I am looking for the mechanism for thermal decarboxylation for any RCOOH, and I am guessing that its possible for the thermal energy to cleave the R-C bond homolytically leaving a R• and •COOH. Then I would guess the R• would add its electron to the H and the O-H bond would undergo homolysis as well leaving the single electrons on the C and now O to form a bond leaving RH and CO2. However I have no idea if this is what actually occurs. Thanks
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$\begingroup$ Related: Decarboxylation of Salicylic acid $\endgroup$– andselisk ♦Commented Dec 27, 2017 at 7:44
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2 Answers
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The decarboxylation of any carboxylic acid can basically take place in four ways:
Unimolecular free radical mechanism (homolytic fission)
However, in case of thermal decomposition there has been no evidence of this type of free radical formation. Although, free radicals are formed in electrolytic or photochemical process. There is also evidence of formation of free radical in ketonic decarboxylation.
Unimolecular heterolytic fission (carbanion formation)
This is the mechanism for most carboxylic acids.
Unimolecular heterolytic fission (carbocation or carbenium ion formation)
This has not been observed in any case.
Bimolecular decarboxylation
This occurs only in case of carboxylic acids that have high electron density on the $\ce{\alpha}$-carbon that might attract the proton from the solution. Examples include anthracene-9-carboxylic acid,2,4,6-trimethylbenzoic acid etc.
[Reference:The mechanism of thermal decarboxylation. B.R. Brown. Quarterly Reviews, Chemical Society. 1951 ; http://pubs.rsc.org/-/content/articlelanding/1951/qr/qr9510500131#!divAbstract]
answered Dec 29, 2017 at 14:01
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Under sufficiently alkaline conditions, i.e. when the acid is deprotonated, the carboxylate can undergo an electron transfer reaction with a suitable partner (oxidant). In the course of this process, the carboxylate is oxidized to an acyloxy radical, which subsequently fragments to yield an alkyl (or alkylaryl) radical and carbon dioxide.
$$\ce{R-COO- ->[-e^-] R-COO\cdot -> R\cdot + CO2}$$
A photochemical variant of this reaction was intensively examined in the group of Axel Griesbeck at the University of Cologne, Germany. For further reading, have a look at
answered Dec 28, 2017 at 11:23
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$\begingroup$ @ShoubhikRajMaiti Nope. The photochemical step involves the generation of a sufficiently powerful oxidant, such as the excited triplet state of a cyclic imide. The electron transfer from the carboxylate might also occur to a ground state oxidant or an electrode, such as in the Kolbe reaction. The outcome might be different - dimerization of the radicals obtained from the carboxylates rather than addition to the imide radical anion - but the decarboxylation via a radical pathway is the same. $\endgroup$ Commented Dec 29, 2017 at 23:57