Question

When sec-butylcyclohexane reacts with bromine in the presence of sunlight, the major product is:

• A.
• B.
• C.
• D.

Hint:

In free radical substitution reactions, the most stable free radical intermediate determines the major product. In this case, the benzylic position is the most stable and thus the bromine will attach there.

Answer 1

The Correct Option is B

The reaction of sec-butylcyclohexane with bromine in the presence of sunlight is an example of a free radical halogenation reaction. This type of reaction proceeds via a radical chain mechanism which includes initiation, propagation, and termination steps.

Steps involved:

1. Initiation: In the presence of sunlight (UV light), bromine molecules (Br₂) dissociate to form two bromine radicals: Br₂ → 2Br•.
2. Propagation: The bromine radical abstracts a hydrogen atom from sec-butyl group of cyclohexane, creating a new radical. The most stable radical will form at the tertiary carbon of the sec-butyl group due to its greater stability compared to secondary or primary radicals.
3. This sec-butyl radical then reacts with another Br₂ molecule, resulting in the formation of the major product where bromine substitutes the hydrogen atom on the tertiary carbon.
4. Termination: Various termination reactions can occur where radicals combine to form non-radical products.

The structure depicted in is the major product formed, where bromine is attached at the tertiary carbon of the sec-butyl group, making it the most stable and thus the major product of the reaction.