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JEE Advanced levelSingle correct optionDifficulty 3 of 3o4084

2-Bromo-3,3-dimethylbutane is (i) treated with /ethanol (E2 conditions), and (ii) warmed in aqueous ethanol without base (E1/SN1 conditions). The principal alkene formed is, respectively,

  1. A(i) 3,3-dimethylbut-1-ene; (ii) 2,3-dimethylbut-2-ene
  2. B(i) 2,3-dimethylbut-2-ene; (ii) 3,3-dimethylbut-1-ene
  3. C(i) 2,3-dimethylbut-2-ene; (ii) 2,3-dimethylbut-2-ene
  4. D(i) 3,3-dimethylbut-1-ene; (ii) 3,3-dimethylbut-1-ene

Answer: A

In E2 no carbocation forms and the only carbon next to C-2 that has hydrogens is the methyl group C-1 (C-3 is quaternary), so 3,3-dimethylbut-1-ene results without rearrangement. Under E1/SN1 conditions the secondary cation undergoes a 1,2-methyl shift to the tertiary 2,3-dimethylbut-2-yl cation, and loss of a proton gives the tetrasubstituted 2,3-dimethylbut-2-ene as the major alkene.
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