Q.46 - Organic Chemistry

Question

Does cis- or trans-1-bromo-4-tert-butylcylohexane react faster in an E2 reaction?

Step-by-Step Solution

Answer

Cis-1-bromo-4-tert-butylcyclohexane reacts faster than trans-1-bromo-4-tert-butylcyclohexane in an E2 reaction.

Orientation of tert-butyl group

The tert-butyl group is bulky, so it prefers to occupy the equatorial position, and it does not suffer from syn-diaxial interaction. Hence it is kept in the equatorial position for both structures.

• When tert-butyl lies in the equatorial position, and the bromide group lies in the axial position, then the conformer is cis. 
• When tert-butyl lies in the equatorial position, and the bromide group lies in the equatorial position, the conformer is trans.

Anti-periplanar orientation

The $\text{β}$ hydrogen must lie in a trans orientation to that of the leaving group (antiperiplanar orientation). In the case of the cis halide, antiperiplanar orientation is maintained, which is not observed for the trans. 

Removal of β hydrogen

It is observed that cis-1-bromo-4-tert-butylcyclohexane reacts faster because the $\text{β}$ hydrogen and the leaving group lies in an antiperiplanar orientation which is not possible in the case of trans-1-bromo-4-tert-butylcyclohexane.

Hence, the E2 reaction is faster for cis-1-bromo-4-tert-butylcyclohexane.

 

The mechanism is shown in the following for better observation of the explanations:

Representation of the elimination reaction scheme for both cis and trans form