Question

Identify 'A' in the following reaction :

• A.

  

• B.

  

• C.

  

• D.

  

Answer 1

The Correct Option is B

The given reaction is a Wolff-Kishner reduction, which is used to reduce carbonyl groups (aldehydes and ketones) to alkanes.

The first step involves the formation of a hydrazone derivative: \[ \text{CH}_3 - \text{CO} - \text{CH}_2 - \text{CH}_3 \xrightarrow{\text{N}_2\text{H}_4} \text{CH}_3 - \text{C}(\text{NHNH}_2) - \text{CH}_2 - \text{CH}_3. \]

In the presence of ethylene glycol and KOH, the hydrazone undergoes decomposition to form: \[ \text{CH}_3 - \text{CH}_2 - \text{CH}_2 - \text{CH}_3. \]

Thus, the product 'A' is butane.

Answer 2

Step 1: Recognize the reagents and the named reaction
The sequence (i) N₂H₄ then (ii) ethylene glycol / KOH is the classical Wolff–Kishner reduction. This reaction converts a carbonyl group (>C=O of aldehydes/ketones) into a methylene group (–CH₂–) under strongly basic, high-temperature conditions.

Step 2: Identify the starting carbonyl compound from the figure
The substrate shown is a simple ketone with two methyl groups on the carbonyl carbon (acetone / propanone): CH₃–C(=O)–CH₃.

Step 3: Outline the Wolff–Kishner pathway (what happens to the carbonyl)
Stage A: Hydrazone formation The ketone reacts with hydrazine to give the hydrazone: CH₃–C(=O)–CH₃ + N₂H₄ ⟶ CH₃–C(=NNH₂)–CH₃ + H₂O.

Stage B: Base-promoted elimination (high temperature, ethylene glycol/KOH) Strong base deprotonates the –NH₂ group(s); subsequent steps expel N₂ gas and water, reducing the carbonyl carbon: … ⟶ CH₃–CH₂–CH₃ + N₂↑ + H₂O.

Step 4: Predict the product for the given substrate
Replacing the >C=O of acetone by –CH₂ converts CH₃–C(=O)–CH₃ into the fully saturated hydrocarbon CH₃–CH₂–CH₃ (propane).

Step 5: Why Wolff–Kishner suits this case
 

• It proceeds under strongly basic conditions (KOH) and high-boiling solvent (ethylene glycol), avoiding acidic media that might cause side reactions.
• The carbonyl is completely removed (no partial reduction), giving a clean methylene.
• Evolution of N₂ gas drives the reaction forward (thermodynamic push).

Step 6: Comparison note (helps avoid common mistakes)
Clemmensen reduction (Zn(Hg)/HCl) also reduces >C=O to –CH₂–, but under strongly acidic conditions. Wolff–Kishner is the basic analogue — ideal if the molecule is acid-sensitive.

Final identification of ‘A’
 

CH₃–CH₂–CH₃ (propane)