AIIMS 2006 Chemistry Addition Reactions Assertion Reason Question

To analyze the assertion and reason in the given question, we first need to clarify the statement regarding the addition of HBr to 2-butene.

Understanding the Assertion

Assertion: "Addition of HBr on 2-butene gives two isomeric products."

2-butene (C₄H₈) is an alkene with the following structural formula:

$$\text{CH}_3\text{CH}=\text{CHCH}_3$$

When HBr adds to 2-butene, it can theoretically lead to two different products depending on the orientation of the addition. However, 2-butene is symmetrical, and thus the addition of HBr will produce only one type of product that can exist in two stereochemical forms (cis and trans), namely 2-bromobutane. Therefore, the assertion that it gives two isomeric products is not entirely true in the context of structural isomerism since both products are stereoisomers of the same compound.

Understanding the Reason

Reason: "Addition of HBr on 2-butene follows Markovnikov's rule."

Markovnikov's rule states that when HX (like HBr) adds to an unsymmetrical alkene, the hydrogen atom (H) will bond to the carbon with the greater number of hydrogen atoms already bonded to it, while the halide (Br) will bond to the carbon with fewer hydrogen atoms.

In the case of 2-butene, both carbons involved in the double bond (C2 and C3) have two hydrogen atoms, making the addition symmetric. Therefore, applying Markovnikov's rule in this case does not lead to different products, as it would in unsymmetrical alkenes. Hence, the statement that the addition follows Markovnikov's rule is misleading.

Conclusion

Given this analysis, both the assertion and the reason are false:

• The assertion is false because the addition does not yield two structural isomers; instead, it yields one compound with stereoisomerism.
• The reason is false because Markovnikov's rule applies to asymmetric alkenes, and it does not correctly apply to 2-butene.

Correct Answer

D) Both Assertion and Reason are false.

Clarification of Other Options:

• Option A: Incorrect because both the assertion and the reason are false.
• Option B: Incorrect for the same reason as option A; both statements are false, thus the reason cannot be a correct explanation.
• Option C: Incorrect because while the assertion is false, the reason is also false.

Summary

In summary, when we consider the addition of HBr to 2-butene, we find that it does not yield two distinct isomeric products. Instead, it produces one compound with stereochemical variations. Moreover, the application of Markovnikov's rule does not change this outcome as the molecule is symmetrical. Therefore, the correct answer is D, as both the assertion and the reason are false.