cyclohexane

Meaning: Cyclohexane is a cycloalkane, C₆H₁₂. In the chair conformation the bond angles are close to 109.5° and all bonds are staggered, so angle strain and torsional strain are minimal. The boat is higher in energy because of eclipsing and flagpole steric repulsion. The twist-boat is a little more stable than the boat, and the half-chair is the highest-energy point on the interconversion path. Each ring carbon has one axial and one equatorial position. A ring flip converts one chair into the other and swaps every axial and equatorial position. It is fast at room temperature.

On the MCAT: A common question asks which chair of a substituted cyclohexane is more stable. A large group such as tert-butyl prefers the equatorial position, because an axial group suffers 1,3-diaxial interactions with the axial hydrogens on the same face of the ring. Expect to rank conformations by energy (chair lowest), to compare the two chairs of a cis or trans disubstituted ring, or to read a Newman projection down a ring C–C bond. In trans-1,4-disubstituted cyclohexane, both groups can be equatorial in one chair.

Common confusion: The flat hexagon drawing suggests 120° angles and large strain, but the real ring is puckered. A ring flip changes axial to equatorial, but it does not change whether a group points up or down, so cis stays cis and trans stays trans.