Q.(i) Explain Wolf-Kishner reduction with reaction.

Wolff-Kishner Reduction: Turning C=O into CH₂

Imagine you have a ketone or an aldehyde — a molecule with a carbonyl group (C=O) — and you want to remove that oxygen entirely, replacing it with two hydrogens. You want to go from C=O to CH₂. That's a reduction, but not the kind that gives you an alcohol. You want to strip the oxygen out completely.

The problem is that simply adding hydrogen gas (H₂) with a metal catalyst usually stops at the alcohol stage. You need a different strategy.

The Wolff-Kishner reduction solves this by first converting the carbonyl into a hydrazone, then using a strong base to break it apart, releasing nitrogen gas (N₂) and leaving behind the CH₂ group. The nitrogen gas bubbles away, driving the reaction forward.

The Intuition: Why It Works

The key insight is that nitrogen is an excellent leaving group when it can form N₂. The reaction proceeds in two main stages:

1. Formation of a hydrazone. Hydrazine (NH₂–NH₂) attacks the carbonyl carbon. After losing water, you get a C=N–NH₂ group — the hydrazone. This is like making an imine, but with an extra NH₂ attached.

2. Base-induced decomposition. A strong base (like KOH or NaOEt) in a high-boiling solvent (like ethylene glycol) deprotonates the hydrazone. The resulting anion rearranges, kicking out N₂ gas. The carbon that was double-bonded to oxygen now picks up two protons from the solvent, becoming CH₂.

The driving force is the formation of the extremely stable N≡N triple bond. That's why the reaction works so cleanly under basic conditions.

The Precise Statement

The carbonyl compound is treated with hydrazine (NH₂NH₂) to form a hydrazone. This intermediate is then heated with a strong base (typically KOH) in a high-boiling solvent like diethylene glycol or ethylene glycol. The product is the corresponding alkane (CH₂ group in place of C=O), along with nitrogen gas and water.

Step-by-Step Mechanism (Simplified)

Step 1: Hydrazone formation (acid-catalysed, but the overall conditions are basic)

The carbonyl oxygen is protonated (if acid is present), making the carbon more electrophilic. Hydrazine attacks, then water is eliminated to give the hydrazone.

Step 2: Deprotonation and elimination

The base abstracts a proton from the NH₂ group of the hydrazone, generating a negatively charged nitrogen. This anion undergoes a series of rearrangements, ultimately expelling N₂ and forming a carbanion intermediate.

Step 3: Protonation

The carbanion picks up a proton from the solvent (or from water present), giving the final alkane.

Important Exam Points