Master Organic Chemistry Reaction Guide

Dihydroxylation of alkenes with cold, dilute KMnO4 to give vicinal diols

Description: Treatment of alkenes with cold, dilute basic KMnO4 leads to 1,2-diols (vicinal diols).


  • The reaction proceeds with “syn” stereochemistry of the alkene, meaning that the two alcohols end up on the same side of the alkene
  • Also note that osmium tetroxide (OsO4) does exactly the same reaction.
  • The purpose of the NaOH is to assist in breaking up the intermediate manganate ester that forms after dihydroxylation. If this manganate ester is allowed to sit around, oxidative cleavage of the diol may occur. This is also why the temperature is kept cold.


Notes: The reaction works well so long as it is kept cold. If higher temperatures are used, cleavage of the diol to give carbonyl compounds is observed. Note that the reaction does not occur with alkynes.

Mechanisms: Potassium permanganate adds to one face of the alkene through a cycloaddition reaction (Step 1, arrows A, B and C) to give a cyclic manganese compound (“manganate ester”). The manganate ester is then reduced with NaHSO3 or KHSO3 and hydrolyzed to the diol through a very long process that is excruciatingly boring to write out and generally not bothered with in Org 1/ Org 2

Notes: The boring part goes something like this: water attacks Mn, transfer proton to O, break Mn–O bond, then add second equivalent of water to Mn, transfer proton to O, break Mn–O bond. This gives the free diol.

Reference: For those curious about the mechanism I strongly suggest you read this paper: “Permanganate Peroxidation of Cyclohexene: Hydroxide Ion And Salt Effect Studies. Taylor, J. E. ; Green, R. Can. J. Chem. 63, 2777 (1985). 

“Good yields of cis-l,2-cyclohexanediol were formed by the reaction of cyclohexene and aqueous potassium permanganate under turbulent stirring conditions only in the presence of low concentrations of sodium hydroxide. Larger amounts had no further benefit or were deleterious.”

Also, apparently under acidic or neutral conditions, more highly oxidized products such as alpha-hydroxy ketones can be formed “without going through the glycols”. See J. Am. Chem. Soc. 1981, 103, 938.  



Comment section

64 thoughts on “Dihydroxylation of alkenes with cold, dilute KMnO4 to give vicinal diols

  1. wud like to know C2H4 + H2O + [O]——-> C2H2(OH)2. Pls explain to me where the water molecule comes from when alkene reacts acidified with potassium manganate (VII). tq

  2. Hi James,
    This is very helpful! Thank you.
    So, when KMnO4 is added to an alkene under high temperatures, then it further oxidizes it correct?
    Does it form aldehyde in terminal alkenes and ketones in other alkenes?? (if so, then what happens to the rest of the alkene?)

  3. I’m getting all confused about the conditions under which KMnO4 reacts.
    For alkynes: I have
    1. water, neutral gives vicinal ketones.
    2. if there’s a base in cold, dilute conditions there’s no reaction.
    3. if there’s a base in warm water you get cleavage. (is this the same as a base in warm and dilute conditions?)

    When you throw in the reactions conditions of alkenes too, I cannot keep all this straight in my head if my life depended on it, which indicates to me that I don’t understand the effect of each of the conditions. Is the difference between the last 2 just dependent on the temperature? What is the significance of the base? How do these compare to reaction conditions of an alkene? Etc., etc. Please help me make sense of all this mess!! Thanks!

    1. Great question. You’d open it up with aqueous acid (H3O+). That protonates the epoxide oxygen, and then water acts as a nucleophile, opening the epoxide.

  4. Can you explain to me thoroughly about the reaction between cyclohexene and cold acidified kmno4 and cyclohexene with hot acidified kmno4? Does the temp affect the product? I’m having a lot of trouble with organic chem.

    1. Yes, the temperature greatly affects the product. At low temperatures you will obtain the 1,2-diol (glycol). At higher temperatures the 1,2-diol will cleave and you will get a reaction product essentially equivalent to ozonolysis.

  5. I am getring confused in the byproduct of the reaction. What will be the difference in byproducts when cis and trans form of aklene is reacted with alkaline KMnO4?

  6. So if I wanted to convert 4-hexen-1-ol to 4-hexanoic acid, would I use potassium permanganate or potassium dichromate and why?

  7. will you please tell me the supporting information regarding this reaction i.e. the no. of moles and the exact reaction conditions and the complete process

  8. What is the difference of using cold dilute neutral KMnO4 with using warm dilute neutral KMnO4 and hot acidic KMnO4 ?

  9. Hi, I wanted to ask you, for how long should the reaction be carried over, to prevent the synthesis of a carboxylic acid?

  10. I don’t understand why when adding KMnO4 /NaOH to 1- cyclohexene you just have one product. Why doesn’t the enantiomer form in this case and in the others it does?

  11. If in the reaction equation, all that’s mentioned is ” dilute KMnO⁴, cold ” and no mention of ” NaOH ” whatsoever, are we going to consider the manganate product as the final product? Or is it too unstable to actually be the final product, and hence presence of NaOH is compulsorily required in the reaction?

    1. Refer to this post:

      In hydroxylation of alkenes you are breaking C-C and forming C-O . Oxygen is more electronegative than carbon, therefore carbon is increasing its oxidation state. It’s a net oxidation because each carbon is oxidized.
      With hydration, you’re also breaking C-C but one carbon is forming C-O and the other is forming C-H. One carbon is oxidized and one is reduced, it’s not a net oxidation.

  12. Hi James,
    I’m wondering why the products of the permanganate oxidation are temperature dependent…By le chateliers, more endothermic reactions should be favored at lower temperatures. But the more exothermic oxidation to form ketones occurs under higher temperatures, while the less exothermic pathway ending in alcohols occurs under lower temperatures?

  13. How does cold KMnO4 behave with double bounds conjugated to a benzene ring, such as seen with styrene? I know that KMnO4 will oxidize styrene to benzoic acid, and until I found this page, was under the impression that it would ALWAYS oxidize to benzoic acid. Does this mean that careful temperature control and monitoring of the reaction form the 1,2-diol as the main product?

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