torsional strain

Meaning: Rotating about a C–C single bond changes the dihedral angle between substituents on the front and back carbons. When bonds are staggered (60° apart), their electron clouds are as far apart as possible. When bonds are eclipsed (0°), the clouds are closest and repel, which raises the energy. In ethane the eclipsed form is about 12 kJ/mol above the staggered form. This is a barrier, not a lock, so rotation stays fast at room temperature.

On the MCAT: Expect Newman projections and graphs of potential energy against dihedral angle. Peaks are eclipsed conformations and troughs are staggered ones. In butane, the lowest-energy form is staggered anti, the gauche form is a higher trough, and the highest peak is the fully eclipsed form with two methyl groups at 0°. In rings, a chair cyclohexane has all bonds staggered, so it has little torsional strain. A boat has eclipsed bonds, so it has more. Cyclopropane has eclipsed hydrogens on all its C–C bonds. Total ring strain is the sum of angle strain, torsional strain and steric strain.

Common confusion: Torsional strain is not steric strain. Torsional strain comes from repulsion between eclipsed bonding electrons, even when the groups are small, like hydrogens. Steric strain comes from bulky groups being forced close together in space, as in gauche butane or 1,3-diaxial interactions. Fully eclipsed butane has both. Angle strain is a third kind, caused by bond angles that deviate from the ideal 109.5°.