chair conformation

Meaning: The chair is the puckered shape cyclohexane adopts that has the lowest energy. Its C–C–C angles are close to 109.5°, so angle strain is essentially absent, and every C–C bond is staggered, so torsional strain is absent too. Each carbon bears one axial bond, parallel to the ring's axis and alternating up and down around the ring, and one equatorial bond, pointing roughly outward from the ring. A ring flip converts one chair into the other and swaps axial and equatorial positions.

On the MCAT: Expect questions that ask you to draw or compare chairs of substituted cyclohexanes and predict which is more stable. A substituent is generally more stable equatorial, because an axial group crowds the other axial groups on the same face, two carbons away (1,3-diaxial interactions). Bulky groups such as tert-butyl strongly favor the equatorial position, so the equilibrium lies toward the chair that places them there. For a disubstituted ring, the larger group usually decides which chair wins. Energy order: chair < twist-boat < boat < half-chair.

Common confusion: A ring flip does not break bonds or turn cis into trans. Each substituent keeps its up or down face, and only the axial or equatorial label changes. Axial does not mean up: axial bonds alternate up and down. The boat is higher in energy because it has eclipsed bonds and a flagpole clash between two hydrogens.