Oxidation of aromatic alkanes with KMnO4 to give carboxylic acids

by James

Description: Treatment of an alkylbenzene with potassium permanganate results in oxidation to give the benzoic acid.

Notes: The position directly adjacent to an aromatic group is called the “benzylic” position.

The reaction only works if there is a hydrogen attached to the carbon.

Examples:

Notes: Note that in example 2 the extra carbons are cleaved to give the same product as in example 1. And in example 3, two benzoic acids are formed. Finally, when no hydrogens are present on the benzylic carbon, no reaction occurs (example 4).

Mechanism: For the purposes of Org 1/ Org 2 the mechanism isn’t considered that important. Manganese acts in mysterious ways. It’s thought that the first step is removal of a hydrogen by one of the oxygens on MnO4(–) in a free-radical reaction. Beyond that, it gets complicated.

{ 43 comments… read them below or add one }

TA April 7, 2012 at 3:05 am

In the second example, there is a three-carbon chain on the benzene, KMnO4 cleaves the chain leaving just one C (carboxylic acid). What is the other product formed then? Does the 2 carbon-chain cleaved off get oxidized too, so you’d have H3C-COOH? Or does it just stay as ethane or what? Essentially, does KMnO4 oxidize both sides of the benzylic hydrogens so it cleaves both sides into having a -COOH group on the cleaved side, or does KMnO4 only oxidize the side with the benzene to have a carboxylic acid?
Thanks for this great and very helpful website!

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james April 8, 2012 at 3:52 pm

Yes, the other product would cleave as a carboxylic acid to give H3C-COOH although it’s quite possible that under these reaction conditions the alpha-position of the carboxylic acid would undergo further oxidation to HOOC-COOH, (oxalic acid) which would then probably oxidize to CO2.

In short, yes – KMnO4 should oxidize both sides.

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Kayla April 10, 2012 at 10:27 pm

You said the mechanism for the KMnO4 causing a carboxylic acid formation to the CH3 of the benzene is too complex for this site. Do you know where I can find the mechanism?

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james April 12, 2012 at 3:05 am

You might start with looking through March’s Advanced Organic Chemistry for this reaction and follow the reference from there. Essentially the first step is that KMnO4 removes a hydrogen from the benzylic position, forming a benzyl radical, and the oxygen then “rebounds” back to the carbon to form C-O. This repeats several times; the overall mechanism can go through several different pathways, but this is the essentials of it.

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Flibbet April 21, 2012 at 10:51 pm

Does H2CrO4 have a similar mechanism/yield the same product?

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james April 22, 2012 at 1:19 am

Yes, I believe that H2CrO4 can also be used for this purpose. Not sure about the mechanism; in both cases, it’s fairly obscure.

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Erica White June 23, 2012 at 12:37 pm

If I have a molecule like methylidene cyclopentane..does KMnO4 react to give a CA??

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Erica White June 23, 2012 at 12:37 pm

If I have a molecule like methylidene cyclopentane..does KMnO4 react to give a CA??

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james June 24, 2012 at 12:00 am

For methylidene cyclopentane, it would cleave the C=C bond, to give cyclopentanone and formic acid. Actually the formic acid would probably become CO2.

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liz July 2, 2012 at 7:32 am

what is the difference in the oxidation products of hexane,cyclohexane,xylene,benzene,benzylchloride,and methyl benzene by KMnO4? i cant understand the reactions

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Bhavna July 4, 2012 at 4:33 pm

Can Kmno4 oxidise methane as well?
If not, why?

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james July 6, 2012 at 9:12 pm

Given enough heat, sure. It would give CO2. But so would plain old combustion.

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Princess August 2, 2012 at 1:30 pm

is it possible to get the actual mechanism involed, using example 1

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james August 4, 2012 at 3:21 pm

It’s a long sequence beginning with hydrogen abstraction and “rebound” of oxygen to carbon. There’s some uncertainty about the exact mechanism, so it hasn’t been shown.

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keven September 13, 2012 at 11:17 pm

should the KMnO4 be acididic or alkaline?

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nabil October 20, 2012 at 9:02 am

what other chemical that can be used to replace KMNO4??

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james October 23, 2012 at 11:17 am

CrO3 is also occasionally used for this reaction.

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Dollur January 6, 2013 at 8:20 am

Explain example 3 again,please?

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james January 7, 2013 at 7:02 pm

In this example both CH2 groups are cleaved to give carboxylic acids. The alkyl C-C bonds are cleaved in the process, resulting in breakage of the cyclohexane ring.

