Q.25 - Organic Chemistry

Question

Name each compound and decide which stereoisomer will react faster in an E2 elimination reaction. Explain your choice.

Step-by-Step Solution

Answer

Cis-1-halo-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-halo-4-tert-butylcyclohexane.

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

 

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

Orientation of the tert-butyl group

The tert-butyl group is bulky, so it prefers to occupy the equatorial position so 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 halide group lies in the axial position, then the conformer is cis. 
• When tert-butyl lies in the equatorial position, and the halide group lies in the equatorial position, then the conformer is trans.

Antiperiplanar 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

Further removal of $\text{β}$ proton takes place to give the elimination product.

 

So based on the explanations, it is observed that cis-1-halo-4-tert-butylcyclohexane reacts faster because the $\text{β}$ hydrogen and the leaving group lies in an antiperiplanar orientation. Thus, the E2 reaction is faster for cis-1-halo-4-tert-butylcyclohexane.

 

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

Reaction scheme of both cis and trans form