MolSynapsequestion bank
MolSynapse / Question bank / Organic Reaction Mechanisms III: Nucleophilic Substitution / SN2 mechanism, rate, stereochemistry, steric effects
JEE Advanced levelOne or more correct optionsDifficulty 3 of 3o3095

cis-4-tert-Butylcyclohexyl tosylate and trans-4-tert-butylcyclohexyl tosylate are each treated with in DMF, and substitution occurs by SN2 (the tert-butyl group is large enough to remain equatorial in all ring conformations that matter). Select the correct statements.

  1. AThe cis tosylate (OTs axial) is substituted faster than the trans tosylate (OTs equatorial).
  2. BThe cis tosylate gives trans-1-azido-4-tert-butylcyclohexane.
  3. CThe cis tosylate gives cis-1-azido-4-tert-butylcyclohexane, because the ring is rigid and substitution proceeds with retention.
  4. DThe trans tosylate is substituted faster because its equatorial leaving group is less crowded.

Answer: A, B

The bulky tert-butyl group stays equatorial, so in the cis isomer the OTs is axial and in the trans isomer it is equatorial. Backside attack on an axial leaving group comes from the equatorial direction, which is open, whereas attack on an equatorial leaving group has to come from the axial direction, which is blocked by the syn-axial hydrogens at C3 and C5. The cis tosylate is therefore faster. SN2 inverts the carbon, so the cis tosylate gives the trans azide (both groups equatorial).
Practice Organic Reaction Mechanisms III: Nucleophilic Substitution More on SN2 mechanism, rate, stereochemistry, steric effects
Previous questionNext question

More questions on SN2 mechanism, rate, stereochemistry, steric effects