Final answer:
In chair conformations of cyclohexane, 'endo' refers to substituents pointing inward, and 'exo' points outward. Large substituents prefer the equatorial position to reduce steric strain. The orientation of substituents is important for understanding the stability and chemical behavior of cyclohexane derivatives.
Step-by-step explanation:
When discussing the chair conformation of cyclohexane, the terms endo and exo refer to the positions that substituents can occupy relative to a specified reference plane or ring. Endo refers to substituents pointing towards the interior of the ring, while exo points away from the ring, towards the exterior. In the context of cyclohexane chair conformations, these terms are often relevant in discussions of Diels-Alder reactions where the endo or exo position of a substituent in a cyclic adduct contributes to reaction stereochemistry.
The chair conformation itself minimizes steric repulsion and is highly favored for cyclohexane. Substituents on a cyclohexane ring in the chair conformation can occupy either axial (pointing straight up or down, perpendicular to the plane of the ring) or equatorial positions (pointing outward around the ring, roughly parallel to the ring plane).
If we consider the more common six-membered ring systems like cyclohexanes or sugars in pyranose form, axial positions are typically less favored for larger substituents due to 1,3-diaxial interactions. These interactions lead to a preference for larger groups to reside in equatorial positions to minimize steric strain and increase molecular stability. Drawing on the mnemonic that alpha and axial start with the letter a, in the case of pyranose rings, an alpha-axial orientation would refer to an OH group at the anomeric carbon pointing axially.