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Q8.1(ix):
What is meant by the following term? Give an example of the reaction. (ix) 2,4-DNP-derivative

Solution :

1. Definition of 2,4-DNP-Derivative

A 2,4-DNP-derivative is the product formed when an aldehyde or a ketone reacts with 2,4-dinitrophenylhydrazine (2,4-DNP). This reaction is a type of nucleophilic addition-elimination reaction (specifically, a condensation reaction) that yields a crystalline solid known as a 2,4-dinitrophenylhydrazone. These derivatives are typically orange, yellow, or red in color and possess sharp melting points, making them highly effective for the identification and characterization of carbonyl compounds [Per the Chemistry of Nucleophilic Addition to Carbonyl Groups].

2. Chemical Mechanism

The reaction proceeds via the attack of the nucleophilic amino group ($-NH_2$) of 2,4-DNP on the electrophilic carbonyl carbon ($C=O$). This is followed by the elimination of a water molecule ($H_2O$) to form a double bond between the carbon and the nitrogen atom of the reagent.

General Equation:

$R_2C=O + H_2N-NH-C_6H_3(NO_2)_2 \rightarrow R_2C=N-NH-C_6H_3(NO_2)_2 + H_2O$

3. Visual Representation of the Reaction

R₂C=O + H₂N-NH-Ar R₂C=N-NH-Ar + H₂O (Where Ar = 2,4-Dinitrophenyl)

4. Specific Example: Reaction with Acetaldehyde

When acetaldehyde (ethanal) reacts with 2,4-dinitrophenylhydrazine, it forms acetaldehyde 2,4-dinitrophenylhydrazone.

Chemical Reaction:

$CH_3CHO + C_6H_3(NO_2)_2NHNH_2 \rightarrow CH_3CH=N-NH-C_6H_3(NO_2)_2 + H_2O$

5. Analytical Utility

Property Description
Color Typically yellow to deep orange-red crystalline solid.
Application Used for qualitative identification of carbonyl groups [By Brady’s Test reagent].
Identification The specific melting point of the derivative confirms the identity of the original aldehyde or ketone.

Final Solution: A 2,4-DNP-derivative is the stable, crystalline hydrazone formed by the condensation of an aldehyde or ketone with 2,4-dinitrophenylhydrazine, represented by the general product $R_2C=N-NH-C_6H_3(NO_2)_2$.


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