Quinone/hydroquinone pair

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 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). 

The electron transfer activity of the quinone/hydroquinone pair has often been described through radical reactions [40] the carbonyl groups can stabilize the created radicals that may accept electrons and become anions, which can transfer electron back and become radicals or can interact with cations in solution. Then, reversible proton transfer leads to phenolic hydroquinone sites ... 

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. 

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... 

The kinetic differences between high-spin (presumably five-co-ordinate) iron(ii) and low-spin (six-co-ordinate) iron(ni) porphyrin complexes in their oxidation or reduction by thequinone-hydroquinone pair have been discussed by Castro et al. In the former case, the authors favour a transition state in which the quinone has entered the inner co-ordination sphere of the iron (Scheme 21) while in the latter an outer-sphere mechanism (Scheme 22) is preferred. The reactivity of the iron(ii) and... 

There are two kinds of redox interactions, in which ubiquinones can manifest their antioxidant activity the reactions with quinone and hydroquinone forms. It is assumed that the ubiquinone-ubisemiquinone pair (Figure 29.10) is an electron carrier in mitochondrial respiratory chain. There are numerous studies [235] suggesting that superoxide is formed during the one-electron oxidation of ubisemiquinones (Reaction (25)). As this reaction is a reversible one, its direction depends on one-electron reduction potentials of semiquinone and dioxygen. 

Hydroquinone or Quinone Benzene Several Past pilot-plant Paired synthesis or anodic oxidation + chemical reduction... 

As we have already seen these lone pairs can form part of the system of n electrons. The difference between chromo-phoric and auxochromic groups is in this way of secondary importance. Also the much discussed question whether a ben-zoid (benzene-like) or a quinonoid (quinone-like) structure should be attributed to dyestuffs becomes, in the light of the resonance theory, an incorrectly chosen alternative. It is the possibility of resonance which is reflected in the multiplicity of the valence structure that forms the true basis for light absorption. An isolated benzoid configuration is just as little a colouring matter as a quinonoid structure compare the uncoloured hydroquinone and the very weakly coloured quinone. 

D Souza has studied assemblies of type 16 [95], which take advantage of the two-point hydrogen bonds formed by hydroquinone-quinone pairing to afford the quinhydrone acceptor. Lifetime measurements using single photon counting point... 

While obviously natural enediynes 2-4 behave in the same way, in dynemicin the cascade of events is different. However, this molecule also displays a delivery unit (the anthraquinone), a safety catch (the epoxide) and a trigger (the quinone). Bioreduction of the quinone unleashes two lone pairs (one on the nitrogen and one on the upper hydroquinone oxygen) which bring about intramolecular epoxide opening. ... 

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