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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3o4108

(E)-But-2-ene and (Z)-but-2-ene are each treated with (anti addition), and each dibromide is then treated with one equivalent of strong base for an anti E2 step giving 2-bromobut-2-ene. Select all correct statements.

  1. AThe dibromide from (E)-but-2-ene is meso-2,3-dibromobutane.
  2. BThe dibromide from (E)-but-2-ene gives (E)-2-bromobut-2-ene.
  3. CThe dibromide from (Z)-but-2-ene gives (E)-2-bromobut-2-ene.
  4. DBoth dibromides could, in principle, give the same alkyne, but-2-yne, by a second dehydrobromination (ignoring isomerisation).

Answer: A, B, D

Anti addition of bromine to the E alkene gives the meso dibromide, and to the Z alkene gives the racemic dibromide. In the meso isomer the anti H/Br conformation puts the two methyl groups cis, giving (E)-2-bromobut-2-ene, while the racemic isomer puts them trans, giving the Z alkene, so the third statement is false. Both dibromides have the same constitution, so a second elimination can give but-2-yne.
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