Potassium Manganate! (AS Edexcel)

Could someone clear up all the things we have to know about potassium manganate (VII)? Looking at mark schemes, they seem to say:

- KMnO4 in alkaline solution, shake with alkene at room temp: brown precipitate produced as it is reduced and alkene becomes a diol. Some textbooks say neutral however, which is correct??

- KMnO4 in acidic solution with alkene, purple goes colourless? Some textbooks say it goes colourless with alochol also?

- KMnO4 (don't know about pH?), purple to colourless with sulphur dioxide. Never heard this one before but saw it on a mark scheme :s-smilie:

Are those correct? :s-smilie:

Any more we need to know?

Reply 1

Tes

I know potassium manganate is like potssium dichromate, except it is a stronger oxidisng agent. It will oxidise alcohols to aldehydes and carcoxylic acids or ketones and get reduced itself.

At lower pH's is disproportionates into permanganate and manganese dioxide and the colour changes (green to brown i think).
3 K2MnO4 + 2 H2O → 2 KMnO4 + MnO2 + 4 KOH

It'll react with an alkene similarly to how potassium dichromate does.

Reply 2

charco

Study Forum Helper

potassium manganate (VII) is a dark purple strong oxidising agent that behaves differently in acid and basic solutions.

In acid:

MnO4(-) + 8H+ + 5e --> Mn2+ + 4H2O

In base:

MnO4(-) + 2H2O + 3e --> MnO2 + 4OH-

With alkenes, as correctly stated, it undergoes an addition reaction that proceeds via a cyclic intermediate producing a diol. It also gives a brown ppt of MnO2

Under acid conditions the final solution appears colourless after reduction as the Mn2+ is very pale pink.

It is strong enough under both acid and base conditions to oxidise alcohols.

In aromatic chemistry, the basic conditions are used to oxidise alkyl sidechains to carboxylic acids (acidic conditions are not used as this may rupture the aromatic ring itself)

Reply 3

RMIM

http://www.chemguide.co.uk/organicprops/alkenes/kmno4.html#top

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.

Reply 4

RMIM

Not sure if it help but got this off a site

Sulphur dioxide is a reducing agent (which reduces ions such as Fe3+, MnO4-, and Cr2O72-)

Reply 5

charco

Study Forum Helper

Sulphur dioxide is a common reducing agent usually used in the form of sodium bisulphite (which releases SO2)

SO2 + 2H2O ---> SO4(2-) + 2e + 4H+

so it reduced MnO4(-) according to the equation:

5[SO2 + 2H2O ---> SO4(2-) + 2e + 4H+]
2[MnO4(-) + 8H+ + 5e --> Mn2+ + 4H2O]
--------------------------------------
5SO2 + 10H2O + 2MnO4(-) + 16H+ + 10e ---> 5SO4(2-) + 10e + 20H+ + 2Mn2+ + 8H2O
-------------------------------------------- cancelling down
5SO2 + 2H2O + 2MnO4(-) ---> 5SO4(2-) + 4H+ + 2Mn2+

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