Water elimination reaction, from carbohydrate

Figure 39 illustrates that the aldehyde group at C-1 and the hydroxyl group at C-5 come rather close together. This proximity facilitates the reaction leading to hemiacetal formation. Aldehydes in general can add hydroxyl compounds to the C=0 bond (cf. Chapt. 1-2). If H—0—H is added, the product is the hydrate of the aldehyde if an alcohol is added the hemiacetal is formed. Full acetals (or simply acetals ) arise from hemiacetals plus alcohols by elimination of water. This reaction is the basis for glycoside formation by carbohydrates (see formulas below). 

Water is not just the solvent in which the chemical reactions of living cells occur it is very often a direct participant in those reactions. The formation of ATP from ADP and inorganic phosphate is an example of a condensation reaction in which the elements of water are eliminated (Fig. 2-22a). The reverse of this reaction— cleavage accompanied by the addition of the elements of water—is a hydrolysis reaction. Hydrolysis reactions are also responsible for the enzymatic depolymerization of proteins, carbohydrates, and nucleic acids. Hydrolysis reactions, catalyzed by enzymes called... 

More complex carbohydrates are formed by combining monosaccharides. For example, sucrose, common table sugar, is a disaccharide formed from glucose and fructose by elimination of water to form a C—O—C bond between the rings, which is called a glycoside linkage (Fig. 22.32). When sucrose is consumed in food, the above reaction is reversed. An enzyme in saliva catalyzes the breakdown of this disaccharide. 

Many carbohydrates have the structural element -CHOH-CHOH-. Radicals such as 1 - 4, just like 18, generated by H-abstraction from such sites can eliminate water [28]. This reaction occurs at a relatively slow rate spontaneously ( water-catalyzed ), but faster upon acid or base catalysis [reactions (l)-(3)]. 

Furan is possibly a carcinogenic substance. It occurs in heated food, especially in roasted coffee. Isotopic dilution analyses with [ H4]-furan as the internal standard yielded 2.5-4.3 mg/kg furan in differently produced coffee powders. Baby food, e.g., carrot mash and potato/spinach mash contained 74 and 75 pg/kg respectively. Furan is formed from amino acids which yield acetaldehyde and glycolaldehyde on thermal degradation (Fig. 9.4). Aldol condensation, cyclization and elimination of water are the reaction steps. Other precursors of furan are carbohydrates, polyunsaturated fatty acids and carotinoids (Fig. 9.4). Furan can also be formed from the thermolysis of ascorbic acid. 

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