free rotation

Meaning: A σ bond has electron density along the bond axis, so the overlap is kept as one end twists. Groups on either side of a C–C single bond can therefore turn past each other, giving different conformations (staggered, eclipsed, anti, gauche). The barrier is small (ethane is about 12 kJ/mol), so rotation is fast at room temperature, though not truly barrier-free. Staggered forms are lower in energy than eclipsed forms because of less torsional strain, and bulky groups add steric strain.

On the MCAT: Rotation is restricted in three common ways. A C=C double bond cannot rotate unless the π bond breaks, so cis/trans (E/Z) isomers are distinct compounds. In rings, the carbons cannot turn freely, so substituents are fixed as cis or trans. Resonance gives the amide (peptide) C–N bond partial double-bond character, which makes it planar and rigid. Passages may show Newman projections and ask which conformation is most stable, or ask whether two drawings are the same molecule.

Common confusion: Students treat conformers as different isomers. Conformers interconvert by rotation alone, with no bonds broken, so they are the same compound. Changing a configuration (R/S, or cis/trans on a double bond) requires breaking bonds. "Free" also does not mean every conformation has equal energy.