cis-1-Bromo-4-tert-butylcyclohexane and trans-1-bromo-4-tert-butylcyclohexane both react with sodium ethoxide in ethanol to form 4-tert-butylcyclohexene. Explain why the cis isomer reacts much more rapidly than the trans isomer.

Elimination reaction often competes with substitution reactions. In this reaction, a substrate (typically an alkyl halide) eliminates one equivalent (unit) of acid to form an alkene. Two possible mechanisms are available for this elimination reaction –E1 and E2 mechanisms.

In an E2 reaction, both groups to be eliminated must be in axial position.

When the bromine is in the axial position in the cis isomer, the tert-butyl substituent is in the more stable equatorial position.

When the bromine is in the axial position in the trans isomer, the tert-butyl substituent is in the less stable equatorial position.

Thus, elimination takes place via the most stable conformer in the cis isomer via the less stable chair conformer in the trans isomer, so the cis isomer undergoes elimination more rapidly.