Question

Compound A formed in the following reaction reacts with B gives the product C. Find out A and B.\[ \text{CH}_3 - \text{C} \equiv \text{CH} + \text{Na} \xrightarrow{} \text{A}\xrightarrow{B} \text{CH}_3 - \text{C} \equiv \text{C} - \text{CH}_2 - \text{CH}_2 + \text{NaBr}(C)\]

• A.

  

• B.

  

• C.

  

• D.

  

Answer 1

The Correct Option is C

This problem requires the identification of an intermediate compound A and a reactant B within a two-step reaction sequence. The initial reactant is propyne (\(CH_3-C \equiv CH\)), and the final products are a substituted alkyne (C) and sodium bromide (\(NaBr\)).

Concept Used:

The solution relies on two fundamental reactions in alkyne chemistry:

1. Acetylide Anion Formation: Terminal alkynes, characterized by a hydrogen atom bonded to a triply-bonded carbon, exhibit weak acidity. Treatment with a strong base, such as sodium metal (\(Na\)) or sodamide (\(NaNH_2\)), results in deprotonation to yield a sodium acetylide salt. In this process, sodium acts as a reducing agent, producing the sodium salt and hydrogen gas.

\[ 2 R-C \equiv CH + 2Na \longrightarrow 2 R-C \equiv C^-Na^+ + H_2(g) \]

2. Nucleophilic Substitution by Acetylide Anion: The acetylide anion (\(R-C \equiv C^-\)) functions as a potent nucleophile. It undergoes nucleophilic substitution (specifically, an \(S_N2\) reaction) with a primary alkyl halide, forming a new carbon-carbon bond and extending the alkyne chain to produce an internal alkyne.

\[ R-C \equiv C^-Na^+ + R'-X \longrightarrow R-C \equiv C-R' + NaX \]

Step-by-Step Solution:

Step 1: Determine the structure of compound A.

The initial reaction involves propyne (\(CH_3-C \equiv CH\)) and sodium metal (\(Na\)). Propyne is a terminal alkyne, and its terminal hydrogen is acidic. Sodium metal will deprotonate the alkyne, forming sodium propynide.

\[ CH_3 - C \equiv CH + Na \longrightarrow CH_3 - C \equiv C^-Na^+ + \frac{1}{2}H_2(g) \]

Consequently, intermediate compound A is identified as sodium propynide, \(CH_3 - C \equiv C^-Na^+\).

Step 2: Determine the structure of compound B.

The provided structure for product C, \(CH_3-C \equiv C-CH_2-CH_2-CH_3\) with a \(CH_3\) branch on the second carbon of the added chain, appears to contain a typographical error. Assuming the intended product is the straight-chain alkyne \(CH_3-C \equiv C-CH_2-CH_2-CH_3\) (hex-2-yne), which is a common outcome in similar examples, the added group would be a propyl group (\( -CH_2-CH_2-CH_3 \)). This implies that reactant B is 1-bromopropane, \(CH_3-CH_2-CH_2-Br\). This assignment aligns with option (1).

Proceeding with the assumption that the depicted product C has a typo and the intended structure is \(CH_3-C \equiv C-CH_2CH_2CH_3\):

The reaction is: \[ CH_3 - C \equiv C^-Na^+ + CH_3CH_2CH_2-Br \longrightarrow CH_3-C \equiv C-CH_2CH_2CH_3 + NaBr \]

Final Computation & Result:

Step 3: Match the identified structures of A and B with the provided options.

• Compound A: sodium propynide (\(A = CH_3 - C \equiv C^-Na^+\))
• Compound B: 1-bromopropane (\(B = CH_3 - CH_2 - CH_2 - Br\))

Evaluating other options:

• Option (2): Incorrect A (propene) and incorrect B.
• Option (3): Incorrect A (propane) and incorrect B.
• Option (4): Correct A but incorrect B (propane).

The correct option is (1) \(A = CH_3 - C \equiv C^-Na^+, B = CH_3 - CH_2 - CH_2 - Br\).