![]() In contrast, acetal forms (glycosides) are not reducing sugars, since with base present, the acetal linkage is stable and is not converted to the aldehyde or hemiacetal. A hemiacetal form is thus a reducing sugar. This means that the cyclic hemiacetal form of a sugar will produce an equilibrium amount of the open-chain aldehyde form, which will then reduce the copper(II) to copper (I) and give a positive test. These reagents are used in basic solution, so that hemiacetals and aldehydes are in equilibrium. Sugars which are oxidized by these reagents are called reducing sugars because they reduce the copper(II) to copper(I). The oxidizing agents used in carbohydrate chemistry are typically copper(II) compounds which are reduced to copper(I) oxide. ![]() Aldehydes are fairly easy to oxidize to carboxylic acids, while acetals (which have no carbonyl group) are quite difficult to oxidize. There is another important difference between the hemiacetal and acetal linkages in sugars and saccharides, and that is their reaction with mild oxidizing agents. Often this improves the water solubility of the alcohol and makes it easier to excrete. Besides the di- and polysaccharides we will look at later, it is very common for glucose (or other sugars) and an alcohol to form an acetal linkage. Conditions can be arranged to produce either the alpha or beta stereochemistry in the glycoside. The conversion between an aldehyde and a hemiacetal is catalyzed either by base or by acid. Keep in mind that the conversion between a hemiacetal and an acetal requires an acid catalyst. The acetal products are called "glycosides." If the sugar used is glucose, they are "glucosides." There are several reasonable mechanisms for these conversions and we will not look at them in detail. If we apply this feature of the scheme to a solution of glucose in methanol (with a trace of acid catalyst included), we get: Let's begin by remembering the reaction sequence which links aldehydes and alcohols, hemiacetals, and acetals.įor our purposes, the key feature is the conversion of a hemiacetal and an alcohol to an acetal, with the concurrent release of a molecule of water. We'll find that these acetal linkages are what holds di- and polysaccharides together. In particular, we'll recall how hemiacetals are converted to acetals. ![]() Today we'll look in more detail at the chemistry of that hemiacetal linkage. We saw that the major stereochemical features of aldohexoses and aldopentoses are usefully described by Fischer projection formulas, but we learned that the structures of these compounds must also be understood as cyclic hemiacetals. Last time we explored the structural characteristics of monosaccharides.
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