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JEE Advanced levelSingle correct optionDifficulty 3 of 3o3153

The ring in 4-tert-butylcyclohexyl tosylate is locked in the chair in which the tert-butyl group is equatorial. The cis and trans isomers are each treated with in DMF (SN2). Which statement is correct?

  1. AThe cis isomer (axial OTs) reacts faster and gives trans-1-azido-4-tert-butylcyclohexane
  2. BThe trans isomer (axial OTs) reacts faster and gives cis-1-azido-4-tert-butylcyclohexane
  3. CThe trans isomer reacts faster and gives trans-1-azido-4-tert-butylcyclohexane
  4. DThe cis isomer (equatorial OTs) reacts faster and gives cis-1-azido-4-tert-butylcyclohexane

Answer: A

With tert-butyl equatorial, the cis-1,4 isomer must have OTs axial and the trans-1,4 isomer has OTs equatorial. The SN2 backside attack on the axial tosylate comes from the equatorial direction, which is less hindered, whereas attack on the equatorial tosylate must come along the axial direction, hindered by 1,3-diaxial hydrogens. Hence cis reacts faster. Inversion places the azide on the face opposite to the former axial OTs, i.e. equatorial, so both groups are equatorial in a 1,4 relationship: the trans isomer.
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