91Ó°ÊÓ

When a non-polar covalent bond cleaves, each atom receives one of the bonded electrons and becomes a free radical.

Free radicals have an unpaired electron and therefore are highly unstable.

This makes the radicals highly reactive and an intermediate in many organic reactions.

Free radicals are stabilized by resonance.

The presence of a radical in the allylic position of an unsaturation causes the homolytic cleavage of a double bond. This is continued to form resonance structures.

The stability of the radical depends on the number of resonance structures possible for the radical.

a. The radical is allylic to a conjugated diene. Therefore, the compound will have three resonance structures.

Resonance in a

b. The radical carbon is allylic to a conjugated triene on one side and a double bond on the other side. Therefore, are two possibilities of resonance.

Resonance possibility 1 in b

Resonance possibility 2 in b

c. The radical is allylic to a double bond. Therefore, two resonance structures are possible.

Resonance in c