Question

Giving chemical reaction write what happens when acidified potassium permanganate solution reacts with potassium iodide solution​

Answer 1

Answer:

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.

Use the BACK button (or HISTORY file or GO menu) on your browser to return to this page later.

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.

Using the reaction to test for carbon-carbon double bonds

If an organic compound reacts with dilute alkaline potassium manganate(VII) solution in the cold 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.

Note:  You will find details of the use of bromine water in testing for carbon-carbon double bonds in the page about the reactions of alkenes with halogens.

Oxidation of alkenes with hot concentrated acidified potassium manganate(VII) solution

This is where it gets complicated! Check with your syllabus to see whether you need to know about it before you go any further. This section was written to cover a statement in the Cambridge International (CIE) A level syllabus.