Final answer:
The size of a substituent on a cyclohexane determines if it can occupy an axial position; larger groups with greater A values prefer the equatorial position to reduce steric strain. There is no exact A value threshold, but generally, substituents larger than isopropyl, such as tert-butyl, avoid the axial position.
Step-by-step explanation:
The subject of your question relates to the conformations of monosubstituted cyclohexanes and the steric hindrance associated with substituents adopting axial positions during a chair flip. Isopropyl groups can adopt an axial position on a cyclohexane chair conformation, but larger groups such as tert-butyl are too sterically hindered.
The degree to which a substituent experiences steric hindrance in the axial position can be quantified using the A value, which measures the energy difference between equatorial and axial conformations of a substituent. While there isn't a strict A value threshold that defines the limit for axial positioning, generally, bulky groups with larger A values will favor the equatorial position to minimize steric strain.
Considering that there is equilibrium between different conformations of substituted cyclohexanes, the most stable conformation at room temperature will be the one where bulky groups are in the equatorial position, reducing 1,3-diaxial interactions.
For a substituent to be too sterically hindered to exist in the axial position, it generally needs to be larger than an isopropyl group, as evidenced by tert-butyl which is known for its considerably larger size and inability to comfortably occupy the axial position due to extreme steric repulsion.