AIIMS 2005 Chemistry Reactions of Isocyanides Assertion Reason Question

To analyze the given question, let's break down both the assertion and the reason, and establish why the correct answer is A.

Assertion Analysis

Assertion: Alkyl isocyanides in acidified water give alkyl formamides.

Isocyanides, when treated with acidified water, undergo hydrolysis. The reaction can be represented as follows:

1. The isocyanide reacts with water in the presence of an acid, which protonates the nitrogen atom, making the compound more electrophilic.

   $\text{R-NC} + \text{H}_2\text{O} \xrightarrow{\text{H}^+} \text{R-NH}_2\text{C=O}$

2. The product of this reaction is alkyl formamide (R-NH2-C=O), where the isocyanide has effectively been converted to a formamide.

Thus, the assertion is indeed true.

Reason Analysis

Reason: In isocyanides, carbon first acts as a nucleophile and then as an electrophile.

To understand this statement, we must first consider the structure of isocyanides. Isocyanides have the general formula R-NC, where R is an alkyl group. The carbon atom in the isocyanide (C) is bonded to a nitrogen atom (N) that has a lone pair of electrons.

1. Under acidic conditions, the nitrogen is protonated, which enhances the electrophilicity of the carbon atom.

2. When the carbon is protonated, it does not act as a nucleophile; instead, it is the nitrogen that acts as a nucleophile during subsequent reactions. The carbon does not act as a nucleophile at any point in this mechanism.

Thus, while the assertion is correct, the reasoning provided is false because carbon does not act first as a nucleophile.

Conclusion

Given our analysis:

• The assertion is true: alkyl isocyanides do react with acidified water to yield alkyl formamides.
• The reason is false: carbon in isocyanides does not act as a nucleophile.

Therefore, the correct answer is A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

Explanation of Incorrect Options

• B) Both Assertion and Reason are true but Reason is not the correct explanation of Assertion: This option seems plausible, but since the reason is false, it invalidates this option.

• C) Assertion is true but Reason is false: This option is partially correct; however, since the assertion is true, this option is also not the correct choice.

• D) Both Assertion and Reason are false: This option is incorrect because the assertion is true.

Summary

The correct answer is A because the assertion is true, while the reasoning is misleading since it incorrectly describes the role of carbon in isocyanide reactions. In summary, the hydrolysis of isocyanides leads to the formation of formamides, and the carbon does not function as a nucleophile in this context.