Editing Carboxylic acid (section)

==Reactions==
[[File:Carboxylic Acid Sunshine Diagram1.svg|class=skin-invert-image|thumb|right|400px|Carboxylic acid [[organic reaction]]s]]
===Acid-base reactions===
Carboxylic acids react with [[Base (chemistry)|bases]] to form carboxylate salts, in which the [[hydrogen]] of the [[hydroxyl]] (–OH) group is replaced with a metal [[cation]]. For example, [[acetic acid]] found in [[vinegar]] reacts with [[sodium bicarbonate]] (baking soda) to form [[sodium acetate]], [[carbon dioxide]], and water:{{cn|date=May 2025}}
:{{chem2|CH3COOH + NaHCO3 → CH3COO−Na+ + CO2 + H2O}}

===Conversion to esters, amides, anhydrides===
Widely practiced reactions convert carboxylic acids into [[esters]], [[amides]], [[Carboxylate|carboxylate salts]], [[acid chlorides]], and [[alcohols]].
Their conversion to [[ester]]s is widely used, e.g. in the production of [[polyester]]s. Likewise, carboxylic acids are converted into [[amide]]s, but this conversion typically does not occur by direct reaction of the carboxylic acid and the amine. Instead esters are typical precursors to amides. The conversion of [[amino acid]]s into [[peptide]]s is a significant biochemical process that requires [[Adenosine triphosphate|ATP]].{{cn|date=May 2025}}

Converting a carboxylic acid to an amide is possible, but not straightforward. Instead of acting as a nucleophile, an amine will react as a base in the presence of a carboxylic acid to give the ammonium [[carboxylate]] salt. Heating the salt to above 100&nbsp;°C will drive off water and lead to the formation of the amide. This method of synthesizing amides is industrially important, and has laboratory applications as well.<ref name=wade2>Wade 2010, pp. 964–965.</ref> In the presence of a strong acid catalyst, carboxylic acids can [[condensation reaction|condense]] to form acid anhydrides. The condensation produces water, however, which can hydrolyze the anhydride back to the starting carboxylic acids. Thus, the formation of the anhydride via condensation is an equilibrium process.{{cn|date=May 2025}}

Under acid-catalyzed conditions, carboxylic acids will react with alcohols to form [[ester]]s via the [[Fischer esterification]] reaction, which is also an equilibrium process. Alternatively, [[diazomethane]] can be used to convert an acid to an ester. While esterification reactions with diazomethane often give quantitative yields, diazomethane is only useful for forming methyl esters.<ref name=wade2 />

=== Reduction ===
Like [[ester]]s, most carboxylic acids can be [[carboxylic acid reduction|reduced]] to alcohols by [[hydrogenation]], or using hydride transferring agents such as [[lithium aluminium hydride]]. Strong alkyl transferring agents, such as [[Organolithium reagent|organolithium]] compounds but not [[Grignard reagents]], will reduce carboxylic acids to ketones along with transfer of the alkyl group.{{cn|date=May 2025}}

The [[Vilsmaier reagent]] (''N'',''N''-Dimethyl(chloromethylene)ammonium chloride; {{chem2|[ClHC\dN+(CH3)2]Cl−}}) is a highly chemoselective agent for carboxylic acid reduction. It selectively activates the carboxylic acid to give the carboxymethyleneammonium salt, which can be reduced by a mild reductant like lithium tris(''t''-butoxy)aluminum hydride to afford an aldehyde in a one pot procedure. This procedure is known to tolerate reactive carbonyl functionalities such as ketone as well as moderately reactive ester, olefin, nitrile, and halide moieties.<ref>{{OrgSynth|title=Reduction of carboxylic acids to aldehydes: 6-Ooxdecanal|prep=CV8P0498|last1=Fujisawa|first1=Tamotsu|last2=Sato|first2=Toshio|collvol=8|collvolpages=498|volume=66|page=121|date=1988|doi=10.15227/orgsyn.066.0121}}</ref>

