Identify 'A' in the following reaction :
The Correct Option is B
Approach Solution - 1
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.
Approach Solution -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)
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