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

What is 2,4-Dinitrophenylhydrazine (2,4 DNP Test)?

Brady’s reagent (2,4-dinitrophenylhydrazine) is a red-orange solid, usually supplied wet to reduce the risk of explosion.

Table of Contents

• 2,4-Dinitrophenylhydrazine Structure
  
• Synthesis of 2,4-Dinitrophenylhydrazine
  
• Brady’s Reagent
  
• Identifying a Carbonyl Compound
  
• 2,4-Dinitrophenylhydrazine Laboratory Test
  
• Reaction with Ethanal
  
• Frequently Asked Questions – FAQs

2,4-Dinitrophenylhydrazine Structure

The structure of 2,4-Dinitrophenylhydrazine is given below.

2,4-DNP derivatives offer a convenient way of separating the components of a mixture of aldehydes and ketones. Rather than attempting to do this directly by fractional distillation, a chemist might take advantage first of the easy separation of 2,4-DNP derivatives by column chromatography. Since their formation from aldehydes and ketones is reversible, hydrolysis of the separated derivatives will then regenerate the original carbonyl compounds.

Synthesis of 2,4-Dinitrophenylhydrazine

Although it’s not a familiarly substituted hydrazine, we can make a reasonable inference that hydrazine H₂N-NH₂ is a good nucleophile. So the synthesis of 2,4-Dinitrophenylhydrazine can be done from the product, it is prepared by reacting hydrazine with 2,4-dinitrochlorobenzene. The electron-accepting effect of the two nitro groups makes this chloride easy to displace.

Since chlorine is ortho, para directing and deactivating, so we should nitrate chlorobenzene. Chlorobenzene is readily prepared from benzene itself, so the synthesis is complete and the reaction is given below.

Brady’s Reagent

An aqueous solution of 2,4-dinitrophenyl hydrazine (DNP) is known as Brady’s reagent. It reacts with carbonyl compounds (aldehydes and ketone) to give a coloured precipitate. These precipitates have a sharp melting point. The melting points of the precipitates confirm the carbonyl compounds.

Benzoic acid is used as an antiseptic in medicines meant for urinary disorders and in vapour form for disinfecting bronchial tubes.

• Acetic anhydride reacts with N₂O₅ to give acetyl nitrate.

(CH₃CO)₂O + N₂O₅ → 2CH₃COONO₂ (Acetyl nitrate)

• The reaction of the acid chloride with water decreases with the increase of C-atoms in alkyl groups.

CH₃COCl > CH₃-CH₂-COCl > CH₃-CH₂-CH₂COCl > ………

Identifying a Carbonyl Compound

The carbonyl compound is simply mixed with an acid solution of Brady’s reagent in methanol. The derivatives are orange coloured crystalline solids called 2,4-dinitrophenylhydrazones. These crystals are filtered off and purified by re-crystallisation.

Their melting temperatures are measured. The original carbonyl compound is then identified by comparing to the tables of melting points of 2,4-dinitrophenylhydrazine derivatives.

Brady’s reagent can also be used as a test for the presence of a carbonyl compound because orange crystals appear when it is added to either an aldehyde or a ketone.

2,4-Dinitrophenylhydrazine Laboratory Test

• Place 5ml of the 2,4-dinitrophenylhydrazine reagent in a test tube.
• Add 10 drops of the unknown compound; sharply tap the test tube with your finger to mix. If crystals do not form immediately, gently heat in a water bath (60^oC) for 5min.
• Cool in an ice bath until crystals form. Collect the crystals by vacuum filtration using Hirsch funnel.
• Allow the crystals to dry on the Hirsch funnel by drawing air through the crystals. Take a melting point and record on the Report sheet.
• The crystals are usually pure enough to give a good melting point. However, if the melting point range is too large, recrystallize from a minimum volume of ethanol.

2,4-Dinitrophenylhydrazine Reaction with Ethanal

2,4-Dinitrophenylhydrazine reacts with both aldehydes and ketones to form a 2,4-dinitrophenylhydrazone. For example with ethanal;

• The reaction is a condensation reaction (water is eliminated). The product is named using the name of the aldehyde or ketone followed by 2,4-dinitrophenylhydrazone, for example, ethanal 2,4-dinitrophenylhydrazone.
• 2,4-dinitrophenylhydrazine can be used as a method for identifying aldehydes and ketones – the 2,4-dinitrophenylhydrazone formed is a solid that can be purified and its melting point determined. Comparison of the melting point with a table of known values could identify the aldehyde or ketone.
• The 2,4-dinitrophenylhydrazone derivatives are all orange/yellow solids.
• The solution of 2,4-dinitrophenylhydrazine is often called Brady’s reagent.