March 28, 2024, 01:11:08 PM
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Topic: The Oxidation of Alcohols by Potassium Permanganate Under Basic Conditions  (Read 2712 times)

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Offline habbababba

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Hello.

I have learned that the oxidation of alcohols to carboxylic acids can be achieved in different ways. Some of these ways include reacting the alcohol  with sulfuric acid (or other acids) and potassium permanganate.

I understand that the role of sulfuric acid is primarily to 'push' the permanganate to reach its highest oxidizing power.

However, I have stumbled upon [http://www.dartmouth.edu/~chem52/handouts/OxidHand.htm]this[/http://www.dartmouth.edu/~chem52/handouts/OxidHand.htm] page where it outlines the oxidation of alcohols into carboxylic acids using the same reagents mentioned above only this time it is performed under basic conditions.

Assuming that the information on this page is valid, this had me thinking about the following:

Looking at the mechanism proposed in the page, it appears to me now that using a strong base actually serves a better role in deprotonating the carbon attached to the hydroxyl group than when using a strong acid, in which case the deprotonating species would be either water or the conjugate base of the strong acid (which should be a weak base) if memory serves. Now I'm stuck between 2 conflicting ideas: the ionic half equations tell us that permanganate can be reduced to a further extent when sulfuric acid is used, so we would favor oxidation under acidic conditions. But the mechanism in the page posted suggests that even though the strong base doesn't 'push' the permanganate to its highest oxidizing power, the strong base can actually do a better job at deprotonating the carbon attached to the hydroxyl group than the other species available in the solution when using sulfuric acid. This idea seems to prefer oxidation under basic conditions.

Anyone can correct my analysis?

Offline orgopete

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Better? I guess that will depend. As pointed out, less reagent is needed under acidic conditions. While hydroxide may be a better nucleophile, I don't know that is a rate limiting step. Is it?

Just for the sake of pointing out how it may not be, permanganate can be used to form a diol or to cleave an alkene. This is how I have analyzed these two reactions. These are typically both run under basic conditions. If cold, a diol can result, while hot results in cleavage. The first step is to form a cyclic manganate ester. I rationalize that at higher temperatures hydroxide can undergo an electrocyclic reaction which cleaves the carbon-carbon bond. I also reason that at low temperatures and in the presence of hydroxide, it can attack the manganate ester resulting in its cleavage. Although hydroxide could also reproduce alkoxide leading to oxidation, it appears to not do so. (I am not saying it could not do so, but rather if alkoxide lead to a faster oxidation that alcohol formation, thus method would not work well.) For this reason, I don't know that basic conditions are necessarily better or not.
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