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Allylic halogenation of butene happens in the following way:

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The RDS of this radical chain mechanism is allylic hydrogen abstraction ($\ce{H_a}$) from the allylic position to give a 1° allylic radical$^1$. This radical is stabilized by resonance to give a 2° allylic radical (which is more stable). Reaction of the radical at carbon 1 gives the major product. Reaction at carbon 3 gives the minor product. The more substituted (and more stable) alkene isomer predominates. The reason for this regioselectivity seems to be that the resonance contributor of the allylic radical with the more substituted double bond dominates.

Now, my question is that if the RDS is the formation of allylic radical then why the formation of major product is decided exclusively by the stability of final product? Can we generalize the statement that the major product will always decided by the stability of final product?

Reference

1: Organic Chemistry- Brown, Iverson, Anslyn, Foote (8E); Page 352

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  • $\begingroup$ It actually depends on the conditions. The major product that the reference gives is thermodynamically favored whereas the other one is kinetically favored $\endgroup$ – Robin Singh Sep 18 '20 at 13:38

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