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 ?
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.