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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:
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.
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:
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:
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):
References:
