Question

Explain the reaction between iodine solution and polysaccharides by giving the structures of related compounds.

Answer 1

It is a charge-transfer complex reaction.

Here there is no chemical reaction between these two reactants.

Prepared by mixing I₂ with KI solution.

And I₂ could not react directly as such but form poly-iodate ions viz. I₅^-or I₇^-.

The negatively charged iodide in these compounds acts as charge donor, the neutral iodine as a charge acceptor.

A solution of iodine (I₂) and potassium iodide (KI) in water has a light orange-brown color.

Once amylose is added, it forms another CT complex.

The polysaccharide acts as a charge donor and the polyiodide as an acceptor.

This new complex absorbs light of a different wavelength than polyiodide, and the color turns dark blue.

The exact structure of the polyiodides inside the amyloid helix is not clear.

These ions are bulky and sit inside the polysaccharide of glucose units and could not get out of these coiled spring of polysaccharide especially amyloses.