Consider the photochemical monobromination of the following alkylcyclohexanes: (I) Methylcyclohexane (II) Ethylcyclohexane (III) Isopropylcyclohexane (IV) sec-Butylcyclohexane In which of these compounds will the major monobrominated product have the bromine atom attached directly to a carbon atom of the cyclohexane ring?

Step-by-step solution

Photochemical bromination is highly regioselective, favoring the formation of the most stable free radical intermediate. We must compare the stability of the tertiary radical on the cyclohexane ring versus the most stable radical on the alkyl side chain for each compound by counting hyperconjugative -hydrogens. (I) Methylcyclohexane: - Ring tertiary radical: 4 -H (from ring) + 3 -H (from methyl) = 7 -H. - Side chain radical (primary): 1 -H (from ring). The ring radical is much more stable, so bromination occurs on