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 identi- fication of sugars.
Preparation: They are obtained by adding a mixture of phenylhydrazinehydrochloride and sodium acetate to the
sugar solution and heating in a boiling water bath for 30 to 45 minutes. The solution is allowed to cool slowly (not under tap) by itself. Crystals are formed. A coverslip preparation is made on a clean slide and seen under the microscope.
Basis of reaction: The reaction involves only the carbonyl carbon (i.e. aldehyde or ketone group) and the next adjacent carbon. Reactions that take place with an aldosugar is shown in Figure 3.8. First phenyl hydrazone is formed and then the hydrazone reacts with two additional molecules of phenylhydrazine to form the osazones. The reaction with a ketose is similar.
Types of Crystals (Fig. 3.9)
• Glucosazone crystals: These are fine, yellow needles in fan-shaped aggregates or sheaves or crosses, typically described as Bundle of Hay. Melting point = 204 to 205°C.
Note: Glucose, mannose and fructose due to simila- rities of structures form the same osazones. But since the structure of galactose differs on C-4, that part of the molecule unaffected in osazone formation, it would form a different osazone.
• Lactosazone crystals: These are irregular clusters of fine needles and look like a Powder puff.
• Maltosazone: These are star-shaped and compared to Sunflower petals.
4. Interconversion of sugars: Glucose, fructose and mannose are interconvertible in solutions of weak
SECTION TWO
alkalinity such as Ba(OH)2 or Ca(OH)2 (Fig. 3.10). These interconversions are due to the fact that all give the same Enediol form, 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 or • Dibasic Saccharic acids or
• 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.
Example
Fig. 3.9: Osazone crystals
Fig. 3.10: Interconversions of sugars in weak alkalinity- Lobry de Bruyn-Van Ekenstein reaction
2. Saccharic or aldaric acid: Oxidation of aldoses with conc. HNO3 under proper conditions converts both aldehyde and primary alcohol groups to –COOH groups, forming dibasic sugar acids, the saccharic or aldaric acids.
Examples
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.
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Fig. 3.12: Hydroxymethyl furfural formation
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.
6. Reduction of sugars to form sugar alcohols: The monosaccharides may be reduced to their corresponding alcohols by reducing agents such as Na-Amalgam. Similarly, ketoses may also be reduced to form keto- alcohol.
Examples
• D-Glucose yields D-Sorbitol. • D-Galactose yields D-Dulcitol. • D-Mannose yields D-Mannitol.
• Ketosugar D-Fructose yields D-Mannitol and D-Sorbitol.
Practical Application
In microbiology sugar alcohols have been used to identify type of bacteria. Different bacteria gives different pattern.
7. Action of acids on carbohydrates: Polysaccharides and the compound carbohydrates in general are hydroly- zed 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.
• Pentoses yield the cyclic aldehyde “furfural” (Fig. 3.11). Twelve percent (12%) HCl has been found most satisfactory for decomposition.
Practical Application
1. The reaction is used for the quantitative determination of pentoses and compound carbohydrates containing pentoses. Furfural can combine with phloroglucinol to form a relatively insoluble compound, furfural phloroglucide, which may be used in estimating the furfural formed in the reaction as a measure of the pentose present.
2. Hexoses are decomposed by hot strong mineral acids to give hydroxymethyl furfural, which decomposes further to produce laevulinic acid, formic acid, CO and CO2 (Fig. 3.12). The furfural products thus formed by decomposition with strong mineral acid can condense with certain organic phenols to form compounds having characteristic colours. Thus it forms basis for certain tests used for detection of sugars.
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.
8. Action with alkalies: With alkalies, monosaccharides react in various ways:
(a) In dilute alkali: The sugar will change to the cyclic α and β forms with an equilibrium between the two isomeric form (See mutarotation).
• On standing: A rearrangement will occur which produce an equilibrated mixture of glucose, fructose and mannose through the common “enediol” form (see interconversion).
• If it is heated to 37°C, the acidity increases, and a series of Enols are formed in which double bond shifts from the oxygen-carbon atoms (Fig. 3.13). (b) In conc. alkali: The sugar caramelises and produces a series of decomposition products, yellow and brown pigments develop, salts may form, many double bonds
SECTION TWO
between C-atoms are formed, and C bond C bonds may rupture.
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– – –.
Practical Application
This reducing action of sugars in alkaline solution is utilised for both qualitative and quantitative determinations of sugars. Reagents containing Cu++ (ic) ions are most commonly used.
These are generally alkaline solution of cupric sulphate containing
• Sodium potassium tartarate (Rochelle salt) and strong alkali NaOH/KOH as in Fehling’s solution (not used now).
• Sodium citrate and weak alkali sodium carbonate as in Benedict’s Qualitative reagent.
Functions of Ingredients
• Sodium citrate/Rochelle salt in the reagents prevent precipitation of cupric hydroxide or cupric carbonate by forming soluble, slightly dissociable complexes
which dissociate sufficiently to provide supply of readily available Cu++ (cupric) ions for oxidation. • The alkali of the reagents enolises the sugars and
thereby causes them to be strong reducing agents. Enolization is better in weak alkali than strong alkali.
Reaction: When a solution of reducing sugar is heated with one of the alkaline copper reagents, the following reactions occur (given in box below).
The Cu++ (ic) ions take electrons from the enediols and oxidize them to sugar acids, and are, in turn reduced to Cu+ (ous) ions. The Cu+ (ous) ions combine with –OH– ions to form
yellow cuprous hydroxide, which upon heating is converted to red cuprous oxide. The appearance of a yellow to red precipitate indicates reduction and the quantity of sugar present can be roughly estimated from colour and amount of precipitate.