Properties of Glucose - Practice Questions & MCQ

1. Glucose, on complete reduction with HI and red phosphorus, finally n-hexane. This indicates that it contains a straight chain of six carbon atoms.
2. It reacts with acetic anhydride and forms penta-acetate derivate. This shows the presence of five hydroxyl groups each linked to a separate carbon atom as the molecule is stable.
3. Glucose combines with one mole of HCN to form a cyanohydrin. These reactions indicate the presence of a carbonyl group, C=O, in the glucose molecule.
4. Mild oxidation of glucose with bromine water gives gluconic acid. Further, glucose also reduces Tollen's reagent and Fehling's solution. These reactions show the presence of an aldehyde group.

1. Glucose does not react with sodium bisulphite. It confirms the absence of free -CHO group.
2. Glucose does not give Schiff's test and DNP test. It confirms the absence of free -CHO group.
3. Glucose pentaacetate does not react with hydroxylamine. It means absence of free -CHO group.

1. Glucoside formation
   Glucose reacts with methanol in the presence of HCl and gives α and β glucoside. Glucoside formation is due to the reaction of alcohol with glucoside -OH group of glucose. β,D glucose forms β,D-methyl glucoside.

2. Reduction
   Monosaccharides can be reduced by various reducing agents such as sodium-amalgam or by hydrogen under high pressure in the presence of catalysts.
   
3. Reaction with nitric acid
   When glucose is oxidised with nitric acid, saccharic acid is formed. Saccharic acid is also known as glucaric acid.
   
4. Ester formation
   They can form esters with carboxylic acids due to the presence of OH groups. For eg. glucose reacts with five molecules of acetic anhydride to form pentaacetate derivative. It indicates that the glucose contains five OH groups.
   

Disaccharides
The disaccharides consist of two molecules of monosaccharides. When hydrolysed with enzymes or dilute acids, they give two molecules of either same or varying monosaccharides. Some examples include,

   Sucrose                   Glucose             Fructose

   Lactose                    Glucose            Galactose

On the basis of the position of linkages between the two monosaccharide units, the disaccharides might be reducing or non-reducing in nature. The resultant disaccharide is non-reducing if the glycosidic linkage involves the carbonyl functions of both monosaccharide units. On the other hand, the resulting disaccharide is the reducing sugar, e.g., maltose and lactose, if one of the carbonyl functions in either of the monosaccharide units is free.

Polysaccharides
Polysaccharides are the carbohydrates having hundreds or even thousands of monosaccharide units joined together by glycosidic linkages, e.g., starch, cellulose, glycogen and dextrins. However, starch and cellulose are the most important polysaccharides.