{{anchor|Barbier-Wieland degradation}}

===Conversion to acyl halides===
The hydroxyl group on carboxylic acids may be replaced with a chlorine atom using [[thionyl chloride]] to give [[acyl chloride]]s. In nature, carboxylic acids are converted to [[thioester]]s. [[Thionyl chloride]] can be used to convert carboxylic acids to their corresponding acyl chlorides. First, carboxylic acid '''1''' attacks thionyl chloride, and chloride ion leaves. The resulting [[oxonium ion]] '''2''' is activated towards nucleophilic attack and has a good leaving group, setting it apart from a normal carboxylic acid. In the next step, '''2''' is attacked by chloride ion to give tetrahedral intermediate '''3''', a chlorosulfite. The tetrahedral intermediate collapses with the loss of [[sulfur dioxide]] and chloride ion, giving protonated acyl chloride '''4'''. Chloride ion can remove the proton on the carbonyl group, giving the acyl chloride '''5''' with a loss of [[hydrogen chloride|HCl]].

[[File:Mechanism of the Reaction of a Carboxylic Acid and Thionyl Chloride.png|class=skin-invert-image|Mechanism for the reaction of a carboxylic acid with thionyl chloride to give an acid chloride|700px]]

[[Phosphorus(III) chloride]] (PCl<sub>3</sub>) and [[phosphorus(V) chloride]] (PCl<sub>5</sub>) will also convert carboxylic acids to acid chlorides, by a similar mechanism. One equivalent of PCl<sub>3</sub> can react with three equivalents of acid, producing one equivalent of H<sub>3</sub>PO<sub>3</sub>, or [[phosphorus acid]], in addition to the desired acid chloride. PCl<sub>5</sub> reacts with carboxylic acids in a 1:1 ratio, and produces [[phosphorus(V) oxychloride]] (POCl<sub>3</sub>) and hydrogen chloride (HCl) as byproducts.{{cn|date=May 2025}}

===Reactions with carbanion equivalents===
Carboxylic acids react with Grignard reagents and organolithiums to form ketones. The first equivalent of nucleophile acts as a base and deprotonates the acid. A second equivalent will attack the carbonyl group to create a [[geminal]] alkoxide dianion, which is protonated upon workup to give the hydrate of a ketone. Because most ketone hydrates are unstable relative to their corresponding ketones, the equilibrium between the two is shifted heavily in favor of the ketone. For example, the equilibrium constant for the formation of [[acetone]] hydrate from acetone is only 0.002. The carboxylic group is the most acidic in organic compounds.<ref>Wade 2010, p. 838.</ref>

===Specialized reactions===
* As with all carbonyl compounds, the protons on the [[alpha-carbon|α-carbon]] are labile due to [[keto–enol tautomerism|keto–enol tautomerization]]. Thus, the α-carbon is easily halogenated in the [[Hell–Volhard–Zelinsky halogenation]].
* The [[Schmidt reaction]] converts carboxylic acids to [[amine]]s.
* Carboxylic acids are decarboxylated in the [[Hunsdiecker reaction]].
* The [[Dakin–West reaction]] converts an amino acid to the corresponding amino ketone.
* In the [[Barbier–Wieland degradation]], a carboxylic acid on an aliphatic chain having a simple [[methylene bridge]] at the alpha position can have the chain shortened by one carbon. The inverse procedure is the [[Arndt–Eistert synthesis]], where an acid is converted into acyl halide, which is then reacted with [[diazomethane]] to give one additional methylene in the aliphatic chain.
* Many acids undergo [[oxidative decarboxylation]]. [[Enzyme]]s that catalyze these reactions are known as [[carboxylase]]s ([[Enzyme Commission number|EC]]&nbsp;6.4.1) and [[decarboxylase]]s (EC&nbsp;4.1.1).
* Carboxylic acids are reduced to [[aldehyde]]s via the [[ester]] and [[Diisobutylaluminium hydride|DIBAL]], via the acid chloride in the [[Rosenmund reduction]] and via the thioester in the [[Fukuyama reduction]].
* In [[ketonic decarboxylation]] carboxylic acids are converted to ketones.
* Organolithium reagents (>2 equiv) react with carboxylic acids to give a dilithium 1,1-diolate, a stable [[tetrahedral intermediate]] which decomposes to give a ketone upon acidic workup.
* The [[Kolbe electrolysis]] is an electrolytic, decarboxylative dimerization reaction. It gets rid of the carboxyl groups of two acid molecules, and joins the remaining fragments together.