Understanding Permanganate Oxidation: MnO2 Formation

[thunderfvck]

Hi.
Okay, first of all, can an alcohol be oxidized by permanganate?...of course it can...
In the case of an alkene the permanganate forms a cyclic intermediate where the double bond once was. The oxygens break off (from the permanganate) and are protonated by an acid or H2O if no acid was added. This forms a diol and MnO2. MnO2, how I loathe you. It also appears in the oxidation of alcohols. But there is no cyclic intermediate formed because there is no diol produced. So I read that the O in the alcohol attacks the Mn in MnO4-, one of the O's in MnO4 is protonated by the H in the OH group (proton transfer)...anyway, look here for a better understanding:
http://www.dartmouth.edu/~chem52/handouts/OxidHand.htm

Everything is nice and dandy until we get to the bottom of that page where it's mentioned how manganate reacts with water to form MnO2, MnO4-, and OH. My question is HOW DOES THIS OCCUR? It was simple with the alkene because the O's just broke off and MnO2 was formed. But I don't see how all of this happens with just water. Help.
Thanks.

 

[Aaron Barnard]

When permanganate (MnO4-) reacts with water, it is first reduced to MnO2 by the transfer of electrons from the water molecules. The MnO2 then reacts with another water molecule to form MnO4-, OH-, and H+. This is the same thing that happens when permanganate is used to oxidize an alcohol, except the alcohol replaces one of the water molecules in the reaction.

 

[ravenprp]

Hi there,

Thank you for sharing your thoughts and questions about permanganate oxidation and MnO2 formation. It's great to see your interest and curiosity in this topic.

To answer your question, let's first review the overall reaction of permanganate oxidation of alcohols. In this reaction, the permanganate ion (MnO4-) acts as an oxidizing agent, meaning it gains electrons and gets reduced. On the other hand, the alcohol molecule loses electrons and gets oxidized. This results in the formation of a carboxylic acid and MnO2.

Now, when water is present in the reaction, it can act as both a nucleophile and a proton donor. This means that it can attack the permanganate ion and also donate a proton to it. In the first step, water attacks the permanganate ion, forming a six-membered ring intermediate with a Mn-O bond. In the second step, the O-H bond in the alcohol molecule is broken, and the resulting O- ion attacks the Mn atom in the intermediate. This leads to the formation of a new Mn-O bond and the release of a carboxylic acid molecule, completing the oxidation of the alcohol.

The Mn-O bond in the intermediate is unstable and can easily break, forming MnO2 and releasing the OH- ion. This reaction is known as hydrolysis and is responsible for the formation of MnO2 in the presence of water.

I hope this explanation helps in understanding how MnO2 is formed during permanganate oxidation with water. If you have any further questions, please do not hesitate to ask. Best of luck in your studies!