Nature of the reaction

Bromine in the presence of sunlight undergoes free radical bromination: $\text{R-H} + Br_2 \xrightarrow{h\nu} \text{R-Br} + HBr$

Bromination is highly selective and prefers abstraction of the hydrogen that gives the most stable radical.

Find the most reactive hydrogen in sec-butylcyclohexane

In sec-butylcyclohexane, the side chain is a sec-butyl group attached to the cyclohexane ring: $\text{cyclohexyl}-CH(CH_3)CH_2CH_3$

The carbon directly attached to the ring in the sec-butyl group is: $-CH(CH_3)CH_2CH_3$

This carbon is tertiary with respect to hydrogen abstraction? Let us check carefully.

That carbon is bonded to:

• cyclohexyl group
• $CH_3$
• $CH_2CH_3$
• one hydrogen

So it is a tertiary carbon bearing one hydrogen.

Removal of this hydrogen gives a tertiary radical, which is the most stable radical available in the molecule.

Why not bromination on the ring?

Hydrogens on the cyclohexane ring are mostly on secondary carbons. Bromination strongly favors: $3^\circ > 2^\circ > 1^\circ$

Hence substitution occurs predominantly at the tertiary hydrogen on the sec-butyl carbon.

Major product

Therefore, bromine replaces the hydrogen on the carbon joining the sec-butyl group to the ring: $\text{cyclohexyl}-CH(CH_3)CH_2CH_3 \rightarrow \text{cyclohexyl}-C(Br)(CH_3)CH_2CH_3$

So the major product is 1-bromo-1-(cyclohexyl)butane-type structure, i.e. bromination at the tertiary side-chain carbon.

Comparison with stored answer

Since the options are not visible in the prompt, the structure cannot be matched directly by inspection. But the correct option must be the one showing Br substituted at the tertiary carbon of the sec-butyl side chain.

The stored correct answer is D. This is consistent if option D corresponds to that structure.