Question

Least energetic conformation of cyclohexane is:
(A) chair conformation
(B) boat conformation
(C) cis conformation
(D) E – Z form

Answer 1

Hint: Cyclohexane conformation is the three-dimensional shape adopted by the cyclohexane molecule in nature. The energetics and the stability of the conformations of cyclohexane is the most widely studied topic in organic chemistry.

Complete Solution :
Let us study the cyclohexane molecule and its conformers for better understanding of the topic and be able to answer the illustration given;
Cyclohexane rings tend to form certain non-planar conformations. They are chair conformer, half-chair conformer, boat conformer, twisted-boat conformer.
Chair conformation-
In this conformation, among all the six carbons, four of them are in the same plane. The angle between the carbon atoms here, is $109.5{}^\circ $ making it free of any bond strain. Thus, this conformer is most stable.

Half-chair conformation-
Here, the bonds are twisted creating some torsional strain. Hence, less stable.

Boat conformation-
Four carbon atoms are coplanar and the other two are displaced in the same direction having hydrogen atoms projected out of the plane. This is a less stable conformer due to torsional strain.

Twist-boat conformation-
These are more stable than the boat shaped conformer.

Thus, the stability trend will go as:
Chair conformer > Twist-boat conformer > Boat conformer > Half-chair conformer.
Now,
     \[Stability\propto \dfrac{1}{Energy}\]

Thus, chair conformers are the least energetic conformer.
So, the correct answer is “Option A”.

Note: Do note that option (C) and (D) can never be the answer. As, cis conformation is the conformation of a single bond that separates two double bonds. And, E – Z form is the naming form of alkenes.
- Also, note that the cyclohexane molecule has the ability to switch within the conformers as stated above except the chair and twist-boat conformations.