oxidising ketones Watch

Cool_Engineer
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I am in ChemEng... Gotta ques about organic chemistry lab

I understand that ketones are not supposed to undergo oxidation reactions with KMnO4 etc however i carried out a practical exp and acetone decolourised KMnO4 and a brown ppte formed - although the reaction took 10 mins or so.
I searched online for whether this is possible and apparently it is, if very stong [O] is used. :confused:

Anyway, does anyone know what the product formed? and the reaction equation for this (acetone with KMnO4)

Thanks
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vinny221
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Acetone is usually used as a weak ligand hence the decolourisation was caused by the formation of a Mn2+ complex. It's more than likely the brown ppt was MnO2 which forms instantaneously when adding the acetone step wise. The MnO2 is very unstable in solution with 2 pi bonds between p orbitals on oxygen and the d orbitals on Mn2+.

As you add the acetone, you replace the H2O ligands in solution hence change the crystal field splitting energy. If the compound goes colourless, the CFSE has become too large to allow transmission of light in the visible region.
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charco
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vigorous oxidation breaks the carbon skeleton and produces CH3COOH as well as HCOOH - this requires extreme conditions such as hot alkaline KMnO4. Side chains on benzene rings can also be oxidised in this way.
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Cool_Engineer
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thanks for the info.

do u know what the full ionic equation is? the exp was carried out under neutral conditions so just the KMnO4.

Coz i tried writing it out but not sure what to do with the acetone part - it seems like im forgetting what i learned in A-level

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Dr Pruss
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(Original post by Cool_Engineer)
I am in ChemEng... Gotta ques about organic chemistry lab

I understand that ketones are not supposed to undergo oxidation reactions with KMnO4 etc however i carried out a practical exp and acetone decolourised KMnO4 and a brown ppte formed - although the reaction took 10 mins or so.
I searched online for whether this is possible and apparently it is, if very stong [O] is used. :confused:

Anyway, does anyone know what the product formed? and the reaction equation for this (acetone with KMnO4)

Thanks
Ketones (or Alkanones to use the organic chemistry terminology from my part of central europe), will oxidise to Alkanals, and then end up as Alkanoic Acids.

Would an alkanoic acid have reacted with the potassium permanganate to create the precipitate observed?.
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Dr Pruss
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Ketones (generic alkanones) will not oxidise directly to acetic acid (alkanoic acid) . They will firstly go through an intermediate phase as an aldehyde or alkanal).

Alkanals or aldehydes are carbonyl bond containing compounds (thus reactive) intermediate organic chemical compounds obtained by oxidation of the primary alkanols are characterised by carbon hydroxy group COH.

Aldehydes are similar to the ketones such as acetone, which also contain a carbonyl group but.........

The difference is that in an aldehyde, the carbonyl group is attached to the end of a chain of carbon atoms, which is not the case with acetone (or the other ketones).

The way I remember it is that the word aldehyde is derived from “alcohol dehydrogenatum” (if my cod-Latin serves me well), or dehydrogenized alcohol.

The generic formula for an aldehyde is R–C=O (C –H). This is translated as a single bond between 1 carbon , therefore a molecuile that can be relatively easily oxidised by capturing a Oxygen atom from a donor such as the permanganate to form a simple alkanoic acid. Re wine does not immediately turn to vinegar it first goes through the taste-bud slamming alkanone and alkanal phase aided by tannins in the grape juice.

In your practical reaction with permanganates and acetone you may notice an acrid ‘vinegar’ smell towards the end of the reaction when you have created an alkanoic acid; as opposed to the ‘sweeter’ intermediate odour of the alkanones.

Not sure if this helps?.
Good luck.


Stop Press....Thanks for yourPM. Glad you have now solved it.
Regards
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