Question

The dihedral angle of the least stable conformer of ethane is

• A. 0°
• B. 120°
• C. 180°
• D. 60°

Answer 1

The Correct Option is D

To determine the dihedral angle of the least stable conformer of ethane, we need to understand conformational analysis.

Ethane is an alkane where each carbon is bonded to three hydrogen atoms, and the carbon-carbon single bond allows for rotation of one CH₃ group relative to the other. As this rotation happens, the spatial arrangement of the hydrogen atoms (conformers) changes, influencing the potential energy of the molecule. 

The dihedral angle is defined as the angle between the planes formed by the atoms in these different conformations:

• Staggered conformation: Here, the dihedral angle is 60°, where the hydrogen atoms on adjacent carbons are as far apart as possible. This conformation is the most stable due to minimal electron repulsion.
• Eclipsed conformation: In this arrangement, the dihedral angle is 0°, where the hydrogens on adjacent carbons overlap. It is the least stable conformation as the electron clouds of the hydrogen atoms repel each other, increasing the energy of the system.

The options given are:

1. 0°
2. 120°
3. 180°
4. 60°

Since the least stable conformer of ethane is the eclipsed conformation, corresponding to a dihedral angle of 0°, the confusion might arise from interpreting the question. The fact to note is:

The question asks for the least stable conformer, which refers directly to the eclipsed form. Hence, the dihedral angle for the least stable conformer is indeed \(0^\circ\), making the correct understanding that 0° is the least stable, whereas 60° is for the staggered stable version.