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What is the probable mechanism for the following reaction?

$\alpha$-Keto acids on heating with conc.$\ce{H2SO4}$ undergo decarboxylation to give monocarboxylic acids:

decarboxylation of alpha-keto carboxylic acid

Also, which of the two carbons (the carbonyl carbon and the carboxylic carbon) is getting eliminated here?

Note: I tried to apply the mechanism for the decarboxylation of $\beta$-keto carboxylic acids in this case too, but couldn't work out one.

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    $\begingroup$ Btw which book is it? The number of oxygen atoms on either side is not balanced. So it's definitely not a simple decarboxylation - it involves oxidation too. $\endgroup$
    – TRC
    Aug 14 at 4:36
  • $\begingroup$ It's Organic Chemistry (II) by O. P. Tandon, GRB Publication. $\endgroup$
    – Nilabja
    Aug 14 at 4:50
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    $\begingroup$ Hi @Nilabja. I voted to reopen your question. Make sure to add your efforts while making the initial draft of your question so that your question won't get closed in the first place and you don't have to go through the effort of making changes and making comments explaining the changes and expanding the comment thread chain. That being said, your previous question was also insta-closed. I don't think the question is bad, so would you like to make some changes? $\endgroup$ Aug 14 at 5:02
  • $\begingroup$ @NilayGhosh Thank You for your help, the question has been reopened. I would keep in mind the suggestions for further questions. Also, as you asked, I have got satisfactory solution to my problem in the previous question. So, I wouldn't like to make any more changes to it, at least for now. $\endgroup$
    – Nilabja
    Aug 14 at 6:10
  • $\begingroup$ All your questions are reall y hard lol $\endgroup$
    – User688539
    Aug 24 at 16:46
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There are two characteristic reactions of $\alpha$-keto acids with sulphuric acid, but none of them produce acetic acid and carbondioxide as the products as indicated in the reaction given in the OP's textbook. Instead, pyruvic acid (the simple $\alpha$-keto acid shown in the textbook reaction) is easily decarboxylated with warm dilute sulphuric acid to give acetaldehyde: $$\ce{CH3-C(=O)-CO2H ->[dil.H2SO4][\Delta] CH3-CHO + CO2}$$ If the reaction is in conc.$\ce{H2SO4}$ as given in the sought reaction, the products are acetic acid and carbonmonoxide: $$\ce{CH3-C(=O)-CO2H ->[conc.H2SO4][\Delta] CH3-CO2H + CO}$$

Mechanism of the first reaction is given in here as follows:

Mechanism of the reaction with pyruic acid in dil. H2SO4

Although the mechanism of this reaction is uncertain yet it occurs only in $\alpha$-keto acids, it is suggested that the $–I$ effect of the $\alpha$-carbonyl group plays an important part in this elimination. You may also find the mechanism of the second reaction in the same site as well:

Mechanism of the reaction with pyruic acid in conc. H2SO4

However, it is also possible for $\alpha$-ketocarboxylic acid to undergo decarboxilation to give corresponding carboxylic acid and $\ce{CO2}$, but need redox conditions similar to pyruate decarboxylation in biological conditions:

$$\ce{CH3-C(=O)-CO2H ->[Oxidation] CH3-CO2H + CO2}$$

For example, this reaction will undergo in the presence of $\ce{H2O2}$ and following mechanism is given (Ref.1):

Mechanism of Decarboxylation of Pyruvic Acid in the Presence of Hydrogen Peroxide

References:

  1. Antonio Lopalco, Gautam Dalwadi, Sida Niu, Richard L. Schowen, Justin Douglas, and Valentino J. Stella, "Mechanism of Decarboxylation of Pyruvic Acid in the Presence of Hydrogen Peroxide," J Pharm Sci. 2016, 105(2), 705–713 (DOI: https://doi.org/10.1002/jps.24653).
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