Brady’s test for aldehydes and ketones

In this practical, students add various liquid aldehydes and ketones to 2,4-dinitrophenylhydrazine solution in a well-plate to form solid derivatives. They then do the same test with methanol and ethanol, showing that the reaction does not occur with alcohols.

The experiment should take around 15 minutes.

Equipment

Apparatus

• Plastic well-plate, 24 wells (eg Sigma ref: M 9655)

For preparation of 2,4-dinitrophenylhydrazine

• Beaker, 100 cm³

Chemicals

• Ethanol
• Propanone
• p-Methoxybenzaldehyde (or other aromatic aldehyde or ketone)
• Methanol
• Ethanal (Acetaldehyde)
• Solution of 2,4-dinitrophenylhydrazine (see preparation notes below) – requires 24 hours to dissolve completely

For preparation of 2,4-dinitrophenylhydrazine

• Deionised water

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout (splash-resistant goggles to BS EN166 3).
• 2,4-dinitrophenylhydrazine, (NO₂)₂ C₆ H₃ NHNH₂ (s), is FLAMMABLE and HARMFUL if swallowed. It is also dangerous in contact with oxidising agents, and can potentially become explosive if allowed to dry out completely. Avoid skin contact.
• Sulfuric acid, concentrated H₂ SO₄ (aq), is CORROSIVE and OXIDISING.
• Propanone is highly FLAMMABLE and IRRITANT to eyes and the respiratory system.
• p-Methoxybenzaldehyde is of low hazard.
• Methanol is highly FLAMMABLE, TOXIC by all routes and causes damage to organs (particularly CNS and optic nerve). Propanol could be used as a safer alternative to methanol. It is FLAMMABLE.
• Ethanal (acetaldehyde) is Highly FLAMMABLE, a carcinogen and an eye/respiratory IRRITANT. Protect face when opening containers and ensure laboratory is well-ventilated. Avoid contact with sulfuric acid – violent polymerisation occurs.

Preparing a solution of dinitrophenylhydrazine

1. Weigh out 0.5 g of 2,4-dinitrophenylhydrazine and dissolve in 10 cm³ of concentrated sulphuric acid in a 100 cm³ beaker. (Leave overnight to allow the compound to dissolve completely.)
2. Add this solution carefully to a solution of 15 cm³ of ethanol and 5 cm³ of deionised water. This gives the yellow-brown stock solution.

The solution can be used for about 10 days but it gradually deteriorates as a precipitate forms.

This procedure is based on that described in L. Harwood and R. Moody, Experimental organic chemistry – principles and practice, p 242. Oxford: Blackwell Scientific, 1989.

Procedure

1. In a well-plate, add 10 drops of 2,4-dinitrophenylhydrazine solution to each of the wells A1–A5 (see diagram below).
2. Carefully add three drops of ethanal to well A1 (ethanal is very volatile!).
3. Repeat adding three drops of the other liquids to wells A2–A5.
4. Observe any changes over the next few minutes.

Observations

Compound	Observation
Ethanal	Immediate yellow precipitate
Propanone	Yellow, crystalline precipitate forms slowly (after 1–2 minutes)
p -Methoxybenzaldehyde (or other aromatic aldehyde or ketone)	Immediate red precipitate
Methanol	No change
Ethanol	No change

Questions

1. Can you write equations for any reactions occurring?
2. Why do you think that these reactions serve such a useful purpose in identifying aldehydes and ketones?
3. Aldehydes and ketones will also form derivatives with hydrazine itself. What is the purpose of using 2,4-dinitrophenylhydrazine in this experiment instead of hydrazine?