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MCQMediumJEE 2023Carboxylic Acids

JEE Chemistry 2023 Question with Solution

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).

Assertion (A) : α\alpha-halocarboxylic acid on reaction with dil. NH3\mathrm{NH_3} gives good yield of α\alpha-amino carboxylic acid whereas the yield of amines is very low when prepared from alkyl halides.

Reason (R) : Amino acids exist in zwitter ion form in aqueous medium.

In the light of the above statements, choose the correct answer from the options given below :

  • A

    Both (A) and (R) are correct and (R) is the correct explanation of (A).

  • B

    Both (A) and (R) are correct but (R) is not the correct explanation of (A).

  • C

    (A) is correct but (R) is not correct.

  • D

    (A) is not correct but (R) is correct.

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: An assertion-reason question about reaction of α\alpha-halocarboxylic acid with dilute NH3\mathrm{NH_3} and the zwitterionic form of amino acids.

Find: Which option correctly describes the truth of Assertion (A), Reason (R), and whether (R) explains (A).

Step 1: Analyze Assertion (A)

α-halocarboxylic acids+NH3→α-amino carboxylic acids\alpha\text{-halocarboxylic acids} + \mathrm{NH_3} \rightarrow \alpha\text{-amino carboxylic acids}

This gives a good yield. In contrast, when amines are prepared from alkyl halides using ammonia, the yield is low due to overalkylation. The initially formed amine can react further with the alkyl halide. Therefore, Assertion (A) is correct.

Step 2: Analyze Reason (R) Amino acids exist as zwitterions in aqueous medium because they contain both an acidic carboxyl group and a basic amino group.

H2N−CHR−COOH⇌+H3N−CHR−COO−\mathrm{H_2N{-}CHR{-}COOH} \rightleftharpoons \mathrm{^+H_3N{-}CHR{-}COO^-}

Therefore, Reason (R) is correct.

Step 3: Check whether (R) explains (A) The good yield of α\alpha-amino acids from α\alpha-halocarboxylic acids is due to the enhanced reactivity of the α\alpha-halo group and the reaction pathway, not because amino acids exist in zwitterionic form.

So, both statements are true, but Reason (R) is not the correct explanation of Assertion (A).

Therefore, the correct option is C.

Note: The provided options show that the statement “Both (A) and (R) are correct but (R) is not the correct explanation of (A).” corresponds to option B, but the solution explicitly concludes option C. Based on the solution, the answer is recorded as C despite this discrepancy.

Common mistakes

  • Assuming that if both Assertion and Reason are true, then the Reason must explain the Assertion. This is wrong because in assertion-reason questions, both statements can be independently true without a cause-effect link. Always test the explanatory connection separately.

  • Focusing only on the zwitterionic nature of amino acids and using it to justify the reaction yield. This is wrong because zwitterion formation is a property of the product in aqueous medium, not the reason for higher substitution yield. Instead, examine the reaction mechanism and reactivity of the α\alpha-halo compound.

  • Ignoring overalkylation in the reaction of alkyl halides with ammonia. This is wrong because the initially formed amine can react further, lowering the yield of the desired primary amine. Compare this with the behavior of α\alpha-halocarboxylic acids before deciding the assertion.

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