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Considering penta-1,4-diene, if allylic substitution is done with NBS, which product will be major, 3-bromo or 1-bromo? In the first case I get stable free radical , in the second case I get conjugated diene. Should I consider stability of free radical first as it is formed first , or the stability of the product which is formed in the end?

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  • $\begingroup$ Continuous conjugation is better than cross conjugation. Therefore, the radical in which continuous conjugation occurs should form the major product. 1 bromo product should be the major. $\endgroup$ – Arishta Jan 6 '18 at 13:27
  • $\begingroup$ Does this mean that here the terminal free radical is more stable due to continuous conjugation than the free radical with cross conjugation? $\endgroup$ – user161158 Jan 6 '18 at 14:03
  • $\begingroup$ Or is it that the end product is more stable? $\endgroup$ – user161158 Jan 6 '18 at 14:03
  • $\begingroup$ What should be seen first? stability of free radical or stability of the end product. $\endgroup$ – user161158 Jan 6 '18 at 14:03
  • $\begingroup$ Is your question, does 3-bromination occur faster than the double bonds move into conjugation to give the radical in the 1- position? $\endgroup$ – Beerhunter Jan 6 '18 at 15:49
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The Main reason for looking at the stability of the intermediate is Hammond's Postulate. Now the intermediate has the electron density spread all over it's body. SO it's reasonable to look at the activation energy of the transition state itself.

When the Br radical attacks at the end position, you have a much sterically favored transition state and the other two bonds are in conjugation and hence we would expect the activation energy would be much lower for this and not for the other transition state where the Br attacks at $3^{rd}$ position.

Hence we can conclude that conjugated diene is most probable product !.

Please correct me if I am wrong.

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