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Question 3b iii)

There are 2 double bonds $\ce{C=C}$ present within each piperic acid molecule, thus when piperic acid reacts with hot excess $\ce{KMnO4}$, it oxidised into a carboxyllic acid. Why is $\ce{CO2}$ produced?

The carbon atom bonded directly to the benzene ring will get oxidised into a carboxylic acid and also that the carbon bonded to the $\ce{COOH}$ group will get oxidized also to a carboxylic acid to form a dicarboxylic acid. What changes will take place to the other 2 carbon atoms.

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this paper stated that $\ce{CO2}$ could be formed as well, how?

Usually when we consider permanganate oxidations we think of

  • oxidation of alkyl benzenes to benzoic acid
  • cleavage and oxidation of a double bond to an acid, through the aldehyde, if there is one hydrogen on the double bond
  • cleavage and oxidation of a double bond to a ketone if there are no hydrogens on the double bond

However, there are a few special cases such as formic acid and oxalic acid, or their corresponding aldehydes. These compounds are oxidized by permanganate to $\ce{CO2}$ (for an idea as to why these compounds react differently than, say, acetic acid, see Scheme 1 here).

With this information in hand we can re-examine the permanganate oxidation of piperic acid.

enter image description here

We see that oxalic acid is one of the primary products produced in this reaction. As discussed above, oxalic acid can then undergo further reaction to produce carbon dioxide.

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  • $\begingroup$ It's my first time to hear about this. so i should learn that in those cases, CO2 is produced, yes?. Is this applied to some aldehydes? $\endgroup$
    – Abmon98
    Commented May 9, 2015 at 23:42
  • $\begingroup$ Yes, you should be aware of the permanganate oxidation of those 2 molecules and their aldehydes. Since this is your first time here, let me add that if this answer was helpful please consider accepting and\or up-voting it, thanks. $\endgroup$
    – ron
    Commented May 9, 2015 at 23:50

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