Oxidative cleavage of alkenes to ketones/carboxylic acids using KMnO4

by James

Description: Alkenes treated with KMnO4 are cleaved into carbonyl compounds. The type of carbonyl compound depends on the substitution pattern of the alkene.

Notes: The reaction is generally run with aqueous acid and heat.

Examples:

Notes: Note that all C–H bonds attached to alkenes are oxidized to C–OH bonds, but alkene carbons with only carbons attached become ketones. Note that the last example is a cyclic alkene that goes to a linear product.

Mechanism: The detailed mechanism for this reaction is generally not considered important for the purposes of Org 1 / Org 2, although the first step is dihydroxylation of the alkene.

 


{ 10 comments… read them below or add one }

vijaykumar

what would be the reaction if sp2 carbon of alkene is attached to both hydrogen atom!!!!

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james

The initial product will be formaldehyde, but this will be oxidized to CO2 under the reaction conditions.

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Stephen Hamilton

In your 4th example you dropped a carbon.

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james

thanks for the spot!

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Nima Omid-Fard

2 Q’s:

1) How come my textbook likes to first add KMnO4 with OH-, then add acid later? Any difference?

2) Can this reaction be done with H2CrO4? I notice that KMnO4 and H2CrO4 do the same job with other reactions, like oxidation of alcohols, aldehydes, or alkyl substituents of benzene rings, so why not with this reaction also?

Reply

james

1) Not sure. Usually KMnO4 is used with acid. However, there’s a second important reaction of KMnO4 – when you treat alkenes with COLD, DILUTE KMnO4, it will form the vicinal diol. Base is often used in the second step to get the alcohol.

2)The reaction is generally not done with H2CrO4. I wouldn’t be surprised if it CAN be done, but H2CrO4 is not generally used – this may be because the reaction is slow.

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Nima Omid-Fard

Thanks for the quick response! Is it possible to have email reminders when you give a response? Thanks!

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james

My software doesn’t currently allow for that – I’m concerned as well it will slow down my site. I’ll try to respond promptly – you can check “recent comments” on the main page

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Nabillionaire

Is the number of carboxylic acids formed in your product, directly dependent on the number of vinylic hydrogens? two vynil h’s = two carboxylic acids? one vynil h = carboxylic acid + ketone??

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james

Well, the maximum number of carboxylic acids would be two. If each carbon of the double bond had a C-H bond then there would be two carboxylic acids. However if one of the carbons has *two* vinylic hydrogens, then that carbon will turn into carbon dioxide.

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