Ascorbic acid as reductant

Y. Hirai, N. Yoza, and S. Ohashi, Flow Injection Analysis of Inorganic Ortho- and Polyphosphates Using Ascorbic Acid as Reductant of Molyb-dopho" phate. Chem. Lett., 5 (1980) 499. 

GREEN SYNTHESIS OF SILVER SOLS INTO POLYELECTROLYTE MATRICES USING ASCORBIC ACID AS REDUCTANT... 

Fig. 12.15 Di ssolution rate of goethite by protonation, complexation with oxalate and reduction by ascorbic acid as a function of pH (Stumm, Furrer, 1987, with permission).

Solution Abbreviating dehydroascorbic acid as D, and ascorbic acid as H2A, we rewrite the reduction as... 

In its biochemical functions, ascorbic acid acts as a regulator in tissue respiration and tends to serve as an antioxidant in vitro by reducing oxidizing chemicals. The effectiveness of ascorbic acid as an antioxidant when added to various processed food products, such as meats, is described in entry on Antioxidants. In plant tissues, the related glutathione system of oxidation and reduction is fairly widely distributed and there is evidence that election transfer reactions involving ascorbic acid are characteristic of animal systems. Peroxidase systems also may involve reactions with ascorbic acid In plants, either of two copper-protein enzymes are commonly involved in the oxidation of ascorbic acid. 

Much interest has also been expressed in tetra-azamacrocyclic compounds, due to their role in the natural reduction of C02 to CH4 by a nickel tetrapyrrole coenzyme found in methane-producing bacteria. Tinnemans et al. used Co(II) tetra-azamacrocyclic complexes with [Ru(bpy)3]2+ as the photosensitizer and ascorbic acid as the sacrificial electron donor in aqueous C02-saturated solutions at acidic pH [33]. Whilst the TON for the total observed products of CO and H2 exceeded 500, they were formed in a ratio of 0.27 1, respectively. 

Grant et al. studied a similar system using [Ni(cyclam)]2+ as the catalyst (cyclam = 1,4,8,11-tetra-azacyclo tetradecane), [Ru(bpy)3]2+ as the photosensitizer, and ascorbic acid as the sacrificial reductant [34], and observed a pH dependence on CO/H2 ratios, with the best ratio of 0.83 1 at pH 5. When Kimura etal. prepared pyridine derivatives of [Ni(cyclam)]2+ [35], the best complex, in C02-saturated ascorbate buffer at pH 5.1 and [Ru(bpy)3]2+ as the photosensitizer, produced 5.8-fold more CO than [Ni(cyclam)]2+. 

Mochizuki et al. synthesized a bimacrocydic Ni(II) complex, [6,6 -bi(5,7-dimethyl-l,4,8,ll-tetraazacyclotetradecane)]-dinickel(II) triflate [36]. In a C02-saturated aqueous solution at pH 4, with [Ru(bpy)3]2+ as the photosensitizer, and ascorbic acid as the sacrificial reductant, the CO/H2 ratio was 15 1 and the rate of CO production was approximately eightfold higher than for [Ni(cyclam)]2+. 

Other Co(II)-complexes that were applied in the photosensitized reduction of C02 to CO (and concomitant H2-evolution) include Co(II)-ethylene glycol dimethyl ether complexes [178], and different tetraaza-macrocyclic Co(II)-complexes such as 27,28. A closely related system, where Ni(II)-tetraaza macrocycle (29) substitutes the cobalt homogeneous complexes in the photosystem including Ru(bpy) + as photosensitizer and ascorbic acid as electron donor, has been reported by Tinnemans [181] and Calvin [182],... 

HG procedures are often advocated as a means of elimination of interferences due to Cl-. However, careful consideration of the gas-liquid separator design is required to minimize or eliminate transport of Cl- to the plasma [75]. The efficiency of AsH3 production is dependent on the acidity, NaBH4 concentration, and prereductant used [76]. The use of cysteine as a reductant has all but replaced traditional reductants such as KI and Kl-ascorbic acid as the hydride yield is... 

This postulation for a reaction pathway to explain the seemingly contradictory data which implicates ascorbic acid as both a reductant and oxidant is not intended to be a final explanation. However, it does seem to fit many of the observations in the literature which under other explanations seem to be almost impossible from a chemical standpoint or simply to be contradictory. 

Other Reducing Agents. 1,4-Dihydropyridines have been successfully employed for the reductive desulfonylation of functionalized sulfones. Of special interest is the desulfonylation of a-nitro sulfones with BNAH under sunlight irradiation to give the corresponding nitro compounds in good yields.90,91 The reaction takes place under mild conditions and tolerates ketones, nitriles, and isolated double bonds (Eq. 120).91 A photo-induced electron-transfer employing ascorbic acid as electron donor is also an efficient approach for the reductive desulfonylation of (3-ketosulfones.197... 

Certain microorganisms, such as Escherichia coli and Staphylococcus albus strains will reduce dehydroascorbic acid. This reduction has been used for the estimation of ascorbic acid after its oxidation in extracts by ascorbic acid oxidase (S27) and also in the chemical reduction methods as a way of reducing dehydroascorbic acid to ascorbic acid. The reduction is not specific for L-ascorbic acid, but interfering compounds are unlikely in natural products. The use of this reducing action of bacteria has been improved by Mapson and Ingram (M8). 

Fine needles, dee 225". Sol in water at tdf. In soln the two carbonyl groups (in position 2 and 3) assume the hydrated form —C(OH),—C(OH)j—. Practically neutral reaction. pKa 3.90. lascorbic acid. Detailed stability data Bogdanski. Bogdanska. Bull. Acad. Polonaise Sci. (II classe) 3, 41 (1955). See also Velisek et al. Coll Czech. Chem. Commute 37, 1465 (1972). Undecomposed dehydroascorbic acid in soln is easily converted to ascorbic acid by reduction with sulfurous acid. Has same antiscorbutic activity in humans as ascorbic acid (upon oral ingestion). 

Reduction of PTM radicals with ascorbic acid. It is quite remarkable that, while the radical PTM- withstands highly reactive reagents as well as reducing organic compounds such as glucose and oxalic acid, nonetheless it is converted rapidly to PTM—H with ascorbic acid (AS) in aqueous (10%) THF at room temperature (Ballester et al., 1978d M. Ballester, J. Riera and M. Casulleras, unpublished). Dehydroascorbic acid (DAS) is isolated, and the stoichiometry requires 1 mol of AS per 2 mol of PTM-. This reduction... 

Whether coenzyme I was concerned in the earlier experiments was not determined and has not mnce been determined. The evidence from these studies was suggestive of the participation of ascorbic acid as a respiratory carrier. It was, however, only suggestive and not conclusive, for the authors did not demonstrate either with their lactate dehydrogenase or hexose diphosphate systems the direct reduction of dehydroascorbic acid to ascorbic acid. 

In general, C3-saturated verdazyls 1 are prepared via oxidation of the readily prepared leucoforms 2 (1963AGE155), and less commonly via the reduction of verdazylium salts 3 using excess ascorbic acid as a mild one-electron reductant (Scheme 1) (1997S17). 

Figure 25 Photoinduced rotation of the terpyridine-containing macrocyclic ring of the copper catenane complex 41 obtained by using p-nitrobenzylbromide as an electron scavenger and ascorbic acid as a chemical reductant. ...

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