Editing Decarboxylation (section)

== In organic chemistry ==
The term "decarboxylation" usually means replacement of a [[carboxyl group]] ({{chem2|\sC(O)OH}}) with a [[hydrogen atom]]:
:{{chem2|RCO2H  ->  RH  +  CO2}}
Decarboxylation is one of the oldest known organic reactions. It is one of the processes assumed to accompany [[pyrolysis]] and [[destructive distillation]]. 

Overall, decarboxylation depends upon stability of the carbanion [[synthon]] {{chem|R|-}},<ref>{{JerryMarch}}</ref><ref>{{Cite web |url=http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch19/ch19-3-4.html|title=Decarboxylation, Dr. Ian A. Hunt, Department of Chemistry, University of Calgary |access-date=2008-12-07 |archive-date=2022-12-21 |archive-url=https://web.archive.org/web/20221221053807/http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch19/ch19-3-4.html|url-status=live}}</ref> although the anion may not be a true [[chemical intermediate]].<ref name=Keton/><ref name=Malon/>  Typically, carboxylic acids decarboxylate slowly, but carboxylic acids with an α [[electron-withdrawing group]] (e.g. β{{nbh}}[[keto acid]]s, β{{nbh}}nitriles, α{{nbh}}[[nitro compound|nitro]] acids, or [[benzoic acid|arylcarboxylic acids]]) decarboxylate easily. Decarboxylation of [[sodium chlorodifluoroacetate]] generates [[difluorocarbene]]:
:{{chem2|CF2ClCO2Na  ->  NaCl  +  CF2  +  CO2}}<ref name=eEROS>{{cite book |doi=10.1002/047084289X.rs058 |chapter=Sodium Chlorodifluoroacetate |title=Encyclopedia of Reagents for Organic Synthesis |date=2001 |last1=Taschner |first1=Michael J. |isbn=0-471-93623-5 }}</ref>  

Decarboxylations are an important in the [[malonic ester synthesis|malonic]] and [[acetoacetic ester synthesis]].  The [[Knoevenagel condensation]] and they allow keto acids serve as a stabilizing [[protecting group]] for carboxylic acid [[enol]]s.<ref>''Organic Synthesis: The disconnection approach'', 2nd ed.</ref>{{page needed|date=April 2024}}<ref name=Malon>{{cite web | publisher = Organic Chemistry Portal | url = https://www.organic-chemistry.org/namedreactions/malonic-ester-synthesis.shtm | title = Malonic Ester Synthesis | access-date = 2007-10-26}}</ref>  

For the free acids, conditions that deprotonate the carboxyl group (possibly protonating the electron-withdrawing group to form a [[zwitterionic]] [[tautomer]]) accelerate decarboxylation.<ref>{{cite web | url = http://www.chemguide.co.uk/organicprops/acids/decarbox.html | author = Jim Clark | year = 2004 | publisher = Chemguide | title = The Decarboxylation of Carboxylic Acids and their Salts | access-date = 2007-10-22}}</ref>  A strong [[base (chem)|base]] is key to [[ketonization]], in which a pair of carboxylic acids combine to [[ketone|the eponymous functional group]]:<ref>{{cite journal|journal=Org. Synth.|first1=J. F.|last1=Thorpe|first2=G. A. R.|last2=Kon|year=1925|title= Cyclopentanone|volume=5|pages=37|doi=10.15227/orgsyn.005.0037}}</ref><ref name=Keton>{{cite journal | last1 = Renz | first1 = M | year = 2005 | title = Ketonization of Carboxylic Acids by Decarboxylation: Mechanism and Scope | journal = Eur. J. Org. Chem. | volume = 2005 | issue = 6| pages = 979–988 | doi = 10.1002/ejoc.200400546 | url = https://www.thevespiary.org/rhodium/Rhodium/Vespiary/talk/files/835-Thermal.Ketonization.Mechanism.and.Scopecc88.pdf | via = The Vespiary }}</ref>
[[File:Barium adipate pyrolysis.png|frameless|center|upright=1.75]]

[[Transition metal]] salts, especially [[copper]] compounds,<ref>{{Cite journal |date=1953 |title=m-Nitrostyrene |url=http://orgsyn.org/demo.aspx?prep=CV4P0731 |journal=Organic Syntheses |volume=33 |pages=62 |doi=10.15227/orgsyn.033.0062|first1= Richard H.|last1=Wiley|first2=Newton R.|last2=Smith|url-access=subscription }}</ref> facilitate decarboxylation via [[metal carboxylate complex|carboxylate complex]] intermediates.  Metals that catalyze [[cross-coupling reaction]]s thus treat aryl carboxylates as an aryl anion synthon; this synthetic strategy is the [[decarboxylative cross-coupling]] reaction.<ref>{{cite journal|last1=Weaver|first1=J. D.|last2=Recio|first2=A.|last3=Grenning|first3=A. J.|last4=Tunge|first4=J. A.|title=Transition Metal-Catalyzed Decarboxylative Allylation and Benzylation Reactions|journal=Chem. Rev.|year=2011|volume=111|issue=3|pages=1846–1913 |doi=10.1021/cr1002744|pmid=21235271 |pmc=3116714}}</ref>  

Upon heating in [[cyclohexanone]], [[amino acid]]s decarboxylate.  In the related [[Hammick reaction]], uncatalyzed decarboxylation of a [[picolinic acid]] gives a [[stable carbene]] that attacks a [[carbonyl]] electrophile.  

[[Oxidative decarboxylation]]s are generally radical reactions.  These include the [[Kolbe electrolysis]] and  [[Hunsdiecker reaction|Hunsdiecker]]-[[Kochi reaction]]s.  The [[Barton decarboxylation]] is an unusual radical reductive decarboxylation.

As described above, most decarboxylations start with a carboxylic acid or its alkali metal salt, but the [[Krapcho decarboxylation]] starts with methyl [[ester]]s.  In this case, the reaction begins with [[halide]]-mediated cleavage of the ester, forming the carboxylate.