Chemguide: Support for CIE A level Chemistry

This statement is about distinguishing between aldehydes and ketones using various oxidising agents.

Before you go on, you should find and read the statement in your copy of the syllabus.

You will find all of this on the page oxidation of aldehydes and ketones.

The most important thing to take from this is that aldehydes are easily oxidised to carboxylic acids (or their salts) by all of the reagents listed: acidified potassium dichromate(VI) solution, Fehling's solution and Tollens' reagent.

On the other hand, ketones are resistant to oxidation by all of them.

In testing, first you have to show that you have an aldehyde or ketone. You can do this with 2,4-DNPH. Then you can do one of these tests to find out which you have got.

If you get a positive result for any of these reactions, you have an aldehyde. A negative result means that you have a ketone.

Do you need the equations for the reactions?

The syllabus asks that you should be able to work out whether you have an aldehyde or a ketone from the results of these tests. That doesn't imply any need to know the equations of the reactions.

Support material for teachers says that you should know the identities of the inorganic products of the Fehling's and Tollens' test (copper(I) oxide and silver respectively).

The support material then goes on to say "The equations for their formation are not too difficult." Does that mean that you have to know them? Probably not.

At the time of writing, no question involving these equations had been asked. Whether or not you find the equations difficult depends on how much time you have spent learning how to write equations from electron-half-equations. If you look at the equations on the page you have read, and find them scary, ignore them! Even if you don't find them scary, I wouldn't spend too much time on them.

The potassium manganate(VII) problem

This isn't mentioned by the syllabus or the support material, but you should be aware of the problem, particularly because the syllabus mentions phenylethanone as one of the ketones you need to know about.

Although you can oxidise aldehydes with potassium manganate(VII) solution, you wouldn't choose to use it to distinguish between aldehydes and ketones.

Potassium manganate(VII) is a powerful enough oxidising agent to break carbon-carbon bonds in ketones, and so you wouldn't get a reliable result.

That is particularly important with the ketone phenylethanone, or any other ketone where the carbonyl group is on a side chain attached to a benzene ring. Acidified potassium manganate oxidises any carbon-containing side chain in these circumstances to a -COOH group attached to the ring.

In these cases, you could get exactly the same colour change whether you had an aldehyde or a ketone. That makes it a useless test.

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