Editing Carboxylic acid

{{short description|1=Organic compound containing a –C(=O)OH group}}
{{redirect|COOH|the Bulgarian DJ and producer Ivan Shopov|Cooh (musician)}}
{{Distinguish|Carbolic acid}}
{{Use dmy dates|date=March 2023}}
[[File:Carboxylic-acid.svg|class=skin-invert-image|thumb|150px|Structure of a carboxylic acid]]
[[File:Carboxylate-resonance-hybrid.png|class=skin-invert-image|thumb|150px|Carboxylate anion]]
[[File:Carboxyl-3D-space-filling-labelled.png|thumb|150px|3D structure of a&nbsp;carboxylic acid]]

In [[organic chemistry]], a '''carboxylic acid''' is an [[organic acid]] that contains a '''carboxyl group''' ({{chem2|\sC(\dO)\sOH}})<ref>{{GoldBookRef|title=carboxylic acids|file=C00852}}</ref> attached to an [[Substituent|R-group]]. The general formula of a carboxylic acid is often written as '''{{chem2|R\sCOOH}}''' or '''{{chem2|R\sCO2H}}''', sometimes as {{chem2|R\sC(O)OH}} with R referring to an [[organyl group]] (e.g., [[alkyl]], [[alkenyl]], [[aryl]]), or [[hydrogen]], or other groups. Carboxylic acids occur widely. Important examples include the [[amino acid]]s and [[fatty acid]]s. [[Deprotonation]] of a carboxylic acid gives a [[carboxylate]] [[anion]].

==Examples and nomenclature==
Carboxylic acids are commonly identified by their [[trivial name]]s. They often have the suffix ''-ic acid''. {{anchor|-oic}}[[IUPAC]]-recommended names also exist; in this system, carboxylic acids have an ''-oic acid'' suffix.<ref>[http://www.acdlabs.com/iupac/nomenclature/79/r79_24.htm Recommendations 1979]. Organic Chemistry IUPAC Nomenclature. Rules C-4 Carboxylic Acids and Their Derivatives.</ref> For example, [[butyric acid]] ({{chem2|CH3CH2CH2CO2H}}) is butanoic acid by IUPAC guidelines. For nomenclature of complex molecules containing a carboxylic acid, the carboxyl can be considered position one of the [[parent chain]] even if there are other [[substituent]]s, such as [[3-chloropropanoic acid]]. Alternately, it can be named as a "carboxy" or "carboxylic acid" substituent on another parent structure, such as [[2-Furoic acid|2-carboxyfuran]].{{cn|date=May 2025}}

The carboxylate anion ({{chem2|R\sCOO−}} or {{chem2|R\sCO2−}}) of a carboxylic acid is usually named with the suffix ''-ate'', in keeping with the general pattern of ''-ic acid'' and ''-ate'' for a [[conjugate acid]] and its conjugate base, respectively. For example, the conjugate base of [[acetic acid]] is [[acetate]].{{cn|date=May 2025}}

