Question

Propose a mechanism for both parts of the Wolff–Kishner reduction of cyclohexanone: the formation of the hydrazone, and then the base-catalyzed reduction with evolution of nitrogen gas.

Accepted Answer

Step 1: Formation of the hydrazone - Start with cyclohexanone (a ketone) and hydrazine (NH₂NH₂). The carbonyl group (C=O) of cyclohexanone reacts with hydrazine in the presence of an acid catalyst. This involves nucleophilic attack by the lone pair on the nitrogen of hydrazine on the electrophilic carbon of the carbonyl group, forming a tetrahedral intermediate. Step 2: Proton transfer - In the tetrahedral intermediate, a proton transfer occurs, leading to the elimination of water (H₂O) and the formation of a double bond between the carbon and nitrogen. This results in the formation of the hydrazone (C=N-NH₂). Step 3: Base-catalyzed reduction - In the second part of the mechanism, the hydrazone is treated with a strong base, such as KOH, and heat. The base deprotonates the terminal nitrogen (NH₂ group), forming a resonance-stabilized anion. Step 4: Nitrogen gas evolution - The resonance-stabilized anion undergoes further deprotonation and rearrangement, leading to the cleavage of the N-N bond and the release of nitrogen gas (N₂). This step is driven by the stability of nitrogen gas as a product. Step 5: Formation of the alkane - The remaining carbon fragment is protonated by water or another proton source, resulting in the formation of cyclohexane, the fully reduced product.

Propose a mechanism for both parts of the Wolff–Kishner reduction of cyclohexanone: the formation of the hydrazone, and then the base-catalyzed reduction with evolution of nitrogen gas.

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Concetti chiave

Wolff–Kishner Reduction

Formation of Hydrazone

Base-Catalyzed Reduction