1. Choose the substituent
2. All chair conformers (chair × rotamer)
Axial chair
Equatorial chair
Each card is a Newman projection looking down the Cα→C1 bond: the substituent carbon is in front, ring carbons C2 and C6 are on the back circle. Rotamers are the three staggered positions of the group. The green-edged card is the lowest-energy conformer; percentages are the share of all molecules at 25 °C.
Bookkeeping ΔG (axial − equatorial)
Experimental A-value
free-energy preference for equatorial
3. Selected conformer
Teaching values: 0.9 kcal/mol per gauche-butane arrangement (a CH3/H 1,3-diaxial contact is the same thing seen from the other side), and 3.7 for a carbon that sits over the ring in a syn-pentane arrangement with C3 and C5. Gauche terms are counted for every four-carbon chain that includes a substituent carbon.
Ideal geometry used in the model
| C–C bond length | 1.54 Å |
| C–H bond length | 1.09 Å |
| Every bond angle | 109.47° |
| All dihedrals | 180° or ±60° |
Every carbon is on the diamond lattice, so the substituent's rotamers are exactly staggered. A real axial tert-butyl group twists and the ring flattens to escape the clash the model shows in red.
Exercise: count the gauche interactions
Every C–C–C–C dihedral at ±60° counts, including the six inside the chair itself. Compare axial and equatorial: the difference × 0.9 kcal/mol is the textbook estimate of the A-value.
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ring C substituent C H
Click atoms to measure.