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I was conducting a lab where we have to identify/justify how functional groups affect the oxidation of a molecule.

I used propene, propanone, and propanoic acid for this reaction. From what I have learned in class, ketones and carboxylic acids cannot get oxidized. However, in my experiment the ketone somehow is not getting oxidized while the propanoic acid is. I tried using butanoic acid just to be sure but it turns out that it gets oxidized as well. I have also made sure that there are no contaminants. I used 4 ml of 100% propanoic acid with 0.003 mol of $\ce{KMnO4}$.

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  • $\begingroup$ How do you know the carboxylic acids are being oxidized? $\endgroup$
    – jerepierre
    Nov 24, 2015 at 20:25
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    $\begingroup$ I know it because when KMnO4 is added it reacts immediately, ie the pink/purple colour goes away suddenly as it did with the propene. It stays pink with the ketone but not with the carboxylic acids. I asked the Chemistry teachers but they don't know any reason either. @jerepierre $\endgroup$
    – Nekhil Nagia
    Nov 24, 2015 at 22:00
  • $\begingroup$ The propanoic acid cation is not being oxidised here to form proponone . An acid/base reaction is occurring where the permanganate is acting as the base. An aldehyde may react with the KMnO4 to form the ketone . $\endgroup$
    – Technetium
    May 9, 2016 at 6:16

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Can you detail the concentrations of both the acid as well as $\ce{KMnO4}$ and also the time it took to de-colourise?

I tried an experiment however, I only have Glacial Acetic Acid, which didn't seem to react ( as the smell was still prominent).

I can suggest you a simple trick to analyse whether it is decomposing. Smell Propionic acid before adding $\ce{KMnO4}$. Then smell it after the colour of $\ce{KMnO4}$ is gone. Keep adding $\ce{KMnO4}$ till the pink colour stay(similar to titration). Then smell again. If the smell is gone then the carboxylic acid is oxidised.

There are 2 possible oxidation products of carboxylic acid:

  1. Ketone
  2. Carbon Dioxide

Both can be pretty obvious. Ketone will smell differently than the acid(more like nail polish remover) and Carbon Dioxide should be visible because of effervescence.

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Could it be that there is an isohypsic (non redox) reaction that changes $\ce{KMnO4}$ into another $\ce{Mn(VII)}$ compound? According to wikipedia, it has been known to happen with other acids.

It would be nice to know if there were other methods available to analyze the reaction mixture (such as TLC).

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