Halocyclohexane

Let's study all halogens together. The covalent radius of F is very small. That's why the conformational energy of fluorocyclohexane is so low (0.25 kcal/mol). When we go down the periodic table, the atomic radius increases. It is thus reasonable that the conformational energy of chlorocyclohexane be higher (0.52 kcal/mol) than that of F. This trend should be observed for Br and I. However, bromocyclohexane bears a conformational energy only slightly higher (0.55 kcal/mol) than that of the chlorine counterpart. The case of iodine is even more puzzling because the size of this atom is huge. We have overlooked one important factor: Halogen-Carbon bond lengths. The larger the halogen, the longer its bond with carbon. This is the clue we need to understand the conformational energies in the series of halocyclohexanes.

The longer the Halogen-Carbon bond, the farther the halogen is from the ring. This is crucial in the axial conformer. The huge iodine stays actually relatively far away from the syn-axial hydrogens at positions 3 and 5 of the ring because of its longer bond with carbon.