Reduction two-electron

Land et al [60] have shown how pulse radiolysis can be used to generate 2-electron reduced species from parent quinones. A significant difference in behaviour of reduced naphtho- and anthraquinones was established during our subsequent studies. Naphthoquinones generated corresponding hydroquinones by complete 2-electron reduction. The fully reduced species readily reacted with oxygen to generate the parent quinone, e.g. naphthazarin. 

Depending on pK values, neutral or anionic forms of the hydroquinone are formed. Then, 

This reaction is quite slow in the case of naphthazarin. However, it becomes faster when oxygen is bubbled, or, in the alkaline solutions. Same reaction in the case of anthraquinone and higher derivatives gives rise to a reduced species which does not react with oxygen, or, reacts infinitesimally slowly. It was inferred that the reduced species could not be a normal hydroquinone. Several studies led to the proposal that a dihydroquinone type of molecule was generated. 

An interesting observation in the two-electron reduction of quinones [37] has been the loss of sulphonate (-SO3 ) group fi om the fiilly reduced quinizarin-2-sulphonate when reduction takes place at pH 7 and the solution is made alkaline. Such -SOs loss was not observed when reduction took place in the alkaline solutions or when quinizarin-6-sulphonate was used. On re-oxidation, quinizarin (and not its 2-sulphonate) was generated. Alkali was ruled out as cause of the -SOs loss. Two alternative mechanisms involving tautomerisation and/or base-induced P-elimination of -SO3 were proposed for this very interesting and rare observation. 

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A voltammogram for the two-electron reduction of M has a half-wave potential of —0.226 V versus the SCE. In the presence of an excess of the ligand L, the following half-wave potentials are recorded... 

The chemistry of the Li—FeS2 system is quite complex. There are at least two steps to the reaction at low discharge rates. The first reaction is an approximately two-electron reduction to a new phase which is a hthiated FeS2 compound. 

Note that flavin coenzymes can carry out either one-electron or two-electron transfers. The succinate dehydrogenase reaction represents a net two-electron reduction of FAD. 

Scheme 10.16 Redox cycle of flavins. The cycle is depicted with a two-electron reduction of flavin by NAD(P)H and two one-electron oxidations.

Two-electron reduction of the quinone provides the hydroquinone 10, which may also be formed by two sequential one-electron reductions. It has also been... 

In polarographic analysis it gives two breaks in the curve, indicating a two electron reduction probably by the eqn ClCH2N02+2e = CH2N02 +CJ- (Ref 5)... 

Bromonitromethane. This article supplements the material in Vol 2, B312-R under Bromo-rnethane and Derivatives CA Registry No 563-70-2. The IR and UV spectra are given in Ref 3. In polarographic analysis a two electron reduction to CH2N02 and Br is indicated (Ref 4). It can be quanty detd by treatment with coned sul-... 

The polarography of sulphones has been fairly extensively studied, sometimes with an analytical bias and sometimes with more theoretical interest. It is generally accepted that a two-electron reduction takes place, to sulphinate ... 

Under different conditions (in aqueous electrolyte) the selectivity of the cleavage reaction may be perturbed by the occurrence51-53 of a dimerization process. Thus, while the major process remains the two-electron reductive pathway, 20% of a dimer (y diketone) may be isolated from the cathodic reduction of PhC0CH2S02CH3. The absence of crosscoupling products when pairs of / -ketosulphones with different reduction potentials are reduced in a mixture may indicate that the dimerization is mainly a simple radical-radical coupling53 and not a nucleophilic substitution. 

Two-electron reduction of 4,4 -(l,3-propanediyl)bis-(iV-methylpyridinium) dibromide with sodium amalgam affords bis-l,4-dihydropyridine 46 <96CL151>. 

EPR spectra of the native D. africanus Fdlll show an almost isotropic signal centered around g = 2.01 similar to the one observed in proteins containing [3Fe-4S] clusters (72). The temperature dependence of this signal and low-temperature MCD spectra and magnetization properties are identical to the ones reported for D. gigas Fdll (93). Upon one-electron reduction ag = 12 signal develops, characteristic of the [3Fe-4S] state. Two-electron reduction elicits an EPR active species with an axial properties and g-values of 1.93 and 2.05, consistent with the presence of a [4Fe-4S] center. 

The reduced alkyl complexes are reoxidized by O2 to the iron(lll) alkyls. The corresponding diamagnetic phthalocyanine iron(ll) alkyl complexes, rFe(Pc)R), were prepared by two-electron reduction of Fe(Pc) by LiAIFl4 to give [Fe(Pc) (actually the Fe(I) phthalocyanine radical anion) followed by reaction with Mel, Etl or i-PrBr. The methyl compound, [Fe(Pc)CHi] was characterized by X-ray crystallography. ... 

Schneemeyer LF, Wrighton MS (1979) Flat-band potential of n-type semiconducting molybdenum disulfide by cyclic voltammetry of two-electron reductants Interface energetics and the sustained photooxidation of chloride. J Am Chem Soc 101 6496-6500... 

Benzoyl-CoA reductase carries out the two-electron reduction of the aromatic ring dnring the anaerobic degradation of benzoate by Thauera aromatica. This involves two-electron transfer from ferredoxin, and a combination of EPR and Mossbaner spectroscopy showed the presence of three different clusters, while inactivation by oxygen was associated with partial conversion of [4Fe-4S] clnsters to [3Fe-4S] clnsters (Boll et al. 2000). 

The third study has employed 4,6-dinitrobenzofuroxan and as metabolic systems the one-electron reductants NADPHxytochrome P450 reductase and ferredoxin NADP(+) reductase and the two-electron reductants DT-diaphorase and Enterobacter cloacae nitroreductase [239]. The compound is activated either by DT-diaphorase or nitroreductase. 

The formation of long-lived Cd e pairs and of metal atoms which are strongly reducing, offers an explanation for the two-electron reduction of solutes, or to express it in other words, for the intermediate storage of electrons on semiconductor particles. The most interesting two-electron reduction is that of water to form Hj Eq. (23). Figure 12 shows experimental results which strongly indicate that formation occurs... 

However, two-electron reduction occurs fractionally to produce H2O2, which may cause deterioration of the polymer electrolyte membrane or gaskets ... 

Arrhenius Plots of Atapp The value of 4 at a given potential E is determined in the same manner as described in Section 10.2.4. Because the eontribution of two-electron reduction to the production of H2O2 (compared with the overall ORR) was very low, we ean evaluate an apparent rate constant at a constant applied potential from the following equation [Wakabayashi et al., 2005a, b] ... 

Au has recently received less attention than Pt as a supported catalyst because of its lower impact in PEMFC energy conversion technology, since the ORR is dominated by a two-electron reduction process, at what is a high overpotential, in acidic media. Nevertheless, it is an important oxygen reduction catalyst in alkaline media, and, in contrast to Pt, is oxide-free in the potential range where oxygen reduction occurs. 

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