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Catherine February 13, 2013 at 12:08 pm

If there is a halogen let’s say attached to our alkyl group will it form carboxylic acid ? and if there is an alkene not directly attached to the benzene but in the side chain will it form carboxylic acid?

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james February 13, 2013 at 3:05 pm

Yes, as long as the carbon has at least one H in addition to the halogen, it will still form a caroboxylic acid – the chlorine will eventually get hydrolyzed. As far as the side chain is concerned, it will be completely cleaved until only the carboxylic acid attached to the ring remains.

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Andrew Lister March 8, 2013 at 7:48 am

Are there any other ways to remove the methyl group where the mechanism is known? i.e. substitute it for a Br or F? and then add a carboxylic acid afterwards?

Thanks

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Joei May 6, 2013 at 4:15 am

I want to know the mechanism of this reaction!

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Alex May 12, 2013 at 5:31 pm

Since in the second example the second product is acetic acid, would the products of reaction n°3 be phtalic acid and oxalic acid?

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james May 16, 2013 at 12:41 pm

Assuming excess KMnO4, it would probably eventually convert to oxalic acid and then 2 equivalents of CO2.

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Benedict June 10, 2013 at 9:00 am

Wouldn’t the methyl group be oxidised to methanoic acid as well?

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Benedict June 10, 2013 at 9:01 am

Oh sorry I forgot that methanoic acid is further oxidised to carbon dioxide and water.

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N June 14, 2013 at 3:21 am

Hello James,

Is it only KMnO4 has the ability to oxidise benzylic carbon to carboxylic acid, is there other oxidant capable of doing this?

Will this oxidation reaction also possible for allylic carbon?

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james June 19, 2013 at 9:02 pm

Great question! Yes, in theory, KMnO4 is useful for such a task. However in practice, other reagents such as SeO2 (among many others) are typically used for this reaction. Google “allylic oxidation”.

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adam September 9, 2013 at 4:37 am

Why is it preferred to use alkali when KMnO4 uses acid in the oxidation of alcohols?

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hannah December 14, 2013 at 1:57 am

my textbook says that reacting cyclopentene with KMnO4 will give you a di-ol…

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Tobi March 28, 2014 at 2:16 am

That’s an alkene. Ocidation of alkanes gives carboxylic acid except oxidation with CrO2Cl2 wich gives an aldehyde. Oxidation of alkenes gives diols and oxidation of alkynes gives a dioc acid

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mluleki March 1, 2014 at 4:20 am

Thanks for this great and very helpful website!!

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ashley April 10, 2014 at 4:58 am

Do we use acidified or alkaline KMnO4? Or it doesn’t really matter?
Great website though! Helped me so much! People like you deserve happiness :’)

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Boo Soon Yew May 4, 2014 at 5:39 pm

Hi Ashley.. I believe if you use alkaline KMnO4, then you will not get the observation “purple solution is decolourized”.. instead you might get “purple solution produces brown ppt”. Acidified KMnO4 is needed to give the “decolourized” observation.

BTW.. a BUG *THUMBS-UP* to those of you who manage this website. It is an EXCELLENT resource for Organic Chem, right till A Level !!! :)

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Ashley April 23, 2014 at 10:30 am

Say there is a halogen attached to the benzene ring, will it remain while the alkyl chain is oxidized? Thank you for this helpful page!

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James April 25, 2014 at 11:01 pm

yep! it will stay.

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Md.Motiar Rahman May 27, 2014 at 12:30 am

Why catalysts work only at side chain carbon atom? Why not carbon-H bond of benzene ring?

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Vaibhav G. June 30, 2014 at 1:55 am

thanks for the great info!
you mentioned that a benzylic H is necessary for this reaction to occur. However i read in Solomons that even PhCOCh2Ch3 reacts and gives Ph COOH .how can this happen?
thanks again.

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James June 30, 2014 at 4:46 pm

That’s an ester hydrolysis, not an oxidation. Sorry, saw an extra oxygen where there wasn’t one.
Yes, hydrogens on carbons adjacent to carbonyls contain weaker C-H bonds than do normal alkanes, so this reaction is possible. They are weaker because of an effect known as “captodative stabilization”, where the free radical initially generated is stabilized by interaction with the orbitals of the carbonyl. I know this is a very technical sounding answer, but it’s not dealt with very much in introductory organic chemistry.

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TuuT July 29, 2014 at 1:07 am

Isn’t PhCOCh2Ch3 an aromatic ketone?

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James July 30, 2014 at 3:39 pm

Thanks for pointing out my error. Corrected above.

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Parakh July 5, 2014 at 8:39 pm

Would this also work with chromium trioxide as the oxidising agent ?

Thanks,
Parakh

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