Answer to Question #86427 in Organic Chemistry for Gayatri

Question

a) By taking a suitable example, give one laboratory test which is commonly used for the detection of aldehydes and ketones. Write the names of two reagents used for distinguishing between aldehydes and ketones.
b) Write chemical reactions for following name reaction:
i) Benzoin condensation
ii) Gattermann synthesis

Answer 1

a) 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.

 i) Benzoin Condensation

ii) Gattermann reaction