not favour the formation of such a crosslinked moiety, unless one crosses the threshold limit of 0.8 V versus SCE. Thus, according to these authors, the middle peak is due to quinone/hydroquinone, whose formation is possible because under acidic conditions the cation radical can be hydrolysed to yield benzoquinone. 

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