Monosaccharide Derivatives: Osazones, Acetylation, Iodo, Interconversions, Oxidation, Study notes of Biochemistry

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Chemical reactions of

monosaccharides

Chemical reactions of monosaccharides

1. Iodocompounds: An aldose when heated with conc. HI loses all of its oxygen and is converted into an iodo- compound.  (^) Since the resulting derivative is a straight chain compound related to normal hexane, thereby suggesting the lack of any branched chains in structure of glucose.
2. Acetylation or ester formation: The ability to form sugar esters, e.g. acetylation with acetyl chloride (CH3– COCl) indicates the presence of alcohol groups. Due to alcoholic –OH groups, it can react with anhydrides and chlorides of many organic and inorganic acids, like acetic acid, phosphoric acid, sulphuric and benzoic acids to form esters of corresponding acids.
3. Osazone formation: It is a useful means of preparing crystalline derivatives of sugars. Osazones have characteristic
   • Melting points
   • Crystal structures, and
   • Precipitation time and thus are valuable in identification of sugars.

4. Interconversion of sugars : Glucose, fructose and

mannose are interconvertible in solutions of weak

alkalinity such as Ba(OH)2 or Ca(OH). These

interconversions are due to the fact that all give the

same Enediolform, which tautomerizes to all three

sugars. This interconversion of related sugars by

the action of dilute alkali is referred to as Lobry de

Bruyn-Van Ekenstein reaction.

5. Oxidation to produce sugar acids: When oxidised under different conditions, the aldoses may form:

• (^) Monobasic Aldonic acids
• (^) Dibasic Saccharic acids
• (^) Monobasic uronic acids containing aldehyde groups thus possessing reducing properties.

1. Aldonic acids: Oxidation of an aldose with Br2 water converts the aldehyde group to a – COOH group aldonic acid Br2 reacts with water to form hypobromous acid, HOBr, which acts as the oxidising agent.

3. Uronic acids: When an aldose is oxidised in such a way that the primary alcohol group is converted to – COOH group, without oxidation of aldehyde group, a uronic acid is formed. They exert reducing action due to presence of free –CHO group.

• (^) Examples

Biomedical importance of D-Glucuronic acid

• (^) In the body D-Glucuronic acid is formed from Glucose in liver by uronic acid pathway, an alternative pathway for glucose oxidation. It occurs as a constituent of certain mucopoly-saccharides. In addition, it is of importance in that it conjugates toxic substances, drugs, hormones and even bilirubin (a break down product of Hb) and converts them to a soluble nontoxic substance, a glucuronide, which is excreted in urine.

7. Action of acids on carbohydrates: Polysaccharides and the compound carbohydrates in general are hydrolyzed into their constituent monosaccharides by boiling with dilute mineral acids (0.5 to 1.0 N) such as HCl or H2SO4. With conc. mineral acids the monosaccharides are decomposed.

• (^) Examples
• (^) Molisch’s test: With α-naphthol (in alcoholic solution) gives red-violet ring. A sensitive reaction but non-specific, given by all sugars.
• (^) Seliwanoff’s test: With resorcinol, a cherry-red colour is produced. It is characteristic of D- fructose.
• (^) Other tests are Anthrone test, Bial-orcinol test, etc.

9. Reducing action of sugars in alkaline solution: All the sugars that contain free sugar group undergo enolisation and various other changes when placed in alkaline solution. The enediol forms of the sugars are highly reactive and are easily oxidised by O2 and other oxidising agents and forms sugar acids. As a consequence they readily reduce oxidising ions such as Ag+. Hg+, Bi+++, Cu++ (cupric) and Fe(CN)6.