Final answer:
In cyclohexane chair conformations, substituents can be classified as axial or equatorial. Bulky substituents prefer an equatorial position to reduce steric strain.
Step-by-step explanation:
To identify each substituent on a cyclohexane as axial or equatorial, one needs to understand the chair conformation of cyclohexane. In this conformation, each carbon atom has two attached hydrogen atoms or substituents, one axial and one equatorial.
The axial substituents are oriented parallel to the axis of the ring, alternating up and down around the ring, while the equatorial substituents extend out roughly around the equator of the molecule, positioned more toward the sides of the ring.
Bulky groups prefer the equatorial position because they experience less steric hindrance compared to the axial position. During the ring flip of cyclohexane, each axial bond becomes an equatorial bond and vice versa.
To determine the stability of cyclohexane substituents, draw both chair conformations and examine the steric interaction between the bulky groups and the neighboring axial hydrogens.
For a monosubstituted cyclohexane, the conformation with the substituent in the equatorial position will generally be more stable and therefore more predominant.
To draw the chair conformation of cyclohexane, start by drawing two parallel slanted lines and connect them to form a puckered hexagon that alternates between carbons above and below the plane.
Then add axial bonds by drawing lines straight up or down from each carbon, and add equatorial bonds by drawing lines outward, following the stereochemistry of the hexagon. Positional numbering of the carbons may be necessary to accurately describe disubstituted cyclohexanes and determine whether substituents are cis or trans to each other.