While I was studying my organic chemistry textbook, I came across the following reactions that need more clarifications:

oxidation of toluene, ethylbenzene and styrene with potassium permanganate in acidic medium

Oxidation of toluene (1) is the only reaction from the textbook, and I understand it.

I have a doubt about the product of ethylbenzene (2) oxidation. I have written the two products — benzoic acid (A) and 2‐phenylacetic acid (B) — which I'm not sure about. Does the ethyl substituent become a carboxylic or does it become a methyl carboxylic group?

For the oxidation of styrene (3) in addition to A and B I would propose one more product: styrene glycol (C) because I was taught that $\ce{KMnO4}$ oxidizes ethylene into diol. So, which of these three compounds will be the main product?

Lastly, are the products of reaction of ethylene with potassium permanganate in basic medium and in acidic medium different or the same? Please correct me if there are any errors.

  • 2
    $\begingroup$ It depends on the reaction conditions. KMnO4 is a very powerful oxidant. If you run it a high temperature with concentrated acid then phenylacetic acid loses CO2 to give toluene which is then oxidised to benzoic acid. Same with styrene, under controlled conditions you can get the diol, push harder and that oxidises further. $\endgroup$
    – Waylander
    Commented Aug 12, 2022 at 6:45
  • $\begingroup$ I do not think that the mechanism is from outside in. The benzylic position is active and ketones are isolated under less forceful conditions. Glycols do form and under acidic conditions will rapidly oxidize further, not much need to push harder. $\endgroup$
    – jimchmst
    Commented Aug 12, 2022 at 18:15
  • $\begingroup$ @Waylander I did some research and I guess your explanation is the correct way to understand this. $\endgroup$ Commented Aug 13, 2022 at 12:11
  • $\begingroup$ @jimchmst Thank you too for the explanation $\endgroup$ Commented Aug 13, 2022 at 12:11

1 Answer 1


After asking the question, I did some research about these reactions, and I think it would be valuable if I share it with the community.

ethene becomes a diol in the presence of basic medium with KMnO4, not when there is an acidic medium. In an acidic medium, it oxidizes completely into a carboxylic acid.

Styrene however, always oxidizes completely into benzoic acid. Same happens to any kind of alkyl (alkane, alkene, alkyne) that is combined with benzene.

I think this happens because, benzene is so rich with electrons and very unsaturated, KMnO4 is really interested in oxidizing benzene, but as we all know benzene will never oxidize under these normal conditions. Instead KMnO4 tries to oxidize the Carbon atom nearest to the benzene, which is not a part of benzene but has substituted to it, and so, Benzoic acid is made.

Hope this helps someone who had the same doubts as me...

  • $\begingroup$ With the proper choice of oxidant and conditions alkyl etc. benzenes can be oxidized to many different compounds with varying degrees of success. This is best experienced by actually running the reactions in the lab or possibly vicariously by reading actual lab results. Hot, acidic, permanganate or dichromate are brute force oxidants that eventually will oxidize almost any carbon compound to CO2 and H2O. $\endgroup$
    – jimchmst
    Commented Dec 30, 2023 at 17:07

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