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What is the mechanism for thermal decarboxylation of a generic carboxylic acid (not special cases like beta-ketocarboxylic acid or malonic acids)?
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Keith Roman
What is the mechanism for thermal decarboxylation of a generic carboxylic acid (not special cases like beta-ketocarboxylic acid or malonic acids)?
The decarboxylation of any carboxylic acid can basically take place in four ways:
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 (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.
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 (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.
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.
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
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.
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
@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 More
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.More
The decarboxylation of any carboxylic acid can basically take place in four ways:
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.
This is the mechanism for most carboxylic acids.
This has not been observed in any case.
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]
The decarboxylation of any carboxylic acid can basically take place in four ways:
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.
This is the mechanism for most carboxylic acids.
This has not been observed in any case.
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]
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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
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
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