A-Values

A-value = conformational free energy difference (kcal/mol) between axial and equatorial placement of a substituent.
Larger A-value → stronger preference for equatorial.
Common values: F 0.3, Cl/Br/I 0.5, OH 0.9, Me 1.7, tBu 5.0

1,3-Diaxial Strain

Axial substituents experience steric repulsion with other axial groups two carbons away.
The lower-energy chair places bulky groups equatorial to minimize this strain.
For multi-substituted rings, sum the A-values of all axial substituents—the chair with the smaller sum is more stable.

Which chair conformer is lower in energy? Click to select.

Chair A

vs.

Chair B

Streak: 0 Best: 0

Correct: ... Incorrect: ...