conformational isomers

Meaning: Conformational isomers (conformers) are arrangements of the same molecule that differ by rotation about single bonds. No bond breaks, so they usually interconvert rapidly at room temperature and cannot be separated. Energy differences come from torsional strain (eclipsed bonds), steric strain (crowded groups), and, in rings, angle strain.

On the MCAT: Expect Newman projections of butane viewed down C2–C3. Anti (methyls 180° apart) is lowest in energy, then gauche (60°), then eclipsed forms, with the methyls fully eclipsed (0°) highest. Energy-versus-dihedral-angle graphs show staggered forms as minima and eclipsed forms as maxima. In cyclohexane, the chair is the most stable conformer. Ring flip swaps axial and equatorial positions, and bulky groups favor equatorial to avoid 1,3-diaxial strain. Questions ask you to rank conformers or to predict which side of a ring-flip equilibrium is favored.

Common confusion: Cis/trans isomers, enantiomers and diastereomers (R/S differences) are configurational isomers, not conformers. Interconverting them requires breaking a bond. Ring flip changes axial and equatorial positions but does not turn cis into trans or change R/S configuration. Rotation is also restricted about double bonds, so the butane-style analysis applies to single bonds.