Aldehydes and ketones both contain the carbonyl group, C=O. Two tests are used together.1. 2,4-Dinitrophenylhydrazine (2,4-DNP), also called Brady's reagent, detects any carbonyl group. Add a few drops of the sample to 2,4-DNP solution:• aldehydes and ketones give a yellow or orange precipitate (a 2,4-dinitrophenylhydrazone derivative)
• other compounds containing C=O, such as carboxylic acids and esters, do notThe precipitate can be filtered, recrystallised and its melting point measured. Comparing this with known values of 2,4-DNP derivatives identifies the particular aldehyde or ketone.2. Tollens' reagent (ammoniacal silver nitrate) distinguishes an aldehyde from a ketone. Warm the sample with Tollens' reagent in a water bath:• an aldehyde gives a silver mirror: the aldehyde is oxidised to a carboxylic acid and Ag+ is reduced to silver
• a ketone gives no reaction, because ketones are not easily oxidisedAg+(aq) + e− → Ag(s) Worked example: two isomers, C4H8O, both give an orange precipitate with 2,4-DNP. Isomer J gives a silver mirror; isomer K does not. J is an aldehyde, e.g. butanal; K is a ketone, butanone.Tollens' reagent is made fresh and warmed in a water bath, not over a flame.
• other compounds containing C=O, such as carboxylic acids and esters, do notThe precipitate can be filtered, recrystallised and its melting point measured. Comparing this with known values of 2,4-DNP derivatives identifies the particular aldehyde or ketone.2. Tollens' reagent (ammoniacal silver nitrate) distinguishes an aldehyde from a ketone. Warm the sample with Tollens' reagent in a water bath:• an aldehyde gives a silver mirror: the aldehyde is oxidised to a carboxylic acid and Ag+ is reduced to silver
• a ketone gives no reaction, because ketones are not easily oxidised