Conformational analysis of substituted cyclohexanes
Cyclohexane is usually drawn as a shorthand bond-line structure as a hexagon (image on the left), however this is far from the actual structure of the molecule. The hexagon suggests a flat molecule with C-C-C angles of 120 degrees (image on the right). This would be a very strained molecule with an unrealistic shape in which all of the adjacent C-H bonds are eclipsed when viewed as Newman projections.
Move the model to see these destabilizing eclipsed interactions in the “flat” cyclohexane form. None of the carbon atoms in the cycle are close to being tetrahedral in shape. We understand that the cyclohexane ring relaxes to minimize angle strain and the destabilizing eclipsed interactions by adopting a chair conformation.
Moving the next model reveals the optimal geometry for the cyclohexane chair in which each carbon atom is now tetrahedral and all of the eclipsing interactions have been eliminated. The relaxed chair structure also reveals the relative axial and equatorial alignment of each attached atom, in this case hydrogens.
Now take a look at the two models of bromocyclohexane. The first has the bromine atom in an axial position where its experiences destabilizing 1,3-diaxial interactions with the co-axial hydrogen atoms situated three ring carbons away. In the second model the bromine atom is equatorial and pointing out into space. This is a much more stable situation and the preferred conformation.