Question

2,4-Dinitrophenylhydrazine is frequently used for making derivatives of ketones and aldehydes because the products (2,4-dinitrophenylhydrazones, called 2,4-DNP derivatives) are even more likely than the phenylhydrazones to be solids with sharp melting points. Propose a mechanism for the reaction of acetone with 2,4-dinitrophenylhydrazine in a mildly acidic solution.

Accepted Answer

Step 1: Protonation of the carbonyl group in acetone. In a mildly acidic solution, the carbonyl oxygen of acetone (CH₃COCH₃) is protonated by a hydronium ion (H₃O⁺), increasing the electrophilicity of the carbonyl carbon. This forms a protonated intermediate, CH₃C(OH⁺)CH₃. Step 2: Nucleophilic attack by 2,4-dinitrophenylhydrazine. The nucleophilic nitrogen atom of the 2,4-dinitrophenylhydrazine (H₂NNH-C₆H₃(NO₂)₂) attacks the electrophilic carbonyl carbon of the protonated acetone, forming a tetrahedral intermediate. Step 3: Proton transfer to stabilize the intermediate. A proton is transferred from the nitrogen of the hydrazine group to the hydroxyl group of the intermediate, converting the hydroxyl group into a better leaving group (water). This step prepares the intermediate for the elimination step. Step 4: Elimination of water. The intermediate undergoes elimination of a water molecule, resulting in the formation of a double bond between the carbon and nitrogen atoms. This forms the imine intermediate, specifically the 2,4-dinitrophenylhydrazone derivative. Step 5: Resonance stabilization of the product. The final product, 2,4-dinitrophenylhydrazone, is stabilized by resonance due to the electron-withdrawing nitro groups on the aromatic ring, which delocalize the lone pair of electrons on the nitrogen atom, contributing to the stability of the product.

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Key Concepts

Nucleophilic Addition

Hydrazone Formation

Acid-Base Catalysis