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

by James

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.


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 permanganage 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.


{ 8 comments… read them below or add one }

Jason January 18, 2012 at 2:23 am

Hi James, How would a diene look with this reaction? Would there be 4 alcohol groups?


james January 18, 2012 at 3:50 am

Yes there would – if you added two equivalents of KMnO4.


ANN April 3, 2012 at 2:58 am

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


james April 3, 2012 at 12:48 pm

I’m not sure I understand the question.


Monique April 25, 2012 at 4:30 pm

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?)


james April 26, 2012 at 8:15 pm
Jennifer May 19, 2012 at 2:35 am

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!


Paul Michino July 20, 2012 at 10:14 am

how do aldehydes and ketones react with cold dilute KMnO4


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