give the action of alkaline potassium permanganate on ethene and propene
Answers
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 with cold dilute potassium manganate(VII) solution
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:
Note: This type of equation is quite commonly used in organic chemistry. Oxygen written in square brackets is taken to mean "oxygen from an oxidising agent". The reason for this is that a more normal equation tends to obscure the organic change in a mass of other detail - as you will find below!
The full equations are given below, although you probably won't need them.
The full equation depends on the conditions.
Under acidic conditions, the manganate(VII) ions are reduced to manganese(II) ions.
Note: If you want to know how to write equations for redox reactions like this you could follow this link, and explore in the redox section of this site.
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Under alkaline conditions, the manganate(VII) ions are first reduced to green manganate(VI) ions . . .
. . . but eventually you get dark brown solid manganese(IV) oxide (manganese dioxide) formed. The overall equation for the formation of this from the manganate(VII) ions is:
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.
Note: You might possibly remember that further up the page it says that potassium manganate(VII) is often made slightly alkaline by adding sodium carbonate solution. Where are the hydroxide ions in this?
Carbonate ions react with water to some extent to produce hydrogencarbonate ions and hydroxide ions. It is the presence of these hydroxide ions that gives sodium carbonate solution its pH in the 10 - 11 region.