Question

Statement I: Staggered conformation of ethane is more stable than the eclipsed conformation.
Statement II: The torsional strain in staggered conformation is more.
Read the above statements and choose the correct answer from the options given below.

• A. Both Statement I and Statement II are false
• B. Both Statement I and Statement II are true
• C. Statement I is true but Statement II is false
• D. Statement I is false but Statement II is true

Hint:

Remember the energy profile for ethane's rotation. The energy is lowest (most stable) at the staggered conformations (60\(^\circ\), 180\(^\circ\), 300\(^\circ\)) and highest (least stable) at the eclipsed conformations (0\(^\circ\), 120\(^\circ\), 240\(^\circ\)). The energy difference is due to torsional strain.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Conformational isomerism in alkanes involves analyzing the relative stability of different spatial arrangements based on repulsive forces between bonds.
Step 2: Key Formula or Approach:
Define torsional strain (repulsion between electron clouds of eclipsing bonds) and relate it to the dihedral angle and stability of Newman projections.
Step 3: Detailed Explanation:
1. Statement I: In the staggered conformation of ethane, the $C-H$ bonds on the two adjacent carbons are as far apart as possible (dihedral angle = $60^{\circ}$). This minimizes electron-electron repulsion. In the eclipsed conformation, they are closest together (dihedral angle = $0^{\circ}$), maximizing repulsion. Thus, the staggered conformation possesses lower potential energy and is more stable. (Statement I is True)

2. Statement II: Torsional strain refers to the repulsion between the bonding electrons of adjacent atoms. Since the staggered form has maximum distance between these bonds, it has the minimum torsional strain. Conversely, the eclipsed form has maximum torsional strain. (Statement II is False)
Step 4: Final Answer:
Statement I is correct, but Statement II is incorrect.