Oxidation, monosaccharides aldonic acid

The monosaccharides formed during pulping are not completely stable. About 10-20% of them are oxidized to aldonic acids by hydrogen sulfite ions. Simultaneously, hydrogen sulfite is reduced to thiosulfate ... 

Acids related to monosaccharides are also common for the formation of natural polysaccharides. There are three types of acids associated with monosaccharides aldonic acids generated by the oxidation of the aldehyde group of a monosaccharide, uronic acids generated by the replacement of the primary alcohol group of a monosaccharide by a carboxyl group, and saccharic acids generated by simultaneous oxidation and replacement. Aldonic acids have a marked tendency to eliminate water and form lactones. The structural formulas of two common hexuronic acids are... 

Reduction of an aldose forms one alditol reduction of a ketose forms two alditols. Br2 oxidizes aldoses, but not ketoses ToUens reagent oxidizes both. Aldoses are oxidized to aldonic acids or aldaric acids. Aldoses and ketoses react with three equivalents of phenyUiydrazine, forming osazones. C-2 epimers form identical osazones. The Kiliani-Fischer synthesis increases the carbon chain of an aldose by one carbon— it forms C-2 epimers. The Ruff degradation decreases the carbon chain by one carbon. The OH groups of monosaccharides react with acetyl chloride to form esters and with methyl iodide/silver oxide to form ethers. 

Much of the chemistry of monosaccharides is the familiar chemistry of alcohols and aldehydes/ketones. Thus, the hydroxyl groups of carbohydrates form esters and ethers. The carbonyl group of a monosaccharide can be reduced with NaBH4 to form an alditol, oxidized with aqueous Br2 to form an aldonic acid, oxidized with HNO3 to form an aldaric acid, oxidized enzymatically to form a uronic acid, or treated with an alcohol in the presence of acid to form a glycoside. Monosaccharides can also be chain-lengthened by the multistep Kiliani-Fischer synthesis and can be chain-shortened by the Wohl degradation. 

There are three possible classes of sugar acids which may be produced by the oxidation of monosaccharides (Figure 9.11). The aldonic acids are produced from aldoses when the aldehyde group at carbon 1 is oxidised to a carboxylic acid. If, however, the aldehyde group remains intact and only a primary alcohol group (usually at carbon 6 in the case of hexoses) is oxidised then a uronic acid is formed. Both aldonic and uronic acids occur in nature as intermediates in... 

Kinetics of oxidation of four pentoses by bromamide-T were conducted in alkaline medium at different temperatures and the overall activation parameters have been calculated.52 Aldonic acids were the oxidation products, and a mechanism was suggested in which formation of the enediol anion of the sugar is the rate-limiting step. As aldoses may undergo epimerization in alkaline solutions, the oxidation of monosaccharides with bromamide-T was also performed in hydrochloric acid solution.53 Kinetic parameters revealed a low reactivity of ketoses relative to aldoses, and indicated that the cyclic forms of the latter are involved in the oxidations. 

Monosaccharides can be oxidized at the aldehyde carbon to give carboxylic acids called aldonic acids. Oxidation at both ends of the carbon chain gives aldaric acids. Reduction of the carbonyl group to an alcohol gives polyols called alditols. The -OH groups in sugars, like those in simpler alcohols, can be esterified or etherified. 

In the second place, oxidation by Fehling s or Tollens reagent cannot be used for the preparation of aldonic acids (monocarboxylic acids) from aldoses. Both Fehling s and Tollens reagents are alkaline reagents, and the treatment of sugars with alkali can cause extensive isomerization and even decomposition of the chain. Alkali exerts this effect, in part at least, by establishing an equilibrium between the monosaccharide and an enediol structure. 

Complexes of Cr111 with ascorbate, which are likely to form during the reactions of Crvl with this major intracellular reductant, have been studied (mainly by Cieslak-Golonka and co-workers),1062-1064 but no definitive structural information has been obtained to date. Partially characterized Crm complexes with monosaccharides or their oxidized derivatives (e.g., aldonic acid) have been isolated from the reactions with either Cr1111092 or CrVI.1093,1094... 

Oxidation of aldoses by chlorites also results in the formation of the corresponding aldonic acids. The aldopentoses react faster than aldo-hexoses, and monosaccharides are oxidized more rapidly than are the disaccharides. The ketoses remain unaffected unless drastic conditions are employed. The principal reaction has been shown to be as follows. 

Aldoses can be distinguished from ketoses by observing what happens to the color of an aqueous solution of bromine when it is added to the sugar. Br2 is a mild oxidizing agent and easily oxidizes the aldehyde group, but it cannot oxidize ketones or alcohols. Consequently, if a small amount of an aqueous solution of Br2 is added to an unknown monosaccharide, the reddish-brown color of Br2 will disappear if the monosaccharide is an aldose, but will persist if the monosaccharide is a ketose. The product of the oxidation reaction is an aldonic acid. 

These reactions may be accompanied by unwanted side-reactions, such as oxidation of hypobromite to bromate or its reduction to bromide. The electro-oxidation of 2,3 4,6-di-0-isopropylidene- -L-sorbofuranose (14) is affected by a number of factors, and here the method of mathematical planning of so-called extreme experiments for obtaining the optimal conditions for electrolysis was utilized the conditions are concentration of sodium bromide, 107.7 g per liter concentration of nickel chloride, 0.71 g per liter concentration of 2,3 4,6-di-0-isopropylidene-L-sorbose, 86 g per liter the amount of electric current passed, 1.912 A-hr/g of the diisopropylidene acetal current density, 4.56 A.dm pH of the solution, 9.83 the expected yield, 91.9 0.7%. A more-detailed survey of the mechanism and kinetics of the electrochemical oxidation of monosaccharides and their derivatives, as well as of the effect of experimental conditions on the yields of aldonic acids and of the di-O-isopropylidene-xylo-hexulosonic acid (15) formed, has been given. ... 

Even though monosaccharides exist largely as cyclic hemiacetals or hemiketals, rapid equilibrium, with trace quantities of carbonyl-containing compounds, means that discussion of the reactions of most sugars must consider both the acyclic and the cyclic structures. Common monosaccharides can be reduced to alkanepolyols (D-manitol, sorbitol) or oxidized to aldaric, aldonic, or uronic acids. 

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