Permanganate-oxalate reaction mechanism

Even though the mechanism of the reaction between oxalate and permanganate is extremely compHcated, titration under acidic conditions is extremely accurate. This is the recommended method for standardi2ation of permanganate solutions. 

The oxidation of oxalic acid by mercuric chloride to give CO2 and mercurous chloride is a classic example of an induced reaction. This reaction is extremely slow unless small quantities of chromic acid and manganous ions are added, whereon facile reduction takes place Addition of permanganate or persulphate and some reducing agents is also effective and the oxidation proceeds readily under photo- or X-irradiation (Eder s reaction). The large quantum yield points to a chain mechanism , which could also function with an inducing oxidant, viz. 

The neutralisation of acids with bases provides many valuable volumetric methods of chemical analysis and redox titrations are useful as well. But here we encounter an important difference between acid/base and redox reactions in solution. Acid/base reactions which involve the transfer of protons are very fast indeed they are usually instantaneous for all practical purposes. In protonic solvents, polar H-X bonds are very labile and undergo rapid proton exchange. For example, if B(OH)3 - a very weak acid - is recrystallised from D20, we obtain a fully-deuterated product. Redox reactions, on the other hand, are often very slow under ordinary conditions. To return to the analogy between acid/base and redox titrations, many readers will be familiar with the reaction between permanganate and oxalic acid the reaction is very slow at room temperature and, for titrimetric purposes, should be carried out at about 60 °C. The mechanism whereby a redox reaction takes place tends to be... 

If permanganate solution is added drop by drop to an acidic oxalate solution, the first drop is decolorized slowly. During the course of the titration the reaction rate becomes progressively more rapid, owing to the catalytic effect of the Mn(II) produced in the reaction. Elucidation of the mechanism has been the object of many studies for over a century.Undoubtedly the last study is yet to be made. The present picture appears to be as follows. 

Very little information is available about the species which react in the further decomposition of ascorbic add. Apparently, both dehydroascorbic acid and 2,3-diketogulonic acid can be oxidized directly, since both oxalylthreonic acid and the same two free acids have been identified. The former would result from oxidative fission of the chain while the lactone ring was intact. Even less is known of the oxidation mechanism. The oxidation occurs with iodine and with acid permanganate (H12) and also with oxygen or peroxide (R23). Recent studies on the similar decomposition of the enols of aryl pyruvates to aryl aldehydes plus oxalic acid identified these reactions as examples of the direct attack... 

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