When drawing molecular structures, particularly chair conformations of cyclohexane, it's essential to understand that there are numerous valid representations. The key to comparing these structures lies in three critical aspects: the distance between substituents, the configuration (cis or trans), and the equatorial versus axial positioning of groups.
The distance between groups is determined by the number of carbon atoms separating them. For example, if two substituents are on adjacent carbons, they are referred to as 1,2-substituents, while those separated by one carbon are termed 1,3-substituents, and those separated by two carbons are 1,4-substituents. Regardless of how the chair is drawn, as long as the distance remains consistent, the representations can be considered equivalent.
Next, the configuration of the substituents is crucial. If both groups are oriented in the same direction (both up or both down), they are classified as cis. Conversely, if one group is up and the other is down, they are trans. This distinction is vital for determining whether two chair conformations represent the same molecule.
For asymmetrical chairs, where different substituents are present, the preference for equatorial versus axial positioning becomes significant. Generally, larger groups prefer the equatorial position to minimize steric strain. When comparing two structures, if one has a larger substituent in the axial position while the other has it equatorial, they are not identical but are conformers, existing in equilibrium with one another.
To illustrate this, consider two representations of a dimethylcyclohexane. If one structure has both methyl groups in the equatorial position and the other has one in the axial position, they are conformers. However, if both structures maintain the same distance between the groups and the same cis or trans configuration, they can be considered the same compound despite their different appearances.
In summary, when evaluating chair conformations, focus on the distance between substituents, their cis or trans configuration, and the equatorial versus axial positioning. By doing so, you can confidently determine whether two drawn structures are identical, conformers, or entirely different molecules.
