OXIDATION OF ALDEHYDES AND KETONES

This page looks at ways of distinguishing between aldehydes and ketones using oxidising agents such as acidified potassium dichromate(VI) solution, Tollens' reagent, Fehling's solution and Benedict's solution.

Background

Why do aldehydes and ketones behave differently?

You will remember that the difference between an aldehyde and a ketone is the presence of a hydrogen atom attached to the carbon-oxygen double bond in the aldehyde. Ketones don't have that hydrogen.

The presence of that hydrogen atom makes aldehydes very easy to oxidise. Or, put another way, they are strong reducing agents.

Because ketones don't have that particular hydrogen atom, they are resistant to oxidation. Only very strong oxidising agents like potassium manganate(VII) solution (potassium permanganate solution) oxidise ketones - and they do it in a destructive way, breaking carbon-carbon bonds.

Provided you avoid using these powerful oxidising agents, you can easily tell the difference between an aldehyde and a ketone. Aldehydes are easily oxidised by all sorts of different oxidising agents: ketones aren't.

You will find details of these reactions further down the page.

What is formed when aldehydes are oxidised?

It depends on whether the reaction is done under acidic or alkaline conditions. Under acidic conditions, the aldehyde is oxidised to a carboxylic acid. Under alkaline conditions, this couldn't form because it would react with the alkali. A salt is formed instead.

Building equations for the oxidation reactions

If you need to work out the equations for these reactions, the only reliable way of building them is to use electron-half-equations.

The half-equation for the oxidation of the aldehyde obviously varies depending on whether you are doing the reaction under acidic or alkaline conditions.

Under acidic conditions it is:

. . . and under alkaline conditions:

Because I am going to be using electron-half-equations quite a lot on the rest of this page, it would definitely be worth following this link if you aren't happy about them.

These half-equations are then combined with the half-equations from whatever oxidising agent you are using. Examples are given in detail below.

Specific examples

In each of the following examples, we are assuming that you know that you have either an aldehyde or a ketone. There are lots of other things which could also give positive results.

Assuming that you know it has to be one or the other, in each case, a ketone does nothing. Only an aldehyde gives a positive result.

Using acidified potassium dichromate(VI) solution

A small amount of potassium dichromate(VI) solution is acidified with dilute sulphuric acid and a few drops of the aldehyde or ketone are added. If nothing happens in the cold, the mixture is warmed gently for a couple of minutes - for example, in a beaker of hot water.

The orange dichromate(VI) ions have been reduced to green chromium(III) ions by the aldehyde. In turn the aldehyde is oxidised to the corresponding carboxylic acid.

The electron-half-equation for the reduction of dichromate(VI) ions is:

Combining that with the half-equation for the oxidation of an aldehyde under acidic conditions:

. . . gives the overall equation:

The problem is that what is important in using these reactions as tests is the colour change in the oxidising agent. In this particular reaction, you have to explain, for example, why the solution turns green. Any equation that you write has got to show the production of the chromium(III) ions.

Using Tollens' reagent (the silver mirror test)

Tollens' reagent contains the diamminesilver(I) ion, [Ag(NH₃)₂]⁺.

This is made from silver(I) nitrate solution. You add a drop of sodium hydroxide solution to give a precipitate of silver(I) oxide, and then add just enough dilute ammonia solution to redissolve the precipitate.

To carry out the test, you add a few drops of the aldehyde or ketone to the freshly prepared reagent, and warm gently in a hot water bath for a few minutes.

Aldehydes reduce the diamminesilver(I) ion to metallic silver. Because the solution is alkaline, the aldehyde itself is oxidised to a salt of the corresponding carboxylic acid.

The electron-half-equation for the reduction of the diamminesilver(I) ions to silver is:

Combining that with the half-equation for the oxidation of an aldehyde under alkaline conditions:

. . . gives the overall equation:

Using Fehling's solution or Benedict's solution

Fehling's solution and Benedict's solution are variants of essentially the same thing. Both contain complexed copper(II) ions in an alkaline solution.

Fehling's solution contains copper(II) ions complexed with tartrate ions in sodium hydroxide solution. Complexing the copper(II) ions with tartrate ions prevents precipitation of copper(II) hydroxide.

Benedict's solution contains copper(II) ions complexed with citrate ions in sodium carbonate solution. Again, complexing the copper(II) ions prevents the formation of a precipitate - this time of copper(II) carbonate.

Both solutions are used in the same way. A few drops of the aldehyde or ketone are added to the reagent, and the mixture is warmed gently in a hot water bath for a few minutes.

Aldehydes reduce the complexed copper(II) ion to copper(I) oxide. Because the solution is alkaline, the aldehyde itself is oxidised to a salt of the corresponding carboxylic acid.

The equations for these reactions are always simplified to avoid having to write in the formulae for the tartrate or citrate ions in the copper complexes. The electron-half-equations for both Fehling's solution and Benedict's solution can be written as:

Combining that with the half-equation for the oxidation of an aldehyde under alkaline conditions:

. . . gives the overall equation: