91Ó°ÊÓ

Torsional strain arises due to the great repulsion between two molecules or atoms that do not have a common bonded atom. Therefore, torsional strain arises in the eclipsed conformation.

Steric strain occurs due to the overlapping of electron clouds of bulky groups (gauche interaction).

a.

The molecule has no eclipsed conformation but gauche interaction. Therefore, it has a steric strain, not a torsional strain.

Steric strain sites

b.

The molecule has an eclipsed conformation, not a gauche interaction (heavy groups interaction). Therefore, it only has a torsional strain.

Torsional strain sites

c.

The molecule has an eclipsed conformation and heavy groups interaction. Therefore, it has both torsional and steric strains.

Torsional strain and steric strain site