Many oxidising agents, like chromate, dichromate, iodine in $\ce{NaOH}$ etc. seem to work via ester formation and elimination. For example, chromic acid will react with the $\ce{-OH}$ of alcohol to form a chromate ester, and then this will undergo an E2-like elimination, with a proton abstracted from carbon and loss of leaving group from oxygen.

However it seems that manganate works via a different mechanism. I think this because here: http://www.masterorganicchemistry.com/2015/05/21/demystifying-alcohol-oxidations/

It says in a footnote at the bottom:

"..main exception you’ll encounter is $\ce{KMnO4}$, which likely proceeds through a C-H abstraction/internal return type mechanism followed by collapse of the hydrate to give the new carbonyl."

I don't quite understand this brief summary of the mechanism, could someone expand on it? I don't quite understand what "internal return type mechanism" is refering to - I kind of understand it refers to something a little like the "intimate ion pair" in the $\mathrm{S_Ni}$ mechanism, but haven't been able to find online a more expanded mechanism - many places seem to show it like the case of chromic acid above, and so I am slightly confused as to what mechanism it really follows.

  • $\begingroup$ March 7th ed. (p 1436) seems to imply that it is a ester formation followed by elimination, the same as it is with CrO3/H2SO4. Unfortunately, I am not quite sure what the phrasing above is referring to. $\endgroup$ Apr 15, 2016 at 19:55
  • $\begingroup$ @orthocresol I have seen ester formation followed by elimination for manganate, but the footnote seems to imply that manganate is somehow different in it's mechanism, but I couldn't understand what the phrasing of it meant either. $\endgroup$ Apr 15, 2016 at 20:27

1 Answer 1


$\ce{KMnO4/OH-}$ Oxidation

KMnO4 Oxidation

$\ce{H2CrO4}$ Oxidation Jones Oxidation

The two main differences are

  1. $\ce{H2CrO4}$ Oxidation occurs in acid, $\ce{KMnO4}$ oxidation occurs in base
  2. The source of the nucleophile in the removal of the metal ester. In $\ce{KMnO4}$ oxidation, the $\ce{Mn}$ ester itself extracts the $\ce{H+}$ from the alcohol carbon, while in $\ce{H2CrO4}$ oxidation the nucleophile is the solvent.
  • 1
    $\begingroup$ Are you sure about your first difference? I am very sure manganate oxidation can take place in both acid and base, depending on how harsh you want the conditions to be. $\endgroup$ Aug 27, 2018 at 23:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.