[[Carbonic acid]], which occurs in [[bicarbonate buffer system]]s in nature, is not generally classed as one of the carboxylic acids, despite that it has a [[moiety (chemistry)|moiety]] that looks like a COOH group. {{cn|date=May 2025}}
{|class = "wikitable"
|+Straight-chain, saturated carboxylic acids (alkanoic acids)
!Carbon<br>atoms
!Common name
!IUPAC name
!Chemical formula
!Common location or use
|-<!--
|0 || [[Oxygen]] || ? || OO || [[Air]]
|- -->
|1 || [[Formic acid]] || Methanoic acid || HCOOH || [[Insect stings]]
|-
|2 || [[Acetic acid]] || Ethanoic acid || {{chem2|CH3COOH}} || [[Vinegar]]
|-
|3 || [[Propionic acid]] || Propanoic acid || {{chem2|CH3CH2COOH}} || Preservative for stored grains, [[body odour]], milk, butter, cheese
|-
|4 || [[Butyric acid]] || Butanoic acid || {{chem2|CH3(CH2)2COOH}} || [[Butter]]
|-
|5 || [[Valeric acid]] || Pentanoic acid || {{chem2|CH3(CH2)3COOH}} || [[Valerian (herb)|Valerian]] plant
|-
|6 || [[Caproic acid]] || Hexanoic acid || {{chem2|CH3(CH2)4COOH}} || [[Goat]] fat
|-
|7 || [[Enanthic acid]] || Heptanoic acid || {{chem2|CH3(CH2)5COOH}} ||Fragrance
|-
|8 || [[Caprylic acid]] || Octanoic acid || {{chem2|CH3(CH2)6COOH}} || [[Coconuts]]
|-
|9 || [[Pelargonic acid]] || Nonanoic acid || {{chem2|CH3(CH2)7COOH}} || [[Pelargonium]] plant
|-
|10 || [[Capric acid]] || Decanoic acid || {{chem2|CH3(CH2)8COOH}} || [[Coconut oil|Coconut]] and [[Palm kernel oil]]
|-
|11 || [[Undecylic acid]] || Undecanoic acid || {{chem2|CH3(CH2)9COOH}} || Anti-fungal agent
|-
|12 || [[Lauric acid]] || Dodecanoic acid || {{chem2|CH3(CH2)10COOH}} || [[Coconut oil]] and hand wash soaps
|-
|13 || [[Tridecylic acid]] || Tridecanoic acid || {{chem2|CH3(CH2)11COOH}} || Plant metabolite
|-
|14 || [[Myristic acid]] || Tetradecanoic acid || {{chem2|CH3(CH2)12COOH}} || [[Nutmeg]]
|-
|15 || [[Pentadecylic acid]] || Pentadecanoic acid || {{chem2|CH3(CH2)13COOH}} || Milk fat
|-
|16 || [[Palmitic acid]] || Hexadecanoic acid || {{chem2|CH3(CH2)14COOH}} || [[Palm oil]]
|-
|17 || [[Margaric acid]] || Heptadecanoic acid || {{chem2|CH3(CH2)15COOH}} || Pheromone in various animals
|-
|18 || [[Stearic acid]] || Octadecanoic acid || {{chem2|CH3(CH2)16COOH}} || [[Chocolate]], waxes, soaps, and oils
|-
|19 || [[Nonadecylic acid]] || Nonadecanoic acid || {{chem2|CH3(CH2)17COOH}} || Fats, vegetable oils, [[pheromone]]
|-
|20 || [[Arachidic acid]] || Icosanoic acid || {{chem2|CH3(CH2)18COOH}} || [[Peanut oil]]
|}

{|class = "wikitable"
|+ Other carboxylic acids
! Compound class
! Members
|-
|unsaturated monocarboxylic acids || [[acrylic acid]] (2-propenoic acid) – {{chem2|CH2\dCH\sCOOH}}, used in polymer synthesis
|-
| [[Fatty acid]]s || medium to long-chain saturated and unsaturated monocarboxylic acids, with even number of carbons; examples: [[docosahexaenoic acid]] and [[eicosapentaenoic acid]] (nutritional supplements)
|-
| [[Amino acid]]s ||the building-blocks of [[protein]]s
|-
| [[Keto acid]]s || acids of biochemical significance that contain a [[ketone]] group; examples: [[acetoacetic acid]] and [[pyruvic acid]]
|-
| [[Aromatic compound|Aromatic]] carboxylic acids || containing at least one aromatic ring; examples: [[benzoic acid]] – the sodium salt of benzoic acid is used as a food preservative; [[salicylic acid]] – a beta-hydroxy type found in many skin-care products; [[phenyl alkanoic acids]] – the class of compounds where a phenyl group is attached to a carboxylic acid
|-
| [[Dicarboxylic acid]]s || containing two carboxyl groups; examples: [[adipic acid]] the monomer used to produce [[nylon]] and [[aldaric acid]] – a family of sugar acids
|-
| [[Tricarboxylic acid]]s || containing three carboxyl groups; examples: [[citric acid]] – found in [[citrus fruit]]s and [[isocitric acid]]
|-
| [[Alpha hydroxy acid]]s || containing a hydroxy group in the first position; examples: [[glyceric acid]], [[glycolic acid]] and [[lactic acid]] (2-hydroxypropanoic acid) – found in sour milk, [[tartaric acid]] – found in wine
|-
| [[Beta hydroxy acid]]s || containing a hydroxy group in the second position
|-
| [[Omega hydroxy acid]]s ||containing a hydroxy group beyond the first or second position
|-
| [[Divinylether fatty acids]] || containing a doubly unsaturated carbon chain attached via an ether bond to a fatty acid, found in some plants
|}

