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The mechanism of permanganate oxidations : pivalaldehyde, benzaldehyde and p-nitro-phenyltrifluoromethylcarbinol. Fleming, Donald George

Abstract

A study has been made of the potassium permanganate oxidation of three organic compounds: pivalaldehyde, p-nitrobenzaldehyde and p-nitrophenyltrifluoromethyl-carbinol. The permanganate-pivalaldehyde reaction has been investigated in the pH range 1 to 13. The results show that the reaction is first order in permanganate and aldehyde, that the oxidation by manganate is slower than that of permanganate by a factor of twenty five, and that the oxidation is general acid-catalyzed. The results in alkaline solution show some increase in rate but are not reproducible, which is most likely due to radical decomposition of the pivalaldehyde. Three mechanisms are discussed: Firstly, a typical general acid-catalysis scheme, secondly, one involving a tetragonal transition state and thirdly, one based on a hydride transfer. Of these, only the first two are able to accommodate the experimental evidence found earlier in the permanganate oxidation of benzaldehyde by Wiberg and Stewart (1). The autocatalysis exhibited by the permanganate benzaldehyde reaction has been studied at low pH. The autocatalysis has also been investigated with p-nitrobenzaldehyde in order to see if there is any substituent effect. Results show that the p-nitro aldehyde has a longer induction period. Moreover, it has been shown that overoxidation of the benzene ring rather than true autocatalysis has occurred. A radical chain mechanism has been put forward in an attempt to explain this observation. The permanganate oxidation of p-nitrophenyltri-fluoromethylcarbinol has been studied in 0.1 M sodium hydroxide in order to determine the mechanism of the reaction. A large enhancement in rate observed compared to rates found by Stewart and Van der Linden (2) in the permanganate oxidation of other substituted phenyltrifluoromethylcarbinols is good evidence for a reaction path involving hydrogen atom abstraction from the alkoxide to the permanganate ion. Such a mechanism is consequently favoured.

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