Identification of Unknown Aldehydes and Ketones

Aldehydes and ketones have a similar structure. Both possess a carbonyl group, which is a carbon double bonded to an oxygen. An aldehyde has at least one hydrogen connected to the carbonyl carbon. The second group is either a hydrogen or a carbon-based group. In contrast, a ketone has two carbon-based groups connected to the carbonyl carbon. Certain reactions undergone by aldehydes and ketones can be used to distinguish them or identify their functional groups based on visible differences in the reaction's outcome.

One such reaction is the DNPH test, which is used to determine whether an aldehyde or ketone is aromatic. In this reaction, 2,4-dinitrophenylhydrazine, or DNPH, attacks the carbonyl of an aldehyde or ketone in an aqueous acidic solution. This condensation reaction produces a hydrazone, which precipitates from the aqueous solution.

When a non-aromatic ketone or aldehyde reacts with DNPH, the precipitate is yellow. However, aromatic ketones and aldehydes give a red-orange precipitate. DNPH can also be used to distinguish alcohols and esters from aldehydes and ketones since DNPH does not react with alcohols or esters. We call this lack of reaction a negative result.

Another useful reaction is used as the iodoform test for methyl ketones, which are ketones that have at least one methyl as a functional group. When a methyl ketone is mixed with iodine under aqueous alkaline conditions, iodine replaces each methyl hydrogen, making an excellent leaving group. Substitution, followed by proton transfer, converts the leaving group into iodoform, which precipitates from the solution as a pale yellow solid.

This reaction also works for acetaldehyde, which is an aldehyde with methyl as its R group. The reaction relies on the unique reactivity of the hydrogens in an alpha-methyl group, so mixing any other ketone or aldehyde with iodine will not make solid yellow iodoform.

Finally, we can distinguish between ketones and aldehydes using the Tollens test. Diamminesilver(1+), or Tollens' reagent, oxidizes aldehydes to carboxylic acids. Tollens' reagent is reduced to elemental silver in the process, which either coats the inner wall of the test tube or forms a black precipitate. Most ketones, however, are not oxidized by Tollens' reagent, so no solid silver will form.

In this lab, you will identify several unknown aldehydes and ketones by performing the DNPH test, the Tollens test, and the iodoform test.