==Physical properties==

===Solubility===
Carboxylic acids are [[polarity (chemistry)|polar]]. Because they are both hydrogen-bond acceptors (the [[carbonyl]] {{chem2|\sC(\dO)\s}}) and hydrogen-bond donors (the [[hydroxyl]] {{chem2|\sOH}}), they also participate in [[hydrogen bond]]ing. Together, the hydroxyl and carbonyl group form the functional group carboxyl. Carboxylic acids usually exist as dimers in nonpolar media due to their tendency to "self-associate". Smaller carboxylic acids (1 to 5 carbons) are soluble in water, whereas bigger carboxylic acids have limited solubility due to the increasing hydrophobic nature of the alkyl chain. These longer chain acids tend to be soluble in less-polar solvents such as ethers and alcohols.<ref name=M&B>{{cite book|last1=Morrison|first1=R.T.|last2=Boyd|first2=R.N.|date=1992|title=Organic Chemistry|publisher=Prentice Hall |edition=6th|isbn=0-13-643669-2}}</ref> Aqueous sodium hydroxide and carboxylic acids, even hydrophobic ones, react to yield water-soluble sodium salts. For example, [[enanthic acid]] has a low solubility in water (0.2 g/L), but its sodium salt is very soluble in water.
:[[File:Solubility in different environments.jpg|class=skin-invert-image|500px]]

===Boiling points===
Carboxylic acids tend to have higher boiling points than water, because of their greater surface areas and their tendency to form stabilized dimers through [[hydrogen bond]]s. For boiling to occur, either the dimer bonds must be broken or the entire dimer arrangement must be vaporized, increasing the [[enthalpy of vaporization]] requirements significantly.{{cn|date=May 2025}}
:[[File:Carboxylic acid dimers.svg|class=skin-invert-image|thumb|Carboxylic acid [[Dimer (chemistry)|dimers]]|alt=|none]]

===Acidity===
Carboxylic acids are [[Brønsted–Lowry acid–base theory|Brønsted–Lowry acids]] because they are proton (H<sup>+</sup>) donors. They are the most common type of [[organic acid]].{{cn|date=May 2025}}

