Quinone-hydroquinone

One of the most exciting discoveries related to quinone/hydroquinone chemistry is thek synthesis by biosynthetic routes (12,13). Using bacterial enzymes to convert D-glucose [50-99-7] (7) to either 1,2- or l,4-ben2enediol allows the use of renewable raw material to replace traditional petrochemicals. The promise of reduced dependence on caustic solutions and the use of transition-metal catalysts for thek synthesis are attractive in spite of the scientific and economic problems still to be solved. 

In the case of mechanism (6) there are materials available which completely prevent chain growth by reacting preferentially with free radicals formed to produce a stable product. These materials are known as inhibitors and include quinone, hydroquinone and tertiary butylcatechol. These materials are of particular value in preventing the premature polymerisation of monomer whilst in storage, or even during manufacture. 

Oxidation-reduction (redox) Inert metal (normally Pt but certain other metals can act in a similar manner) in a solution containing two species that give rise to a redox system. E depends on of the system and the relative activities of the oxidised and reduced forms. Quinone-hydroquinone QH4O2 -1- 2H+ -1- 2e-CjH4(OH)2, which is thus pH dependent Fe - -/Fe + Mn04-/Mn +... 

Polypyrrole shows catalytic activity for the oxidation of ascorbic acid,221,222 catechols,221 and the quinone-hydroquinone couple 223 Polyaniline is active for the quinone-hydroquinone and Fe3+/Fe2+ couples,224,225 oxidation of hydrazine226 and formic acid,227 and reduction of nitric acid228 Poly(p-phenylene) is active for the oxidation of reduced nicotinamide adenine dinucleotide (NADH), catechol, ascorbic acid, acetaminophen, and p-aminophenol.229 Poly(3-methylthiophene) catalyzes the electrochemistry of a large number of neurotransmitters.230... 

One of the attractions of aprotic solvents is that the electron transfer behaviour of many compounds is much simpler than in protonic media. However, this is not always so for example, the quinone/hydroquinone couple is very simple in aqueous solution but it is complicated in aprotic solvents by the number of protonation equilibria which no longer lie well to one side as they do in aqueous solution (Bessard et al., 1970). 

Efforts to achieve a retardation of cross-linking in elastomers are based on the general assumption of a radical mechanism for retardation cross-linking and the possibility of its inhibition by a deactivation of the reactive macromolecular radical [33]. These compounds generally contain one or more labile hydrogen atoms, which after, donation of this atom, will form relatively inactive radicals. Typical antirad agents are quinones, hydroquinones, and aromatic amines (phenyl and napthylamines). 

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... 

Kinetic Parameters of Quinone, Hydroquinone Redox Couple at a Platinum Interface0... 

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. 

The excited-state lifetime of the uncomplexed fluorophore M is unaffected, in contrast to dynamic quenching. The fluorescence intensity of the solution decreases upon addition of Q, but the fluorescence decay after pulse excitation is unaffected. Quinones, hydroquinones, purines and pyrimidines are well-known examples of molecules responsible for static quenching. 

P-3, consisting of a free-base porphyrin with an appended quinone, is weakly fluorescent due to PET. Complexation with substituted hydroquinones induces fluorescence enhancement because of the unfavorable conditions for PET from the porphyrin to the quinone-hydroquinone entity. 

The value of E for the quinone-hydroquinone couple is 0.699 V. Look at the list of electrode potentials given in Appendix 3 and decide which redox couples would be powerful enough to oxidize hydroquinone to quinone. 

The reduced form of any of the couples shown in Appendix 3 which have an of about 0.9 V or more, i.e. about 0.2 V higher than the of the quinone-hydroquinone couple, would be suitable. Good choices would be permanganate or ceric ion. Bromine will also oxidize hydroquinone, but its chemical reactivity precludes its use in such an application. 

The role of ubiquinone (coenzyme Q, 4) in transferring reducing equivalents in the respiratory chain is discussed on p. 140. During reduction, the quinone is converted into the hydroquinone (ubiquinol). The isoprenoid side chain of ubiquinone can have various lengths. It holds the molecule in the membrane, where it is freely mobile. Similar coenzymes are also found in photosynthesis (plastoquinone see p. 132). Vitamins E and K (see p. 52) also belong to the quinone/hydroquinone systems. 

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. 

Anderson B, Oglesby F Corneal changes from quinone-hydroquinone exposure. AMA Arch Ophthalmol 59 495-501, 1958... 

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). 

Thus, by a combination of oxidation by lignin peroxidases, Mn(II)-dependent peroxidases and other active oxygen species and reductions of some aromatic aldehydes, acids and ketones to the corresponding benzylic alcohols, all aromatic rings in the lignin polymer can be either converted to ring opened products or to quinones/hydroquinones. These products are then further metabolized to CO2 by a currently unknown mechanism. 

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