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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3st165

cis-1,3,5-Trimethylcyclohexane is treated with CH3/CH3 1,3-diaxial interaction = 15.4 kJ/mol and ignoring all other interactions (each axial methyl has no other axial neighbours on its face besides the other two methyls). Select the correct statements.

  1. AThe all-equatorial chair is more stable than the all-axial chair
  2. BThe all-axial chair contains three CH3/CH3 1,3-diaxial interactions
  3. CThe all-axial chair is less stable than the all-equatorial chair by 46.2 kJ/mol under these assumptions
  4. DThe compound is chiral

Answer: A, B, C

In the cis-1,3,5 isomer all methyls are on the same face, giving (eq,eq,eq) or (ax,ax,ax). In the all-axial chair the pairs C1/C3, C3/C5 and C1/C5 are each syn-diaxial, giving 3 interactions = 3 x 15.4 = 46.2 kJ/mol relative to the strain-free all-equatorial chair. The molecule has mirror planes through each methyl carbon and is achiral (D false).
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