Carboxylic acids are typically [[weak acid]]s, meaning that they only partially [[Dissociation (chemistry)|dissociate]] into [[Hydronium|{{chem2|[H3O]+}}]] [[cation]]s and [[Carboxylate|{{chem2|R\sCO2−}}]] [[anion]]s in neutral [[Water (molecule)|aqueous]] solution. For example, at room temperature, in a 1-[[molarity|molar]] solution of [[acetic acid]], only 0.001% of the acid are dissociated (i.e. 10<sup>−5</sup> moles out of 1&nbsp;mol). Electron-withdrawing substituents such as [[Trifluoromethyl group|trifluoromethyl]] ({{chem2|\sCF3}}) give stronger acids (the p''K''<sub>a</sub> of acetic acid is 4.76 whereas trifluoroacetic acid, with a [[Trifluoromethyl group|trifluoromethyl substituent]], has a p''K''<sub>a</sub> of 0.23). Electron-donating substituents give weaker acids (the p''K''<sub>a</sub> of formic acid is 3.75 whereas acetic acid, with a [[Methyl group|methyl substituent]], has a p''K''<sub>a</sub> of 4.76){{cn|date=May 2025}}
{|class = "wikitable"
! Carboxylic acid<ref>{{cite book | editor-last= Haynes |editor-first=William M. | year = 2011 | title = CRC Handbook of Chemistry and Physics | edition = 92nd | publisher = [[CRC Press]] | isbn = 978-1439855119|pages=5–94 to 5–98|title-link=CRC Handbook of Chemistry and Physics}}</ref>
! [[Acid dissociation constant|p''K''<sub>a</sub>]]
|-
|[[Formic acid]] ({{chem2|HCO2H}}) || 3.75
|-
|[[Chloroformic acid]] ({{chem2|ClCO2H}}) || 0.27<ref name=metabolites>{{cite web | url = https://hmdb.ca/metabolites/HMDB0250109 | title = Chlorocarbonic acid | work =  Human Metabolome Database }}</ref>
|-
|[[Acetic acid]] ({{chem2|CH3CO2H}})|| 4.76
|-
|[[Glycine]] ({{chem2|NH2CH2CO2H}})||2.34
|-
|[[Fluoroacetic acid]] ({{chem2|FCH2CO2H}}) || 2.586
|-
|[[Difluoroacetic acid]] ({{chem2|F2CHCO2H}})|| 1.33
|-
|[[Trifluoroacetic acid]] ({{chem2|CF3CO2H}})|| 0.23
|-
|[[Chloroacetic acid]] ({{chem2|ClCH2CO2H}})|| 2.86
|-
|[[Dichloroacetic acid]] ({{chem2|Cl2CHCO2H}})|| 1.29
|-
|[[Trichloroacetic acid]] ({{chem2|CCl3CO2H}})|| 0.65
|-
|[[Benzoic acid]] ({{chem2|C6H5\sCO2H}})||4.2
|-
|[[2-Nitrobenzoic acid]] (''ortho''-{{chem2|C6H4(NO2)CO2H}})||2.16
|-
|[[Oxalic acid]] ({{chem2|HO\sC(\dO)\sC(\dO)\sOH}}) (first dissociation)
| 1.27
|-
|[[Oxalic acid#Acid-base properties|Hydrogen oxalate]] ({{chem2|HO\sC(\dO)\sCO2−}}) (second dissociation of oxalic acid)
|4.14
|-
|}
[[Deprotonation]] of carboxylic acids gives carboxylate anions; these are [[resonance stabilized]], because the negative charge is delocalized over the two oxygen atoms, increasing the stability of the anion. Each of the carbon–oxygen bonds in the carboxylate anion has a partial double-bond character. The carbonyl carbon's partial positive charge is also weakened by the −<sup>1</sup>/<sub>2</sub> negative charges on the 2 oxygen atoms.{{cn|date=May 2025}}

===Odour===
Carboxylic acids often have strong sour odours. [[Ester]]s of carboxylic acids tend to have fruity, pleasant odours, and many are used in [[perfume]]. {{cn|date=May 2025}}

=== Characterization ===
Carboxylic acids are readily identified as such by [[infrared spectroscopy]]. They exhibit a sharp band associated with vibration of the C=O carbonyl bond (''ν''<sub>C=O</sub>) between 1680 and 1725&nbsp;cm<sup>−1</sup>. A characteristic ''ν''<sub>O–H</sub> band appears as a broad peak in the 2500 to 3000&nbsp;cm<sup>−1</sup> region.<ref name=M&B/><ref>{{cite journal |last1=Smith |first1=Brian |title=The C=O Bond, Part VIII: Review |url=https://www.spectroscopyonline.com/view/co-bond-part-viii-review |website=Spectroscopy |series=November 2018 |date=November 2018 |volume=33 |pages=24–29 |access-date=12 February 2024}}</ref> By <sup>1</sup>H [[Nuclear magnetic resonance|NMR]] spectrometry, the [[hydroxyl]] hydrogen appears in the 10–13 ppm region, although it is often either broadened or not observed owing to exchange with traces of water.{{cn|date=May 2025}}

==Occurrence and applications==
Many carboxylic acids are produced industrially on a large scale. They are also frequently found in nature. Esters of fatty acids are the main components of lipids and polyamides of [[amino acid|aminocarboxylic acids]] are the main components of [[protein]]s.{{cn|date=May 2025}}

