Question

Assertion: Staggered conformation of ethane is most stable while eclipsed conformation is least stable.
Reason: Staggered form has the least torsional strain and eclipsed form has the maximum torsional strain.
(A) Both assertion and reason are correct and reason is the correct explanation for assertion.
(B) Both assertion and reason are correct but reason is not the correct explanation for assertion.
(C) Assertion is correct but reason is incorrect.
(D) Both assertion and reason are incorrect.

Answer 1

Hint: Conformations: They are the isomers of the same alkane which are formed due to slight change in the structure, specially due to the carbon-carbon bond changes i.e. bond angle, bond length, etc. Conformations of alkane create two stages staggered and eclipsed.

Complete step by step solution:
We know that bonds are not stable. They rotate in the compound. For example: In ethane the centre atom is carbon and by the carbon one methane and three hydrogen atoms are attached. So ethane generally shows two conformations: one is staggered and second is eclipsed.
Conformations: They are the isomers of the same alkane which are formed due to slight change in the structure, specially due to the carbon-carbon bond changes i.e. bond angle, bond length, etc.
Eclipsed form: The form in which the atoms are close to each other. They have high energy because repulsion is more between the atoms and the group of atoms. This is not the stable state for molecules.
Staggered form: The form in which the same or heavier atoms are opposite to each other. They have less energy because repulsion is less among the atoms or group of atoms. This is more stable than eclipsed form. In staggered form there are two types: Anti and gauche staggered.
Anti-staggered form: In this form the angle between the same atoms or the heavier atoms are \[{180^ \circ }\] i.e. they are exactly placed opposite to each other.
Gauche-staggered form: In this form the angle between the same atoms or group of atoms are $ {60^ \circ } $ .
Torsional strain: It is defined as the repulsion between electrons in bonds that do not share an atom.
Here we are given with the compound ethane. And staggered conformation of ethane is most stable while eclipsed conformation is least stable because staggered form has the least torsional strain and eclipsed form has the maximum torsional strain. So, both assertion and reason are correct and reason is the correct explanation for assertion.
Hence option A is correct.

Note:
Generally gauche-staggered form is the more stable form than eclipsed but less stable than anti-staggered form. So the general order for the stability in gauche, anti and eclipsed is as:
Eclipsed $ < $ Gauche $ < $ Anti-staggered.