Suppose we have a alkene( ex- 2- butene) we try to subject it to allylic bromination using NBS/CCl4 and then the mixture is separated using fractional distillation. According to me as the reaction proceeds with a free radical pathway we must get three products namely cis , trans 1-bromobut-2-ene and the rearranged product but the answer is given as that the fractions obtained will be only 2 . Where am I going wrong ?

  • $\begingroup$ Is your but-2-ene starting material an E/Z mixture or single isomer? $\endgroup$ – Waylander Jan 6 '19 at 10:59
  • $\begingroup$ Not specified and that is the main problem $\endgroup$ – Aditya Garg Jan 6 '19 at 11:03

You are quite correct that this is a radical bromination, know as the Wohl–Ziegler reaction. The mechanism is here Wohl–Ziegler. The key point is that the radical intermediate at the allylic position is stabilised by resonance with the double bond giving a 3-centred intermediate. This can brominate at either end (favouring the more substituted), but it does not isomerise the double bond. So I think the question setters are refering to 3-bromobut-1-ene and 1-bromobut-2-ene as the possible products without concerning themselves with the E/Z isomers.

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    $\begingroup$ The OP suggested (E)-"ex"-2-butene as a substrate (bp 0.8 oC). Given that the W-Z NBS reaction is normally done in refluxing CCl4 (bp 77 oC) to enhance the solubility of NBS. There may be difficulty in containing the substrate in the reaction vessel. Dry ice/actone condenser??? $\endgroup$ – user55119 Jan 6 '19 at 23:30
  • $\begingroup$ A poorly chosen example substrate, perhaps. $\endgroup$ – Waylander Jan 7 '19 at 8:49

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