Carboxylic acids are used in the production of polymers, pharmaceuticals, solvents, and food additives. Industrially important carboxylic acids include [[acetic acid]] (component of vinegar, precursor to solvents and coatings), [[acrylic acid|acrylic and methacrylic acids]] (precursors to polymers, adhesives), [[adipic acid]] (polymers), [[citric acid]] (a flavor and preservative in food and beverages), [[ethylenediaminetetraacetic acid]] (chelating agent), [[fatty acid]]s (coatings), [[maleic acid]] (polymers), [[propionic acid]] (food preservative), [[terephthalic acid]] (polymers). Important carboxylate salts are soaps.{{cn|date=May 2025}}

==Synthesis==

===Industrial routes===
In general, industrial routes to carboxylic acids differ from those used on a smaller scale because they require specialized equipment.
* Carbonylation of alcohols as illustrated by the [[Cativa process]] for the production of acetic acid. Formic acid is prepared by a different carbonylation pathway, also starting from methanol.
* Oxidation of [[aldehyde]]s with air using cobalt and manganese catalysts. The required aldehydes are readily obtained from alkenes by [[hydroformylation]].
* Oxidation of hydrocarbons using air. For simple alkanes, this method is inexpensive but not selective enough to be useful. Allylic and benzylic compounds undergo more selective oxidations. Alkyl groups on a benzene ring are oxidized to the carboxylic acid, regardless of its chain length. [[Benzoic acid]] from [[toluene]], [[terephthalic acid]] from ''para''-[[xylene]], and [[phthalic acid]] from ''ortho''-[[xylene]] are illustrative large-scale conversions. [[Acrylic acid]] is generated from [[propene]].<ref>{{cite book|last1=Riemenschneider|first1=Wilhelm|date=2002|chapter=Carboxylic Acids, Aliphatic|title=Ullmann's Encyclopedia of Industrial Chemistry|publisher=Wiley-VCH|location=Weinheim|doi=10.1002/14356007.a05_235|isbn=3527306730}}.</ref>
* Oxidation of ethene using [[silicotungstic acid]] catalyst.
* Base-catalyzed dehydrogenation of alcohols.
* Carbonylation coupled to the addition of water. This method is effective and versatile for alkenes that generate secondary and tertiary [[carbocation]]s, e.g. [[isobutylene]] to [[pivalic acid]]. In the [[Koch reaction]], the addition of water and carbon monoxide to [[alkenes]] or [[alkynes]] is catalyzed by strong acids. Hydrocarboxylations involve the simultaneous addition of water and [[Carbon monoxide|CO]]. Such reactions are sometimes called "[[Walter Reppe|Reppe chemistry]]."
:{{chem2|[[Acetylene|HC\tCH]] + CO + H2O → [[Acrylic acid|CH2\dCH\sCO2H]]}}
* Hydrolysis of [[triglyceride]]s obtained from plant or animal oils. These methods of synthesizing some long-chain carboxylic acids are related to [[soap making]].
* [[Fermentation (biochemistry)|Fermentation]] of ethanol. This method is used in the production of [[vinegar]].
* The [[Kolbe–Schmitt reaction]] provides a route to [[salicylic acid]], precursor to [[aspirin]].

