2,4 DNP Test (2,4-Dinitrophenylhydrazine)

The 2,4-Dinitrophenylhydrazine (2,4-DNP) test is a qualitative test used to detect the presence of a carbonyl group in organic compounds, specifically in ketones and aldehydes. When 2,4-DNP reacts with a carbonyl compound, it forms a yellow, orange, or red precipitate called a hydrazone, which indicates a positive result. This color change occurs due to the formation of a stable compound between the carbonyl group and 2,4-DNP.

1.0What is 2,4-Dinitrophenylhydrazine (DNPH) or Brady’s Reagent

Brady’s reagent is a solution of 2,4-Dinitrophenylhydrazine (DNPH) in a solvent, typically methanol or ethanol, often acidified with sulfuric or hydrochloric acid. The reagent is named after the chemist, Brady, who popularized its use in detecting carbonyl compounds.

• It is primarily used to detect the presence of carbonyl groups (C=O) in aldehydes and ketones.
• DNPH in Brady’s reagent reacts with carbonyl compounds to form a hydrazone derivative, which precipitates out as a solid, often a yellow, orange, or red precipitate.
• This color change and precipitation reaction provide a clear, visual indication of the presence of a carbonyl group, distinguishing it from other functional groups.

2.0Structure of 2,4-Dinitrophenylhydrazine (DNPH)

2,4-Dinitrophenylhydrazine (DNPH) is an organic compound with the chemical formula C_6​H₆​N₄O₄​. Its structure consists of a benzene ring substituted with two nitro (-NO₂​) groups at the 2 and 4 positions and a hydrazine (-NH₂​) group at the 1 position. The two nitro groups increase the electron-withdrawing nature of the molecule, enhancing its reactivity with carbonyl compounds.

The structure can be represented as follows:

The electron-withdrawing nature of the nitro groups makes the hydrazine portion of DNPH more nucleophilic, allowing it to react with carbonyl groups in aldehydes and ketones readily.

3.0Identifying a Carbonyl Compound with DNPH

DNPH is an effective qualitative reagent for identifying carbonyl compounds, specifically aldehydes and ketones. The procedure is as follows:

• Mechanism: When DNPH reacts with the carbonyl group in aldehydes or ketones, a nucleophilic addition occurs, followed by dehydration to form a stable hydrazone product.
• Visual Indicator: The formation of a bright yellow, orange, or red precipitate indicates the presence of a carbonyl group in the compound being tested. The specific color of the precipitate can sometimes help distinguish between aldehydes and ketones.
• Identification of Carbonyl Compound: The precipitate can be collected and purified, and its melting point can be measured. This melting point can be compared to a database of known DNPH derivatives to help identify the specific aldehyde or ketone.

4.02,4-Dinitrophenylhydrazine Laboratory Test Procedure

The DNPH test is widely used in laboratories for the detection of aldehydes and ketones. Here’s a typical laboratory procedure:

1. Materials Required: DNPH (as Brady’s reagent), test carbonyl compound, pipette, test tubes, and solvent (methanol or ethanol).
2. Procedure:

• Place a small amount of Brady’s reagent in a test tube.
• Add a few drops of the test carbonyl compound (aldehyde or ketone).
• Shake or gently agitate the mixture and observe.

3. Result:

• The formation of a yellow, orange, or red precipitate confirms the presence of a carbonyl compound.
• The intensity and color of the precipitate may provide additional clues regarding the specific identity of the compound.

4. Chemical reaction involved:

5.0Reaction of 2,4-Dinitrophenylhydrazine with Ethanal 

Ethanal (Acetaldehyde) is a simple aldehyde with the formula CH₃O. When Ethanal reacts with DNPH, the following steps occur:

• Step 1: DNPH attacks the carbonyl group of Ethanal, leading to the formation of an unstable intermediate.
• Step 2: The intermediate undergoes dehydration to form ethanal-2,4-dinitrophenylhydrazone, a stable, colored precipitate.

Reaction:

This reaction forms a yellow or orange precipitate, confirming the presence of an aldehyde.

6.0Synthesis of 2,4-Dinitrophenylhydrazine

The synthesis of 2,4-Dinitrophenylhydrazine typically involves the following steps:

• The starting material is benzene, which undergoes nitration.
• Nitration: Benzene is treated with a mixture of concentrated nitric acid and sulfuric acid to introduce nitro groups at the 2 and 4 positions on the ring, forming 1,3-dinitrobenzene.
• Reduction to Hydrazine: The nitro groups are reduced to amine groups (-NH₂​), which are then treated to introduce a hydrazine (-NH₂​) moiety.

After these steps, DNPH is formed as a bright orange or red solid, stable under normal laboratory conditions.