Using 2,4-Dinitrophenylhydrazine
- 2,4-dinitrophenylhydrazine (also known as 2,4-DNPH) is a reagent which detects the presence of carbonyl compounds (compounds with -C=O group)
- The carbonyl group of aldehydes and ketones undergoes a condensation reaction with 2,4-dinitrophenylhydrazine
- A condensation reaction is a reaction in which two molecules join together and a small molecule (such as H2O or HCl) is eliminated
- The product formed when 2,4-DNPH is added to a solution that contains an aldehyde or ketone is a deep-orange precipitate which can be purified by recrystallisation
- The melting point of the formed precipitate can then be measured and compared to literature values to find out which specific aldehyde or ketone had reacted with 2,4-DNPH
Ketones and aldehydes react with 2,4-DNPH in a condensation reaction
The test tube on the left shows a negative 2,4-DNPH test and the tube on the right shows a positive test
Examiner Tip
The 2,4-DNPH test is especially useful as other carbonyl compounds such as carboxylic acids and esters do not give a positive result