Radical addition of HBr to conjugated dienes

Radical addition of HBr to an alkene depends upon the bromine atom adding in the first step so that the more stable radical is formed. If we extend this principle to a conjugated diene, e.g. buta-1,3-diene, we can see that the preferred secondary radical will be produced if halogenation occurs on the terminal carbon atom. However, this new radical is also an allylic radical, and an alternative resonance form may be written. 

A hydrogen atom is abstracted from HBr in the following step of the chain reaction to produce the addition product. Depending upon which resonance 

This is comparabie to the eiectrophiiic addition of HBr to butadiene (see Section 8.2), though the addition is in the reverse sense overaii, in that Br adds before H in the radicai reaction, whereas H adds before Br in the ionic mechanism. As with the electrophilic addition, we shall nsnally obtain a mixtnre of the two prodncts. 

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