Which of the following conformations of butane will be the most stable?
Choices:
A. A gauche conformation
B. An eclipsed conformation
C. The anti conformation
D. None of the above
The Correct Answer and Explanation is:
The correct answer is: The anti conformation
Explanation:
Butane (C₄H₁₀) is a straight-chain alkane composed of four carbon atoms. When examining the conformations of butane, we focus primarily on the rotation about the central carbon-carbon single bond (C2–C3). The spatial arrangement of the atoms around this bond leads to different conformations, which vary in their stability due to steric hindrance and torsional strain.
There are three primary types of conformations for butane:
- Anti conformation:
- In this staggered conformation, the two methyl groups (–CH₃) are positioned 180° apart.
- This is the most stable conformation of butane.
- It has the lowest energy because it minimizes both torsional strain and steric hindrance, as the bulky methyl groups are as far apart as possible.
- Gauche conformation:
- Also a staggered conformation, but the methyl groups are 60° apart.
- It is less stable than the anti conformation because of steric strain resulting from the proximity of the two bulky methyl groups.
- Gauche interactions slightly raise the energy of the molecule compared to anti.
- Eclipsed conformation:
- In this conformation, atoms on adjacent carbons are directly aligned, resulting in the highest torsional strain.
- It has the highest energy and is therefore the least stable.
- Specifically, when the two methyl groups eclipse each other, the steric hindrance is greatest.
Therefore, among the given options, the anti conformation is the most stable due to its optimal spatial arrangement that minimizes repulsion between atoms.
Summary:
- Anti conformation: Most stable (lowest energy)
- Gauche conformation: Less stable than anti
- Eclipsed conformation: Least stable (highest energy)