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Oxidation of Alcohols by KMnO4

Oxidizing agents such as KMnO4, PDC, Jones reagent, TEMPO, etc. have been used globally over the century. These oxidizing agents convert many families of hydrocarbons into their respective alcohols. KMnO4 is one of the strong oxidizing agent used for the conversion of primary or secondary alcohols into aldehydes and ketones in controlled conditions.  Many other functional groups such as 1-2 diols, oximes, thiols, sulfides, etc. can also be oxidized by this oxidant.

Now the question is Why KMnO4 acts as a strong oxidizing agent? In KMnO4 if we look into the oxidation state of Mn we found that the manganese is in +7 oxidation state. As we know that the atomic number of Mn is 25 therefore, the electronic configuration of Mn is 1s2,2s2,2p6,3s2,3p6,4s2,3d5. Now the Mn is in +7 oxidation state which suggests the removal of 7 electrons from the 4s and 3d orbitals (i.e. 4s0, 3d0). The maximum capacity to remove electrons has been reached for Mn+7 and now it can only take up the electrons and become reduced (i.e. lower down the oxidation state) and hence acts as an oxidizing agent. 

KMnO4 primarily used for the oxidation of primary alcohols into their respective carboxylic acids. However, if secondary alcohol is treated with KMnO4, it converts into ketone. Tertiary alcohol does not react with  KMnO4. This KMnO4 can react with alcohols in different medium such as an aqueous, basic, and acidic medium.

The majority of reaction generally proceeds under aqueous and basic medium under controlled conditions. The oxidation of carbon can be understood by counting the number of oxygen bonds attached to the carbon atoms. The electrode potential of KMnO4 under acidic and basic condition was reported to be +1.51V and +1.23V respectively, which makes KMnO4 as a strong oxidizing agent in an alkaline medium. It has to be noted that the rate of oxidation depends on the pH of the solution and the temperature at which the reaction proceed. Both these factors affect the yield and purity of the desired compound.

To perform oxidation under aqueous solutions other solvents such as ethanol, methyl cyanide, pyridine, tertiary butanol, acetic acids are also used which helps to solubilize the KMnO4 and proceed the reaction in forward directions.

This Strong KMnO4 oxidant not only oxidized alcohols but can also oxidize alkenes, unsaturated esters, Alkynes under controlled pH, amines, aliphatic nitro compounds, thiols, etc.

Mechanism


Lets understand some examples with the help of above mechanism.


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