Why Must Alkene Oxidative Cleavage Solutions be Basic and Hot?

[Clari]

Alkenes are oxidatively cleaved to alts of carboxylic acids, by hot basic permanganate solutions. However, I don't understand why the solutions need to be basic and hot? Why can't it be cold or acidic?

Please tell me.

 

[redwraith94]

Hot reactions proceed faster than cold reactions simply because the molecules have more energy, and also they collide more often. acidic or basic usually has to do with some sort of catalytic reaction IDK much aboot organic chem, so I will leave that question to other members.

 

[movies]

I think that an excess of OH- in solution facilitates the reaction by cleaving the reduced manganese species off of the organic substrate. It's probably also necessary to initiate the final oxidation from aldehyde to carboxylic acid. In acidic solution then there wouldn't be any good nucleophiles, just water.

 

[The Bob]

A primary alchol can become a carboxylic acid but the alchol needs an acidificed dichromate solution (e.g. potassium dichromate solution) or sulphuric acid (well simply concentrated acid). If I have understood the question, you are asking how to make a carboxylic from an alchol. So I would say it has to be acidic.

To make an alchol from an alkene does need a base for the electrophilic addition.

The Bob (2004 ©)

 

[movies]

In the case of a chromium oxidation you need the acid to facilitate making chromic acid, which is the active oxidizing agent. It's a different mechanism.

 

[The Bob]

That would explain it then. That is the only mechanism I have come across at the moment.

The Bob (2004 ©)

 

[movies]

I read that acidic KMnO₄ can also oxidize alcohols and aldehydes to carboxylic acids, so maybe the "basic" part isn't so important for that. I also read that hydroxide accelerates the cleavage of the manganese species after the dihydroxylation step of the alkene cleavage process. So, I would guess that the nucleophile is most important there.

The same effect is true with OsO₄. If you use a stoichiometric amount of the osmium, then you have to do a workup that is capable of cleaving the osmium-organic complex before you can isolate the dihydroxylation product.