Monday, January 28, 2008

IIT JEE Revision - Reactions of alkenes with KMnO4

Oxidation of Alkenes with potassium permanganate

Alkenes react with cold dilute potassium permanganate solution(alkaline) to form 1,2-diols called glycols. The glycols contain two -OH groups on adjacent carbon atoms.

Ethene gives ethanediol (glycol).
Propene gives propane-1,2 diol (propylene glycol)

The alkaline potassium permanganate (known as Baeyer's reagent) has bright pink colour. Glycols have no colour and the pink colour will disappear after reaction. So Baeyer's test is used to find the presence of double bond.

Reaction with hot potassium permanganate: In this case, the alkene gets split up at the double bond forming acids or ketones.

=C(R-R') carbon attached to two alkyl groups gets oxidized to ketone
=C(R-H) carbon attached to one alkyl group and one hydrogen gets oxidized to carboxylic acid
=C(H-H) carbon attached to two hydrogen atoms gets oxidized to CO2

1 comment:

Gautham said...

A little more clarity would be better, even though its for revision, 1 or 2 examples might be illustrated.