Does KMnO4 Oxidize Alpha-Beta Unsaturated Aldehydes? [closed]








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I suspect that since $\ce{KMnO4}$ is a strong oxidizing agent it will react with the $\alpha,\beta$-unsaturated part of the carbonyl forming an epoxide. Are there any secondary reactions that one might have to worry about? Or how can I tell if my oxidizing agent will oxidize or not in a systematic way? Take as a hypothetical example, the oxidation with $\ce{KMnO4}$ of cinnamaldehyde.




As Waylander predicted, in aqueous solutions, the first reaction would have been the oxidation of aldehyde to corresponding carboxylic acid (in here, cinamaldehyde to cinnamic acid). When that happens, the resulting $\alpha,\beta$-carboxylic acid would undergo degradation reaction as explained by following example (Ref.1):


[![KMnO4 oxidation of cinnamic acid](https://i.stack.imgur.com/lDP9H.png)](https://i.stack.imgur.com/lDP9H.png)


According to the authors:



> 
> Permanganate ion reacts rapidly with cinnamic acid in aqueous perchloric acid solutions to form an intermediate which appears yellow and which exhibits an absorption maximum at 415 nm. The intermediate then decomposes at a slower rate to give benzaldehyde and manganese(III) as products.
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> 
> 


According to the results, the first reaction is clearly a second order for *trans*-cinnamic acid with $$k\_2 = (1.56 \pm 0.03) \times 10^3 \ \pu{M-1 sec-1}$$. The decomposition of this intermediate is a somewhat slower first-order reaction with:
$$k\_1 = (2.60 \pm 0.02) \times 10^{-2} \ \pu{sec-1}.$$


The nature of these reactions was further defined by a study of the kinetic isotope effects (Ref.1). When the solution of reaction products was made basic by the addition of sodium hydroxide, a brown precipitate of $\ce{MnO2}$ was indeed obtained, confirming that the intermediate decomposes to give benzaldehyde
and $\ce{Mn^{III}}$ as the initial products (since $\ce{Mn^{III}}$ is known to disproportionate to $\ce{Mn^{II}}$ and $\ce{MnO2}$ under alkaline conditions; Ref.2).


**Note:** It is recommended to also read Ref.3 for alkene oxidation by $\ce{KMnO4}$.


**References:**


1. Donald G. Lee and James R. Brownridge, "Oxidation of cinnamic acid by permanganate ion. Spectrophotometric detection of an intermediate," *J. Am. Chem. Soc.* **1973**, *95(9)*, 3033–3034 (ODI: <https://doi.org/10.1021/ja00790a060>).
2. R. Stewart, In *Oxidation in Organic Chemistry, Part A: A series of monographs,*; Kenneth B. Wiberg, Ed., Academic Press: New York, N. Y., 1965, p 1—68 (ISBN: 0127480501).
3. Kenneth B. Wiberg and Richard D. Geer, "The Kinetics of the Permanganate Oxidation of Alkenes," *J. Am. Chem. Soc.* **1966**, *88(24)*, 5827–5832 (ODI: <https://doi.org/10.1021/ja00976a025>).


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