How can aldehydes and ketones be distinguished?
Firstly note that in general aldehydes are more reactive than ketones. This is a result of two effects:
1. Ketones are more sterically hindered.
2. Alkyl groups are electron donating and so reduce the partial positive charge on the carbonyl carbon.
As a consequence of this difference in reactivity aldehydes are oxidised more easily than ketones and so, by selecting a sufficiently weak oxidising agent, we can distinguish the two functional groups by oxidising one but not the other.
Fehling's Test
The test begins as two separate solutions - Fehling's A and Fehling's B. The first is a light blue CuSO4 solution while the second is a solution of a chelate and sodium hydroxide.
Equal volumes of the two solutions are mixed and the sample is added. The resulting solution is heated.
Aldehyde - The aldehyde is oxidised and a brick red Cu(I) oxide precipitates out,
Ketone - No reaction occurs.
The Silver Mirror Test
This test makes use of Tollen's reagent which contains the complex [Ag(NH3)2]+. It is easily made by mixing aqueous ammonia with aqueous silver nitrate.
Aldehyde - Upon heating with Tollen's reagent solid silver metal is produced as Ag+ is reduced to Ag.
Ketone - No reaction occurs.
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