2,4-Dinitrophenylhydrazine Test

What is a 2,4-DNP reagent?

The 2,4-dinitrophenylhydrazine reagent is a reddish-orange aqueous solution, also known as Brady’s reagent.

2,4-DNP Test

Addition reactions of derivatives of ammonia are important from the point of view of identification of carbonyl compounds. Addition is generally followed by elimination resulting in the formation of unsaturated compounds.

In reaction with 2,4-dinitrophenylhydrazine (2,4-DNP), they form the respective precipitate of 2,4–dinitrophenylhydrazones.

Aldehydes & ketones react with 2,4-dinitrophenylhydrazine reagents to give a yellow or red-orange precipitate. This test is used for the identification of both aldehyde and ketone compounds. Yellow precipitates indicate non-conjugated ketones or aldehydes, whereas red-orange precipitates indicate conjugated systems. This test differentiates ketones and aldehydes from alcohols and esters with which 2,4-DNP does not react, and thus no precipitate is formed. The aldehyde and ketone derivatives are crystalline materials with very well-defined melting points reported in the literature and can be used to identify specific compounds.

Experiment Description

2,4-Dinitrophenylhydrazine can be used to analyze aldehydes & ketones by forming yellow-orange precipitates qualitatively. Let’s confirm.

Aim of Experiment

To identify the presence of carbonyl compounds using the 2,4-dinitrophenylhydrazine test.

Material Required

One Test Tube Holder
2,4-Dinitrophenylhydrazine reagent
Test tubes (as per need)
Compound to be tested

Procedure

Take 2-3 drops of the liquid compound in a test tube.
Add a few drops of an alcoholic solution of 2,4-dinitrophenylhydrazine.
The appearance of a yellow-orange precipitate confirms the presence of a carbonyl group.
If precipitation does not appear at room temperature, warm the mixture in a water bath for a few minutes and cool it.

Precautions

Always use freshly prepared Brady’s reagent to perform the test.
Do not heat the reaction mixture directly over the flame if precipitation does not appear.

FAQs on

Q. Is reagent 2,4-DNP basic or acidic?

Ans: 2,4-DNP is basic in nature. It is mainly due to lone pairs of electrons present on the free amine of nitrogen atoms. This nitrogen atom acts as Lewis acid.

Q. What is Brady’s reagent?

Ans: An aqueous solution of 2,4-dinitrophenyl hydrazine (DNP) is known as Brady’s reagent.

Q. How to identify the presence of aldehydes or ketones?

Ans: Aldehydes and ketones can easily be identified using the chemical reactions of aldehydes and ketones with 2,4-DNP (popularly called as 2,4-DNP test). It reacts with carbonyl compounds (aldehyde and ketone) to give a coloured precipitate.

Q. Why are 2,4-Dinitrophenylhydrazones better derivatives than Phenylhydrazones?

Ans: 2,4-Dinitrophenylhydrazones are, for different reasons, safer derivatives than phenylhydrazones. First, higher molecular masses exist in these derivatives, increasing the volume of the substance to be checked. Heavier-mass derivatives also have a greater chance of becoming solid.

Q. Does amines react with Brady’s reagent?

Ans: Aldehydes & ketones react with 2,4-DNP (basic) to give yellow, orange, or reddish-orange precipitates, whereas amines do not. Amines are basic in nature.