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JEE Advanced levelSingle correct optionDifficulty 3 of 3o4068

Two stereoisomeric 2-isopropyl-5-methylcyclohexyl chlorides (Cl on C-1, isopropyl on C-2, methyl on C-5) undergo E2 with /ethanol. In the more stable chair of chloride P all three substituents are equatorial. In the more stable chair of chloride Q the isopropyl and methyl groups are equatorial and Cl is axial. Which statement is correct?

  1. AQ reacts faster than P but gives only the disubstituted alkene, because the hydrogen on C-2 is equatorial in Q
  2. BQ reacts faster than P and gives mainly the trisubstituted alkene (double bond C-1=C-2); P gives only the disubstituted alkene (double bond C-1=C-6) because only one axial hydrogen is anti to Cl in its reactive chair
  3. CP and Q give the same alkene mixture at the same rate, since the Saytzeff rule always governs
  4. DP reacts faster than Q because all-equatorial is more stable, and P gives mainly the trisubstituted alkene

Answer: B

E2 needs an axial leaving group and an axial anti hydrogen. In Q, Cl is already axial and H on C-2 and C-6 are both axial, so Saytzeff elimination toward the isopropyl-bearing carbon is possible and fast. In P, Cl must first flip to axial, which also flips isopropyl to axial; then C-2 carries no axial hydrogen and only the C-6 hydrogen is anti, giving the disubstituted alkene slowly.
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