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MCQEasyJEE 2025Alkanes (Properties, Halogenation)

JEE Chemistry 2025 Question with Solution

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

Four structural product options for bromination of sec-butylcyclohexane, numbered 1 to 4, showing bromine attached at different positions on the side chain or cyclohexane ring.
  • A

    Option 1

  • B

    Option 2

  • C

    Option 3

  • D

    Option 4

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: sec-butylcyclohexane reacts with bromine in the presence of sunlight.

Find: the major product of free radical bromination.

The reaction between sec-butylcyclohexane and bromine in the presence of sunlight is a free radical halogenation. Sunlight initiates radical formation, and bromination occurs preferentially at the position that gives the most stable free radical intermediate.

The relevant stability order is:

tertiary radical>secondary radical>primary radical\text{tertiary radical} > \text{secondary radical} > \text{primary radical}

In sec-butylcyclohexane, abstraction of hydrogen from the tertiary carbon of the sec-butyl group forms the most stable radical intermediate. Therefore bromine substitutes at that tertiary carbon.

Structure of the major brominated product of sec-butylcyclohexane with bromine attached at the tertiary carbon of the sec-butyl substituent.

Thus, the structure shown as option 2 is the major product. Therefore, the correct option is B.

Radical Mechanism View

Given: reaction of sec-butylcyclohexane with bromine under sunlight.

Find: which product forms predominantly.

This reaction proceeds through a radical chain mechanism.

  1. Initiation:
Br2→2Br∙\text{Br}_2 \rightarrow 2\text{Br}\bullet
  1. Propagation: a bromine radical abstracts the hydrogen that leads to the most stable carbon radical. In this molecule, that radical is formed at the tertiary carbon of the sec-butyl group.
  1. The resulting tertiary radical reacts with another bromine molecule to give the brominated product at that same carbon.

Hence the major product is the one with bromine attached at the tertiary carbon of the sec-butyl substituent, which corresponds to option 2. The correct option is B.

Common mistakes

  • Choosing bromination at a secondary carbon of the side chain is incorrect because bromination is highly selective and favors formation of the more stable tertiary radical. Check radical stability before selecting the product.

  • Assuming substitution occurs randomly on the cyclohexane ring is incorrect because the major product is determined by the most stable radical intermediate, not by arbitrary hydrogen replacement.

  • Confusing free radical bromination with an ionic substitution mechanism is wrong because sunlight indicates a radical chain process. Use radical initiation and radical stability to predict the product.

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