How can aldehydes and ketones be distinguished?

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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 CuSO₄solution 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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