Question

Draw Newman and Sawhorse projections for the eclipsed and staggered conformations of ethane. Which of these conformations is more stable and why?

Answer 1

Sawhorse and newman projections of ethane:


Conformation in which hydrogen atoms attached to two carbons are aligned exactly behind the $H$ of first carbon as the projection of its bonds with $H$ atoms, is called eclipsed conformation and conformation in which hydrogens are as far apart as possible is known as the staggered conformation.

Sawhorse projections for the eclipsed and staggered conformations:



Newman’s projections for the eclipsed and staggered conformations:


Comparing the stability between staggered and eclipsed conformation:

Among staggered and eclipsed conformation of ethane, staggered conformation is more stable than eclipsed conformation as:

(i) In a staggered form of ethane, the electron clouds of carbon-hydrogen bonds are as far apart as possible. Thus, there are minimum repulsive forces, minimum energy/ torsional strain and maximum stability of the molecule

(ii) Whereas, when the staggered form changes into the eclipsed form, the electron clouds of the carbon – hydrogen bonds come closer to each other resulting in an increase in electron cloud repulsions.

(iii) Thus, the staggered form has the least torsional strain and the eclipsed form have the maximum torsional strain.