This page reviews the tests that are used to identify carbonyl compounds and to distinguish between aldehydes and ketones.
Tests for carbonyl compounds Test : Add the unknown substance to a solution of 2,4-dinitrophenylhydrazine(2,4-DNPH). Observation : Carbonyl compounds turn from green to an orange precipitate. Test : Using the infra-red spectrum. Observation : They have a strong C=O absorption at a wavenumber around 1700cm-1 | Tests for aldehydes Test : Warm with a solution of silver nitrate and ammonia (ammoniacal silver nitrate), also known as Tollens' reagent. Observation : Aldehydes produce a silver mirror due to the silver ion becoming reduced to the silver metal. The aldehyde is refluxed to a carboxylic acid. Test : Warm with Fehling's (Benedict's) solution. Observation : Aldehydes produce a red precipitate. Ketones stay blue. |
