Question

Torsional strain in eclipsed conformations of a molecule is due to:
A. repulsion between the eclipsed carbons
B. repulsion between aligned electron pairs of the eclipsed bond
C. repulsions between aligned H
D. repulsions between C-H bond of eclipsed conformation

Answer 1

Hint: We know that the different spatial arrangements of atoms of groups of atoms which can be converted from one form to another by the rotation around the single bonds are known as conformations or the conformational isomers.

Complete step by step answer:
In the eclipsed conformation, the hydrogen atoms of both the carbon atoms present are facing each other which means that the carbon hydrogen bond pairs of the two carbon atoms are very close to each other. The rotation around the central bond leads to the increase in the potential energy of the molecule. The increase in the potential energy of the molecule is due to the repulsion between the electrons present in the bond. This increase in the potential energy of the molecule is known as torsional strain.
We can say that the torsional strain depends on the rotations around the central bond. For example let’s consider the staggered and eclipsed conformation of ethane.

From the structure of ethane we can say that the torsional strain in the staggered conformation is less than the torsional strain in the eclipsed configuration. Thus the staggered conformation is more stable than the eclipsed configuration because of the torsional strain.

Hence the correct answer is option (b) i.e. torsional strain in eclipsed conformations of a molecule is due to repulsion between aligned electron pairs of the eclipsed bond.

Note: The rotation of carbon along the single bond is not free thus the potential energy of the molecule changes as the rotation occurs along the carbon-carbon single bond. The potential energy of the staggered configuration is minimum.