Question

Can the iodine test distinguish between amylose and amylopectin? What Would you look for? Can sucrose act as a reducing sugar? Explain your answer. In the acid-catalyzed hydrolysis of starch, how can you tell when the hydrolysis of starch is complete? Why does the test work this way? What is the monosaccharide that results at the end? Polysaccharides generally do not give a positive test with Fehling's solution. Why?

Answer 1

1. Amylose gives blue colour with iodine which disappears on heating. Amylopectin gives a red brown colour with iodine.

2.Sucrose is a non reducing sugar.

Non reducing sugars do not contain free aldehydic or ketonic group, however, they contain acetal or ketal structures which are stable towards alkalies and hence not converted into free carbonyl group.

3. Starch is colourless amorphous powder and is insoluble in water. When boiled with dilute acid, it yields dextrin, maltose and ultimately glucose. Glucose is soluble in water. Thus when the insoluble residue dissolves, we can say that the hydrolysis is complete. The monosaccharide that results at the end is glucose.

4. Polysaccharides generally do not give a positive test with Fehling's solution as they do not contain free aldehdic or ketonic group and are non reducing in nature.