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

To determine the major product when sec-butylcyclohexane reacts with bromine in the presence of sunlight, we need to understand the mechanism of the reaction.

1. Type of Reaction: The reaction between sec-butylcyclohexane and bromine in the presence of sunlight is a free radical halogenation. Sunlight initiates the formation of free radicals, leading to the substitution of hydrogen atoms in the aliphatic chain with bromine atoms.
2. Stability of Radicals: During free radical halogenation, the bromine radicals will preferentially abstract the hydrogen from the position that forms the most stable radical. For cyclohexane derivatives, tertiary radicals are more stable than secondary, which in turn are more stable than primary radicals:
   • Tertiary radical > Secondary radical > Primary radical
3. Application to Sec-Butylcyclohexane: In sec-butylcyclohexane, the tertiary carbon of the sec-butyl group is most likely to form a radical because it is more stable than other possible radicals. The abstraction of a hydrogen atom from the tertiary carbon of the sec-butyl group gives the most stable radical intermediate.
4. Formation of Major Product: Once the most stable radical is formed, a bromine atom can attach to it, leading to the formation of the major product. This substitution occurs at the tertiary carbon of the sec-butyl group.
5. Conclusion: Thus, the major product of the reaction is the one where bromine is substituted at the tertiary carbon of the sec-butyl group.

This matches the image provided in the options, confirming it as the major product.

Answer 2

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.