Addition of 2,4-Dinitrophenyl hydrazine to Aldehydes and Ketones

Demonstration Experiment on Video

Addition of 2,4-Dinitrophenyl hydrazine to Aldehydes and Ketones

Objectives: Nucleophilic Addition to the Carbonyl Function, Addition-Elimination

Peter Keusch

German version

Chemicals:
2,4-dinitrophenyl hydrazine
acetone
benzaldehyde
conc HCl
2 N HCl

Apparatus and glass wares:
2 conical measures, 350 mL
porcelain dish 50 mL
beaker 800 mL
2 glass stirring rods
2 snap-cap vials 10 mL

Reagent solution:
1.2 g of 2,4-dinitrophenyl hydrazine are doused with 12 mL of conc. HCl in a porcelain dish (fume hood!). The formed light yellow hydrochloride is mixed to produce a slurry that is poured into 600 mL of 2 N HCl while stirring. The hydrochloride should be perfectly dissolved.

Hazards and safety precautions:

2,4-dinitrophenyl hydrazine may cause irritation and may be harmful if swallowed. Skin and eye irritant.
Benzaldehyde may cause a skin allergy. It can affect you when breathed.

Acetone is highly flammable. Irritating to eyes.

Conc. HCl is extremely corrosive. Contact with the eyes or skin can cause serious permanent damage. Toxic by inhalation - the concentrated solution releases dangerous quantities of hydrogen chloride vapour.

Safety glasses and protective gloves should be worn. Good ventilation required. The reagent solution should be prepared inside a fume hood!

Experimental procedure:

Two conical measures are each filled with 300 mL of the hydrochloric acid solution of 2,4-dinitrophenyl hydrazine. 10 mL of acetone and benzaldehyde, respectively, are slowly added to the dinitrophenyl hydrazine solutions while stirring.

Results:

Copious crystalline precipitates are formed. Acetone gives a yellow precipitate and benzaldehyde gives a orange yellow precipitate.

Video clip (Download RealPlayer .rm file)

Discussion:

The formation of this precipitate is a positive test for the carbonyl group of ketones and aldehydes. The ketone or aldehyde is converted to its hydrazone by reaction with dinitrophenyl hydrazine.

A positive test is a yellow, orange, or red precipitate. Small crystals made from unconjugated aldehydes and ketones give precipitates toward the yellow end of the scale. Large crystals made from conjugated compounds tend to be more red.

Hydrazones have a sharp melting point and can assist in identifying carbonyl compounds,

References:
Demonstration Experiment on Video Addition of Bisulfite to Aldehydes
Demonstration Experiment on Video Schiff's Reagent
Microscale Projection Experiment Test for Aldehydes - Schiff's Reagent

Index of Lecture Experiments