Q 11-11-19 P - Organic Chemistry (Mcmurry)

Question

Which isomer would you expect to undergo E2 elimination faster, trans-1-bromo-4-tert-butylcyclohexane or cis-1-bromo-4-tert-butylcyclohexane? Draw each molecule in its more stable chair conformation, and explain your answer.

Step-by-Step Solution

Answer

The cis isomer of 1-bromo-4-tert-butylcyclohexane reacts faster under E2 elimination..

Chair conformation

Chair conformation is the other possible cyclohexane structure in which the carbons at 2, 3, 5, and 6 lie in the same plane. The carbon at the 1st position lies above the plane, and the carbon at the 4^th position lies below the plane.

The Newman projection of the reacting conformation

The tert-butyl group is always placed in the equatorial position in the more stable conformation. The cis isomer of 1-bromo-4-tert-butylcyclohexane reacts faster under E2 elimination because the arrangement of –H and –Br are anti periplanar, and it favors E2 elimination.

More stable conformations of cis and trans isomers of 1-bromo-4-tert-butylcyclohexane.