ALKENES and POTASSIUM MANGANATE(VII)

This page looks at the reaction of the carbon-carbon double bond in alkenes such as ethene with potassium manganate(VII) solution (potassium permanganate solution).

Oxidation of alkenes

Experimental details

Alkenes react with potassium manganate(VII) solution in the cold. The colour change depends on whether the potassium manganate(VII) is used under acidic or alkaline conditions.

If the potassium manganate(VII) solution is acidified with dilute sulphuric acid, the purple solution becomes colourless.

If the potassium manganate(VII) solution is made slightly alkaline (often by adding sodium carbonate solution), the purple solution first becomes dark green and then produces a dark brown precipitate.

Chemistry of the reaction

We'll look at the reaction with ethene. Other alkenes react in just the same way.

Manganate(VII) ions are a strong oxidising agent, and in the first instance oxidise ethene to ethane-1,2-diol (old name: ethylene glycol).

Looking at the equation purely from the point of view of the organic reaction:

The full equation depends on the conditions.

Under acidic conditions, the manganate(VII) ions are reduced to manganese(II) ions.

Under alkaline conditions, the manganate(VII) ions are first reduced to green manganate(VI) ions . . .

. . . and then further to dark brown solid manganese(IV) oxide (manganese dioxide).

This last reaction is also the one you would get if the reaction was done under neutral conditions. You will notice that there are neither hydrogen ions nor hydroxide ions on the left-hand side of the equation.

Complications

(. . . and you thought this was already complicated enough?)

The product, ethane-1,2-diol, is itself quite easily oxidised by manganate(VII) ions, and so the reaction won't stop at this point unless the potassium manganate(VII) solution is very dilute, very cold, and preferably not under acidic conditions.

That means that this reaction has little use as a way of preparing ethane-1,2-diol. Its only real use is in testing for carbon-carbon double bonds - and even then it isn't very good!

That page deals with the oxidation of alcohols by acidified potassium dichromate(VI) solution - a slightly less powerful oxidising agent than potassium manganate(VII). The essential chemistry will be the same, although manganate(VII) ions eventually oxidise ethane-1,2-diol all the way to carbon dioxide and water. You only need read the first part of that page about oxidation of primary alcohols.

Using the reaction to test for carbon-carbon double bonds

If an organic compound reacts with dilute alkaline potassium manganate(VII) solution to give a green solution followed by a dark brown precipitate, then it may contain a carbon-carbon double bond. But equally it could be any one of a large number of other compounds all of which can be oxidised by manganate(VII) ions under alkaline conditions.

The situation with acidified potassium manganate(VII) solution is even worse because it has a tendency to break carbon-carbon bonds. It reacts destructively with a large number of organic compounds and is rarely used in organic chemistry.

You could use alkaline potassium manganate(VII) solution if, for example, all you had to do was to find out whether a hydrocarbon was an alkane or an alkene - in other words, if there was nothing else present which could be oxidised.

It isn't a useful test. Bromine water is far more clear cut.