The reactions of carbonyl compounds

1 Solubility in water

There is the possibility of forming a hydrogen bond between a lone pair on the carbonyl oxygen atom and one of the hydrogen atoms in water. We should expect, therefore, that the smaller aldehydes and ketones would be soluble, but those with longer, non-polar carbon chains are not. This is the case. However, the video clearly shows that the propanone is miscible, but the propanal is not. Butanone is also miscible with water. I can't think of an obvious reason why the propanal is not soluble. You might like to think about this one further.

2 Oxidation with sodium dichromate(VI)

We saw in Unit 2 that primary alcohols are first oxidized to aldehydes and then to carboxylic acids by acidified sodium dichromate. The reaction that we see here is the second stage of this process. This reaction takes place quite readily. We see the orange to green colour change as the dichromate(VI) ion is reduced to the chromium(III) ion. This is a popular observation in both exams and practical assessments. Propanal is oxidized to propanoic acid. The full equation with the acidified dichromate(VI) is fairly complicated, but we can simplify it by using [O] to represent the oxygen from the oxidizing agent:

CH₃CH₂CHO + [O] ⇒CH₃CH₂COOH

Ketones are not readily oxidized, and so no change is seen on warming with the acidified sodium dichromate(VI) solution.

3 Oxidation with Benedict's solution

We see from the video that the propanone had no effect on the Benedict's solution, but the propanal produced the brick-red precipitate of copper(I) oxide. The propanal is oxidized to propanoic acid again, resulting in the reduction of the Benedict's solution. This is frequently used as a test to distinguish between aldehydes and ketones, and is another popular source of exam questions.

4 Oxidation with Tollen's reagent

This is exactly the same principle as the last two experiments. Tollen's reagent contains the Ag⁺ ion which is a very weak oxidising agent. The aldehyde is readily oxidised and so reduces the Ag⁺ ion to silver metal which appears as a beautiful silver mirror on the inner surface of the test tube. As the ketone is not readily oxidised it has no effect on Tollen's reagent. This provides yet another way to distinguish an aldehyde from a ketone.

5 Reaction with Brady's reagent

This is used as a test for aldehydes and ketones. An addition-elimination reaction occurs between the carbonyl group and the amine group of the hydrazine. Brightly coloured precipitates are formed known as 2,4-dinitrophenylhydrazones. These compounds can be separated and recrystallized to give pure solids. Determination of the melting point of these carbonyl derivatives enables us to identify the original aldehyde or ketone.

The reaction with propanal:

The reaction with propanone:

6 Reaction with iodine

Aldehydes and ketones both react in a similar way with iodine. Hydrogen iodide is produced and a hydrogen atom on a carbon adjacent to the carbonyl group is substituted for an iodine atom. As the iodine is used up the brown colour disappears. We see on the video that as the propanone is miscible with the water it mixes and reacts much faster than the propanal which formed a separate layer from the acid catalyst.

CH₃CH₂CHO + I₂ ⇒ CH₃CHICHO + HI

CH₃COCH₃ + I₂ ⇒ CH₃COCH₂I + HI