Which of the following is the most stable chair conformation of bromocyclohexane? Br H IV V. Br Br Br H OA II B: III C D. IV OE V
The Correct Answer and Explanation is:
The most stable chair conformation of bromocyclohexane is:
Answer: C. III
Explanation
Cyclohexane is a six-membered ring that commonly adopts a chair conformation to minimize angle and torsional strain. In this conformation, each carbon atom has one axial (straight up/down) and one equatorial (angled outward) substituent. The equatorial position is generally more stable due to less steric hindrance.
When a substituent such as bromine (Br) is attached to cyclohexane, its size and steric interactions play a key role in determining the most stable conformation. Bromine is a relatively large atom, so it prefers the equatorial position to minimize 1,3-diaxial interactions with axial hydrogens on the same side of the ring. These interactions, if bromine were axial, would create steric hindrance and raise the energy of the conformation.
In the case of bromocyclohexane, two chair conformations are possible, as the ring can flip, interchanging the axial and equatorial positions of all substituents. In one conformation, Br is axial and H is equatorial; in the other, Br is equatorial and H is axial.
Since the bromine atom is much bulkier than hydrogen, the chair conformation where Br is equatorial and H is axial is significantly more stable. This conformation avoids the unfavorable 1,3-diaxial interactions that would occur if Br were axial.
Thus, among the given options (I–V), the structure labeled III, where bromine occupies the equatorial position, represents the most stable chair conformation of bromocyclohexane.
In summary, bulky groups prefer the equatorial position in the chair conformation to reduce steric strain, and for monosubstituted cyclohexanes like bromocyclohexane, the most stable form is the one where the bulky substituent (Br) is equatorial. Therefore, the correct answer is:
👉 C. III
