Chemguide: Support for CIE A level Chemistry

Learning outcome 17: Carbonyl compounds

17.1: Aldehydes and ketones

Statement 17.1.1

This statement is about the formation of aldehydes and ketones by the oxidation of alcohols.

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

Background work

Start by reading the page introducing aldehydes and ketones.

This covers naming and physical properties, together with a brief introduction to the reactivity of the compounds.

Statement 17.1.1(a) Is about the formation of aldehydes from primary alcohols.

Statement 17.1.1(b) Is about the formation of ketones from secondary alcohols.

You will find all this on the page about making aldehydes and ketones.

CIE is happy for you to use the simple versions of the equations with oxygen in square brackets.

Notice that when you make an aldehyde, you use an excess of the primary alcohol so that there isn't enough oxidising agent present to oxidise the aldehyde further to make an acid. You also distil off the aldehyde as it is formed.

When you make a ketone, the proportions don't matter, and you simply distil off the ketone.

CIE mentions the use of acidified potassium manganate(VII) for these reactions as well as the more usual acidified potassium dichromate(VI) described on the page you have read, and so you have to know about it.

The potassium manganate(VII) equations will be exactly the same as the potassium dichromate(VI) ones if you simplify them with [O].

The colour changes will be different:

  • The solution turns from orange to green during the potassium dichromate(VI) reaction.

  • The solution turns from purple to colourless during the potassium manganate(VII) reaction.

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© Jim Clark 2020