===Laboratory methods===
Preparative methods for small scale reactions for research or for production of fine chemicals often employ expensive consumable reagents.
* [[Oxidation of primary alcohols to carboxylic acids|Oxidation of primary alcohols]] or [[aldehyde]]s with strong [[Oxidizing agent|oxidants]] such as [[potassium dichromate]], [[Jones reagent]], [[potassium permanganate]], or [[sodium chlorite]]. The method is more suitable for laboratory conditions than the industrial use of air, which is "greener" because it yields less inorganic side products such as chromium or manganese oxides.<ref>{{Cite journal |last1=Mohammadpoor-Baltork |first1=Iraj |last2=Sadeghi |first2=Majid M. |last3=Adibi |first3=Abol-Hassan |date=2001-10-31 |title=Efficient, Solvent-Free Oxidation of Organic Compounds with Potassium Dichromate in the Presence of Lewis Acids |journal=Molecules |language=en |volume=6 |issue=11 |pages=900–908 |doi=10.3390/61100900 |doi-access=free |issn=1420-3049 |pmc=6236395}}</ref>
* Oxidative cleavage of [[olefin]]s by [[ozonolysis]], [[potassium permanganate]], or [[potassium dichromate]].
* Hydrolysis of [[nitrile]]s, [[ester]]s, or [[amide]]s, usually with acid- or base-catalysis.
* Carbonation of a [[Grignard reagent]] and [[organolithium]] reagents:
:{{chem2|RLi + [[Carbon dioxide|CO2]] → RCO2−Li+}}
:{{chem2|RCO2−Li+ + [[Hydrogen chloride|HCl]] → RCO2H + [[Lithium chloride|LiCl]]}}
* [[Halogenation]] followed by hydrolysis of [[methyl ketone]]s in the [[haloform reaction]]
* Base-catalyzed cleavage of non-enolizable ketones, especially [[aryl]] ketones:<ref>{{cite journal|title=Carboxylation of Aromatic Compounds: Ferrocenecarboxylic Acid|author=Perry C. Reeves|journal=Org. Synth.|year=1977|volume=56|page=28|doi=10.15227/orgsyn.056.0028}}</ref>
:{{chem2|R\sC(\dO)\s[[Aryl|Ar]] + H2O → R\sCO2H + ArH}}

===Less-common reactions===
Many reactions produce carboxylic acids but are used only in specific cases or are mainly of academic interest.
* Disproportionation of an [[aldehyde]] in the [[Cannizzaro reaction]]
* Rearrangement of diketones in the [[benzilic acid rearrangement]]
* Involving the generation of benzoic acids are the [[von Richter reaction]] from nitrobenzenes and the [[Kolbe–Schmitt reaction]] from [[phenol]]s.

==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.

==Carboxyl radical==
The carboxyl [[free radical|radical]], •COOH, only exists briefly.<ref>{{Cite journal|title = Infrared Spectrum and Structure of Intermediates in Reaction of OH with CO|author = Milligan, D. E.|author2=Jacox, M. E.|year = 1971|journal = Journal of Chemical Physics|volume = 54|pages = 927–942|doi=10.1063/1.1675022|issue = 3|bibcode = 1971JChPh..54..927M}}</ref> The [[acid dissociation constant]] of •COOH has been measured using [[electron paramagnetic resonance]] spectroscopy.<ref>The value is p''K''<sub>a</sub>&nbsp;=&nbsp;−0.2&nbsp;±&nbsp;0.1. {{Cite journal|title = ESR Measurement of the p''K''<sub>a</sub> of Carboxyl Radical and Ab Initio Calculation of the Carbon-13 Hyperfine Constant|last1= Jeevarajan |first1=A. S. |last2=Carmichael |first2=I. |last3=Fessenden |first3=R. W.|year = 1990|journal = Journal of Physical Chemistry|volume = 94|pages = 1372–1376|doi=10.1021/j100367a033|issue = 4}}</ref> The carboxyl group tends to dimerise to form [[oxalic acid]].{{cn|date=May 2025}}

==See also==
* [[Acid anhydride]]
* [[Acid chloride]]
* [[Amide]]
* [[Amino acid]]
* [[Ester]]
* [[List of carboxylic acids]]
* [[Dicarboxylic acid]]
* [[Pseudoacid]]
* [[Thiocarboxy]]
* [[Carbon dioxide]] (CO<sub>2</sub>)

==References==
{{Reflist}}

==External links==
{{Wiktionary|carboxyl}}
* Carboxylic acids pH and titration [http://www2.iq.usp.br/docente/gutz/Curtipot_.html – freeware for calculations, data analysis, simulation, and distribution diagram generation]
* [https://agrocode.com/en/glosario_terminos/polyhydroxycarboxylic-acids/ PHC.]

{{Functional Groups}}
{{Organic reactions}}
{{Authority control}}

{{DEFAULTSORT:Carboxylic Acid}}
[[Category:Carboxylic acids| ]]
[[Category:Functional groups]]