cis- and trans-1,2-Dimethylcyclohexane template

Chairs of cis-1,2-dimethylcyclohexane (a,e ⇌ e,a, equal energy) and trans-1,2-dimethylcyclohexane (e,e strongly favoured over a,a), with the wedge drawings and CIP labels.

What the template shows

• cis = both CH₃ on the same face (one axial, one equatorial); trans = opposite faces (both equatorial or both axial)
• Ring flips of the two diastereomers
• C1 axial, C2 equatorial
• C1 equatorial, C2 axial
• cis (1R,2S): meso
• favours right
• diaxial: four 1,3-diaxial clashes, minor
• diequatorial: major (about 99 %)
• trans (1S,2S): chiral
• cis/trans is fixed by the bonds; axial/equatorial changes with the ring flip. A ring flip never turns cis into trans.
• cis-1,2: axial/equatorial ⇌ equatorial/axial, two chairs of equal energy (they are mirror images; the compound is meso).
• trans-1,2: the diequatorial chair (one gauche CH₃/CH₃, about 3.8 kJ mol⁻¹) is far more stable than the diaxial one (four 1,3-diaxial CH₃/H, about 15 kJ mol⁻¹).
• So trans-1,2-dimethylcyclohexane is the more stable diastereomer, by about 7.5 kJ mol⁻¹.
• Wedge = toward you, hash = away from you · every part of this figure is editable

Drawn with 16 atoms, 43 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 16 Textbook
• Eliel & Wilen, Stereochemistry of Organic Compounds, Wiley 1994, ch. 11 Textbook