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MCQEasyJEE 2024Alkynes (Acidic Nature, Reactions)

JEE Chemistry 2024 Question with Solution

The final product A, formed in the following multistep reaction sequence is: CH3−C≡CH+Na→A\text{CH}_3 - \text{C}\equiv\text{CH} + \text{Na} \rightarrow A B→CH3−C≡C−CH2−CH2−CH3+NaBrB \rightarrow \text{CH}_3 - \text{C}\equiv\text{C} - \text{CH}_2 - \text{CH}_2 - \text{CH}_3 + \text{NaBr} Choose the correct answer from the options below:

  • A

    A = CH3–C≡CNa\text{CH}_3\text{–C}\equiv\text{CNa}, B = CH3–CH2–CH2–Br\text{CH}_3\text{–CH}_2\text{–CH}_2\text{–Br}

  • B

    A = CH3–CH2–CH2Br\text{CH}_3\text{–CH}_2\text{–CH}_2\text{Br}, B = CH3–C≡C–CH3\text{CH}_3\text{–C}\equiv\text{C–CH}_3

  • C

    A = CH3–C≡CNa\text{CH}_3\text{–C}\equiv\text{CNa}, B = CH3–C≡CH\text{CH}_3\text{–C}\equiv\text{CH}

  • D

    A = CH3–C≡CNa\text{CH}_3\text{–C}\equiv\text{CNa}, B = CH3–CH2–CH3\text{CH}_3\text{–CH}_2\text{–CH}_3

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: The sequence shown is

CH3−C≡CH+Na→A\text{CH}_3-\text{C}\equiv\text{CH} + \text{Na} \rightarrow A

and then

A+B→CH3−C≡C−CH2−CH2−CH3+NaBrA + B \rightarrow \text{CH}_3-\text{C}\equiv\text{C}-\text{CH}_2-\text{CH}_2-\text{CH}_3 + \text{NaBr}

Find: The identities of A and B.

From the solution, the correct option is stated as A and also as Option (1).

This means

A=CH3−C≡CNaA = \text{CH}_3-\text{C}\equiv\text{CNa}

and

B=CH3−CH2−CH2−BrB = \text{CH}_3-\text{CH}_2-\text{CH}_2-\text{Br}

The final product formed is

CH3−C≡C−CH2−CH2−CH3\text{CH}_3-\text{C}\equiv\text{C}-\text{CH}_2-\text{CH}_2-\text{CH}_3

Therefore, the correct option is A.

Discrepancy Noted in the solution

Given: The question is about formation of an alkyne salt and its reaction with an alkyl bromide.

Find: Which option matches the sequence.

The solution contains reaction discussion about hydration, oxidation with CrO3\text{CrO}_3, and Wolff-Kishner reduction using NH2NH2\text{NH}_2\text{NH}_2 and KOH\text{KOH}, which is unrelated to the given question. However, the same the solution explicitly concludes that the correct answer is Option (1) and also labels the correct option as A.

Using that explicit conclusion:

A=CH3−C≡CNa,B=CH3−CH2−CH2−BrA = \text{CH}_3-\text{C}\equiv\text{CNa}, \qquad B = \text{CH}_3-\text{CH}_2-\text{CH}_2-\text{Br}

So the correct option is A.

Common mistakes

  • Mistake: treating Na as if it adds across the triple bond. Why it is wrong: in this context the terminal alkyne hydrogen is replaced to form the sodium acetylide. What to do instead: identify A as CH3−C≡CNa\text{CH}_3-\text{C}\equiv\text{CNa}.

  • Mistake: choosing B as an alkane or alkyne instead of an alkyl bromide. Why it is wrong: the product side shows formation of NaBr, so the second reactant must supply Br. What to do instead: select B as CH3−CH2−CH2−Br\text{CH}_3-\text{CH}_2-\text{CH}_2-\text{Br}.

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