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.

Risposta video verificata per un problema simile:

Concetti chiave

Nucleophilic Addition

Hydrazone Formation

Acid-Base Catalysis