Dinitrophenylhydrazine: Structure, Brady’s reagent, synthesis & Sample Questions

2,4- Dinitrophenyl hydrazine is a chemical compound belonging to the family of aromatic hydrazines, mostly referred to as 24-DNPH. The chemical formula of this compound is C₆H₃(NO₂)₂NHNH₂. It is formed by a substitution reaction on hydrazine. The structure of this compound comprises a benzene ring with one hydrazine group and the ortho and para positions occupied by nitro groups.

Key Terms: Hydrazine group, carbon, compounds, benzene, Reagents, hydrazine, nitro groups, substitution reaction, aromatic hydrazines, Benzene ring, substitution reaction

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Structure Of 2, 4 DNP

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Planar representation of the structure of 2,4-DNPH

Structure of DNP obtained through X-ray analysis

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Physical Properties

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• The compound in its solid-state exhibits a reddish-orange shade. 
• Due to the sensitivity of the solid towards pressure, the compound is mostly kept as a moist powder for laboratory purposes. 
• The container with the damp powder should be stored inside another container containing a small amount of water to maintain the level of moisture content in the compound. 
• The other physical properties of 2,4 DNPH include its slight solubility in water and a relatively high melting point of 471-475 K. 
• The presence of an aromatic benzene ring in the compound makes it carcinogenic. In addition to that, the compound is flammable.

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Brady's Reagent

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2,4-DNPH finds extensive use in instructional laboratories as a reagent chosen for performing qualitative analysis of different organic compounds. An aqueous solution of 2,4- DNPH with the addition of methanol and concentrated sulphuric acid is used in the identification process of ketones and aldehydes. This mixture is known as Brady’s reagent or Borche’s reagent. The objective of using this reagent is due to the reaction which leads to the formation of hydrazone, which forms a reddish yellow precipitate. The shade of the colour of precipitate depends on the type of carbonyl compound which is being put to test.

RR'C=O (generic ketone) + C6H3(NO2)2NHNH2 → C6H3(NO2)2NHN=CRR' + H2O

DNP test

The above reaction between a generic ketone and 2,4-DNPH features an addition-elimination reaction with the separation of water, forming the hydrazone. This is because hydrazine (H₂N-NH₂) being a powerful nucleophile, gets easily attached to the carbonyl group followed by the elimination process of the water molecule. 2,4-DNPH does not tend to react with compounds with lone pair electrons adjacent to the carbonyl group, which leads to delocalisation of the bonds by resonance. Eg- amides, Carboxylic Acids, etc.

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Synthesis of 2,4-Dinitrophenyl Hydrazine

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2,4- DNPH is prepared by treating 2,4-dinitrochlorobenzene with hydrazine in an acidic environment of sulphuric acid for promoting the nucleophilic addition of hydrazine to the benzene ring by replacement of the chlorine. Due to the high electronegativity of the chlorine atom as well as the electron drawing tendency of the nitro groups, the ring carbon atom joined to the chlorine is subjected to nucleophilic attack by hydrazine. The reaction intermediate product then separates the chlorine atom as a result of steric hindrance which then binds itself to the protons available in the acidic medium producing hydrogen chloride.

Synthesis of 2,4-DNPH

Reaction mechanism of 2,4-DNPH synthesis

The reason behind conducting the reaction in an acidic environment is to facilitate the detachment of the chlorine atom from the transition compound. The transition compound which is formed as the principal reaction intermediate is supposed to form an intramolecular hydrogen bonding that can hinder the detachment of the chlorine atom from the compound.

Intramolecular hydrogen bonding in reaction intermediate

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DNP Test Procedure

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The DNP test is done to detect aldehydes and ketones. It is a common test that is extensively preferred in laboratories as one of the most reliable methods. 

• The procedure includes pouring 5ml of 2,4-DNPH solution in a test tube and then adding around 10-12 drops of the test sample to it. 
• The mixture is to be mixed thoroughly. Generally, crystals are seen to form instantaneously, but if that is not the case, then the experimental setup is heated in a water bath at 60 degrees centigrade for around 5-7 minutes. 
• After that, the apparatus is provided with an ice bath till crystals are seen to form. 
• After drying and proper dehumidification, the melting point of the derivative crystals is analysed and matched with the charts for identification purposes. 
• The crystals formed as a result of this process are 2,4-dinitrophenylhydrazones. 
• These compounds have typical coloured crystals with definite melting points that can reliably help in detecting aldehydes and ketones.

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Reaction with Ethanal 

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2,4-Dinitrophenylhydrazine in a solution of methanol and sulphuric acid reacts with aldehydes and ketones to form orange precipitates within a few minutes. Two of such typical reactions include ethanal and propanal, producing hydrazones. The reaction given below represents the process by which Brady's reagent reacts with ethanal.

Brady's reagent reacts with ethanal

The end product formed as a result of this reaction is 2,4-dinitrophenylhydrazone. The hydrazone formed as a result of this reaction produces yellowish-orange crystals of characteristic sharp melting point. The above reaction follows the mechanism of a condensation reaction where there is a union of the molecules with the elimination of a water molecule from the end product.

Elimination of a water molecule from the end product

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Things to Remember

• 2,4-dinitrophenyl hydrazine is an aromatic hydrazine
• It is the most essential component of Brady’s reagent
• Brady’s reagent is used in laboratories for the detection of aldehydes and ketones
• The dry state of 2,4-DNPH is explosive, hence stored as a wet powder
• 2,4-DNPH reacts with ethanal and propanal to produce hydrazones with yellowish-orange crystals of characteristic melting point.

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