Question

Why is the free single bond rotation restricted in the case of cyclic compounds?

Answer 1

Because `C-C` bond rotation is constrained by the ring structure.

Explanation:

Get a set of molecular models and make models of ethane and cyclohexane. The methyl residues of ethane can rotate with respect to each other around the `C-C` vector; of course it is energetically unfavourable if the hydrogens are eclipsed.

Now compare the cyclohexane structure. A degree of conformational change is possible for cyclohexane: boat; twist-boat; chair. But this conformational change is tightly constrained by the ring, and `C-C` cannot rotate with respect to each other. Please do not think that models are childish. You would always see a set of models on the desks of distinguished professors of organic chemistry. It is an aid to visualization and understanding.