AIIMS 2003 Chemistry Addition Reactions Assertion Reason Question

To address the question, we will analyze both the assertion and the reason provided, and we'll explore the chemistry of the reaction in detail.

Assertion:

Trans-2-butene on reaction with Br₂ gives meso-2,3-dibromobutane.

Reason:

The reaction involves syn-addition of bromine.

Explanation:

1. Understanding the Assertion:

   • Trans-2-butene ($C_4H_8$) is an alkene that has the following structure: $$\text{CH}_3 - \text{CH} = \text{CH} - \text{CH}_3$$ where the two methyl groups (CH₃) are on opposite sides of the double bond (trans configuration).
   • When trans-2-butene reacts with bromine ($Br_2$), it undergoes an addition reaction. This reaction is characterized by the addition of bromine atoms across the double bond.
   • The bromine adds to the double bond through a cyclic bromonium ion intermediate, which leads to the formation of 2,3-dibromobutane. However, due to the stereochemistry involved in the reaction, the product is meso-2,3-dibromobutane, which is achiral. The structure of meso-2,3-dibromobutane can be represented as: $$\text{Br} - \text{CH}_3 - \text{C}(\text{Br}) - \text{CH}_3$$
   • The key point here is that the meso compound has a plane of symmetry, making it optically inactive.

2. Understanding the Reason:

   • The statement that the reaction involves syn-addition of bromine is crucial. In the context of bromination, syn-addition means that both bromine atoms add to the same side of the double bond. In the case of trans-2-butene, this syn-addition results in the formation of the meso compound because the bromine atoms add to the same face of the molecule, which aligns the stereochemistry appropriately to achieve the meso configuration.

3. Correct Answer Justification:

   • The assertion is true because trans-2-butene does indeed yield meso-2,3-dibromobutane upon reaction with $Br_2$.
   • The reason is also true because the mechanism of the reaction involves syn-addition.
   • Therefore, both the assertion and reason are true, and the reason correctly explains the assertion. This corresponds to option C, which states that both the assertion and reason are true, and the reason is a correct explanation for the assertion.

Why Other Options are Incorrect:

• Option A (Only Assertion is true): This is incorrect because the reason is also true and supports the assertion.
• Option B (Only Reason is true): This is incorrect because the assertion itself is true; thus, both cannot be true in this case.
• Option D (Both Assertion and Reason are false): This is incorrect as both statements are true.

Conclusion:

The correct answer is C because both the assertion and reason are true, and the reason accurately explains why trans-2-butene yields meso-2,3-dibromobutane through syn-addition of bromine. The stereochemical outcome emphasizes the importance of understanding reaction mechanisms and the resultant configurations in organic chemistry.