2,4-DNP Test for Aldehydes and Ketones

2,4-DNP Test for Aldehydes and Ketones

Aldehyde or Ketone

Standards
Cyclohexanone, Benzophenone, and Benzaldehyde

Procedure
Add a solution of 1 or 2 drops or 30 mg of unknown in 2 mL of 95% ethanol to 3 mL of 2,4-dinitrophenylhydrazine reagent. Shake vigorously, and, if no precipitate forms immediately, allow the solution to stand for 15 minutes.
The 2,4-dinitrophenylhydrazine reagent will already be prepared for you.

Positive test
Formation of a precipitate is a positive test.

Complications

Some ketones give oils which will not solidify.
Some allylic alcohols are oxidized by the reagent to aldehydes and give a positive test.
Some alcohols, if not purified, may contain aldehyde or ketone impurities.