Carbonyl groups of monosaccharides

Oxidation and reduction reactions occur at the carbonyl group of monosaccharides, so they all begin with the monosaccharide drawn in the acyclic form. We will confine our discussion to aldoses as starting materials. 

The stmcture of monosaccharides is often written in the acycHc form although only very minor amounts of it ever occur in that form. Because the interconversions are rapid, the carbonyl groups of sugars can and do react both as if they are free and as if they are in a hemiacetal ring form. 

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. 

The presence of alcohol and, in some cases, an aldehyde group makes monosaccharides susceptible to oxidation, whereas the presence of a carbonyl group makes monosaccharides susceptible to reduction. Because monosaccharides are the fundamental carbohydrate, you need to know what happens in the many reactions in which they re involved. The following sections are here to help you out with that. Welcome to the nitty-gritty of monosaccharide oxidation and reduction ... 

Even though the acyclic form of a monosaccharide may be present in only trace amounts, the equilibrium can be tipped in its favor by Le ChStelier s principle (Section 9.8). Suppose, for example, that the carbonyl group of the acyclic form reacts with a reagent, thus depleting its equilibrium concentration. The equilibrium will then shift to compensate for the loss, thus producing more of the acyclic form, which can react further. 

The chemistry of carba sugars is quite similar to that of cyclitols both groups lack the latent carbonyl groups of their saccharide counterparts, and therefore fail to exhibit many of the characteristic properties of monosaccharides. Thus carba sugars and cyclitols do not form hydrazones or osazones, nor do they mutarotate, or reduce heavy-metal salts in base, in contrast to their oxidation products, the inososes. 

An aldehyde sugar forms an intramolecular hemiacetal when the carbonyl group of the monosaccharide reacts with a hydroxyl group on one of the other carbon atoms. 

Problem 18.19. The carbonyl group of a monosaccharide can be reduced to an alcohol group by various reducing agents, e.g., H2 in the presence of a metal catalyst or NaBH4 (Sec. 14.6). Show the product(s) of the reduction of D-fructose by NaBH4. 

Problem 18.20. The yields in reactions of the carbonyl group in monosaccharides (e.g., the reactions described in Problems 18.18 and 18.19) are very high even though the amount of the open-chain carbonyl structure is extremely low (< 0.2 percent). How can this occur ... 

It is also an aldose. Like many monosaccharides, the carbonyl group of glucose may react with one of its hydroxyl groups. This creates an equilibrium between open chain and ring forms. These two forms of glucose are shown below ... 

Monosaccharides have the general formula C H2 0 , with one of the carbons being the carbonyl group of either an aldehyde or a ketone. The most common monosaccharides have from three to nine carbon atoms. The suffix indicates that a molecule is a carbohydrate, and the prefixes trir, tetr-, pent-, and so forth, indicate the number of carbon atoms in the chain. Monosaccharides containing an aldehyde group are classified as aldoses those containing a ketone group are classified as ketoses. 

An alditol is a polyhydroxy compound formed by reduction of the carbonyl group of a monosaccharide to a hydroxyl group. 

We learned earlier that alcohols undergo rapid and reversible addition to the carbonyl group of aldehydes and ketones, to form hemiacetals (review Sec. 9.7). This can happen intramolecularly when the hydroxyl and carbonyl groups are properly located in the same molecule (eqs. 9.14 and 9.15), which is the situation in many monosaccharides. Monosaccharides exist mainly in cyclic, hemiacetal forms and not in the acyclic aldo- or keto-forms we have depicted so far. 

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