2 4 Dinitrophenylhydrazine - Structure, Preparation, Brady’s Reagent, FAQs

2,4-dinitrophenylhydrazine is generally abbreviated as 2,4-DNP also known by the Brady’s reagent. This is generally present in solid form and orange-red in color. It is very reactive in nature so to avoid the risk of explosion we keep it in water or we can say it is stable in wet conditions as compared to dry one.

2,4-Dinitrophenylhydrazine Structure

2,4-DNP structure can be shown as follows where DNP full form is dinitrophenylhydrazine

As the name suggests 2,4-dinitro i.e. presence of two nitrogen groups and phenyl hydrazine is represented by benzene ring with This group is called phenyl hydrazine.

The main use of 2,4-DNP and its derivatives gives us a method of separating the components of compounds containing aldehydes and ketones mixture. Aldehyde and ketone can easily separate out with the help of fractional distillation but we can easily separate 2,4-DNP and its derivatives with the help of column chromatography. One more advantage of this is that formation from aldehydes and ketones is reversible in nature and separated derivatives will be regenerated easily to their original carbonyl compounds with the help of hydrolysis of those derivatives.

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Preparation of 2 4 Dinitrophenylhydrazine from Chlorobenzene

2,4-DNP is not said to be commonly known substituted hydrazine but it can be prepare with the help of resonating phenomenon with this we can easily make a reasonable inference which suggests that hydrazine represented by the molecular formula acts as a good nucleophile. We can prepare the 2,4-DNP with the reaction of hydrazine with 2,4-dinitrochlorobenzene. Presence of two nitro groups makes the compound available for electron-accepting which makes it easy to displace these two chloride ions. The reaction can be shown as follows:

In this reaction we can see that 2,4-dinitrochlorobenzene reacts with hydrazine and one chloro group replaces it with hydrazine group.

Mechanism of Synthesis of 2,4-DNP

Mechanism can be explained by the following three steps:

1. Chlorobenzene is prepared from benzene by chlorination where the chlorine group is said to be ortho and para directing group and deactivating in nature.

2. After that we nitrate chlorobenzene with sulphuric and nitric acid. As chlorine is para or ortho directing then first para position is already blocked then two nitro groups will attack on ortho and para positions as nitro is also said to be ortho para directing group.

3. After this 2,4-dinitrochlorobenzene reacts with hydrazine which replaces the chlorine group with hydrazine and gives us the desired product.

The mechanism can be shown as follows:

Brady’s Reagent

Aqueous solution of 2,4-dintrophenyl hydrazine or 2,4-DNP is known as Brady’s reagent or borsches reagent. This reagent reacts with carbonyl compounds where carbonyl compounds are those which contain carbonyl group like aldehyde and ketone and carbonyl group is represent as while in aldehyde carbonyl group is present as while in ketone is there and gives colored precipitate with them. These precipitates generally show sharp melting points and these sharp melting points will confirm the presence of carbonyl compounds in them.

2,4-DNP test is given by only those compounds which are aldehyde and ketones in nature as when aldehyde and ketones reacts with 2,4-DNP then it forms yellow, orange or reddish orange precipitates with 2,4-DNP but when alcohols which do not have carbonyl compound gets react with 2,4-DNP the reaction will not get succeed or we can say no reaction takes place. This test is also used to check the presence of alcohol in the body. Benzaldehyde and acetone can be distinguished by this test.

How can we Identify Carbonyl Compounds with the help 2,4-DNP?

Carbonyl compound in methanol gets easily or we can say simply mixed with an acid solution of Brady’s reagent. The derivatives or product formed by this reaction is of orange color and crystalline solids which are further known by the name 2,4-dinitrophenylhydrazones. Crystals of 2,4-dintrophenylhydrazone are further filtered and purified by the process called re-crystallisation.

After drying, the melting point of the crystal has to be noted and compare the melting point with 2,4-dinitrophenyl hydrazines. If it got matched or we can say it gives colored precipitates then the carbonyl group gets confirmed.

The reaction can be shown as:

The main advantage of Brady’s test is that it is used to test whether the carbonyl compound is present or not and the presence of carbonyl compound in the form of aldehyde or ketone will give orange colored precipitates.

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2,4-Diphenylhydrazine Laboratory test

This test is generally known by its abbreviation called 2,4-DNP test. The full form 2 4 DNP test is 2,4-diphenylhydrazine laboratory test. Let us consider the test by first showing its image

This test can be explained by the following points:

1. Put 5 ml of 2,4-dinitrrophenylhydrazine in a test tube.

2. Now in this add 10 drops of any unknown compound and shake the test tube vigorously to mix them well and it will form crystals. If crystals were not formed then heat the test tube gently in a water bath about C for just 5-10 minutes.

3. After heating, cool the solution in an ice bath until you find the crystals. When crystals appear, wait some time to settle it down and then filter it through a Hirsch funnel like the funnel showing in image.

4. Now allow the crystals to dry with the help of Hirsch funnel by drawing air through crystals. Now after drying the crystals note down the melting point.

5. Take care that the crystals are dried properly and pure to note the accurate melting point. If you find the range of melting point is large then recrystallize the crystal with ethanol.

Reaction of 2,4-DNP with Ethanal

2,4-DNP is able to react with aldehydes as well as ketones which further produces 2,4-dinitrophenylhydrazone as a product. The reaction between 2,4-DNP and ethanal which is an aldehyde can be known as elimination reaction as in this reaction water molecules get eliminated and these reactions generally give orange and yellow colored solid compounds or precipitates of yellow/orange color. The melting point of these types of compounds is quietly sharp and the solution of 2,4-diphenylhydrazine is known by the name Brady’s reagent.

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NCERT Chemistry Notes :

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