Illustrated Glossary of Organic Chemistry

Torsional strain (Pitzer strain): Strain caused by the close approach of atoms or groups separated by three covalent bonds. In the molecule W-X-Y-Z, atoms W and Z may experience torsional strain if a particular conformation (such as an eclipsed conformation) brings these atoms into close proximity. Torsional strain can cause a resistance to bond rotation, and can influence a barrier to rotation. (Student confusion over the definitions of torsional strain versus other types of strain was the impetus to start the Illustrated Glossary of Organic Chemistry.)

Sawhorse projections:

Newman projections:

Eclipsed conformation
More strain

Anti-staggered conformation
Less strain

When the chlorine atoms of 1,2-dichloroethane are aligned (an eclipsed conformation), the chlorine atoms experience torsional strain. The eclipsed hydrogen atoms also experience torsional strain (albeit less than the chlorine atoms because hydrogen has a smaller atomic radius than chlorine). This torsional strain is relieved when carbon-carbon bond rotation changes the molecule into a staggered conformation (such as the anti-staggered conformation shown here). Verify this with molecular models.