Question

How can the iodine test be used to distinguish between starch and fructose?

Why must the Seliwanoff's test be time controlled?

Answer 1

Starch is polysaccharide, whereas fructose is monosaccharide.

Starch is made up of amylose and amylopectin, amylose is water soluble whereas amylopectin is water insoluble

Iodine can form inclusion compound with amylose which is blue colour, whereas fructose being monosachride does not form inclusion compound hence it is colourless with iodine

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Selwinoff's test Shows positive test for: Ketoses

Reactions:

The test reagent dehydrates ketohexoses to form 5-hydroxymethylfurfural. 5-hydroxymethylfurfural further reacts with resorcinol present in the test reagent to produce a red product within two minutes (reaction not shown).

Aldohexoses react to form the same product, but do so more slowly.

Aldohexose undergo tautomerisation to ketone in acidic solution slowly so they show positive selwinoff test over longer period

half ml of a sample solution is placed in a test tube. Two ml of Seliwanoff's reagent (a solution of resorcinol and HCl) is added. The solution is then heated in a boiling water bath for two minutes.

A positive test is indicated by:The formation of a red product.