Structure of a carboxylic acid
Carboxylate anion
3D structure of a carboxylic acid

In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group (−C(=O)−OH)[1] attached to an R-group. The general formula of a carboxylic acid is often written as R−COOH or R−CO2H, sometimes as R−C(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 acids and fatty acids. Deprotonation of a carboxylic acid gives a carboxylate anion.

Examples and nomenclature

Carboxylic acids are commonly identified by their trivial names. They often have the suffix -ic acid. IUPAC-recommended names also exist; in this system, carboxylic acids have an -oic acid suffix.[2] For example, butyric acid (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 substituents, such as 3-chloropropanoic acid. Alternately, it can be named as a "carboxy" or "carboxylic acid" substituent on another parent structure, such as 2-carboxyfuran.

The carboxylate anion (R−COO or R−CO2) 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.

Carbonic acid, which occurs in bicarbonate buffer systems in nature, is not generally classed as one of the carboxylic acids, despite that it has a moiety that looks like a COOH group.

Straight-chain, saturated carboxylic acids (alkanoic acids)
Common Name IUPAC Name Chemical formula Common location or use
1 Formic acid Methanoic acid HCOOH Insect stings
2 Acetic acid Ethanoic acid CH3COOH Vinegar
3 Propionic acid Propanoic acid CH3CH2COOH Preservative for stored grains, body odour, milk, butter, cheese
4 Butyric acid Butanoic acid CH3(CH2)2COOH Butter
5 Valeric acid Pentanoic acid CH3(CH2)3COOH Valerian plant
6 Caproic acid Hexanoic acid CH3(CH2)4COOH Goat fat
7 Enanthic acid Heptanoic acid CH3(CH2)5COOH Fragrance
8 Caprylic acid Octanoic acid CH3(CH2)6COOH Coconuts
9 Pelargonic acid Nonanoic acid CH3(CH2)7COOH Pelargonium plant
10 Capric acid Decanoic acid CH3(CH2)8COOH Coconut and Palm kernel oil
11 Undecylic acid Undecanoic acid CH3(CH2)9COOH Anti-fungal agent
12 Lauric acid Dodecanoic acid CH3(CH2)10COOH Coconut oil and hand wash soaps
13 Tridecylic acid Tridecanoic acid CH3(CH2)11COOH Plant metabolite
14 Myristic acid Tetradecanoic acid CH3(CH2)12COOH Nutmeg
15 Pentadecylic acid Pentadecanoic acid CH3(CH2)13COOH Milk fat
16 Palmitic acid Hexadecanoic acid CH3(CH2)14COOH Palm oil
17 Margaric acid Heptadecanoic acid CH3(CH2)15COOH Pheromone in various animals
18 Stearic acid Octadecanoic acid CH3(CH2)16COOH Chocolate, waxes, soaps, and oils
19 Nonadecylic acid Nonadecanoic acid CH3(CH2)17COOH Fats, vegetable oils, pheromone
20 Arachidic acid Icosanoic acid CH3(CH2)18COOH Peanut oil
Other carboxylic acids
Compound class Members
unsaturated monocarboxylic acids acrylic acid (2-propenoic acid) – CH2=CH−COOH, used in polymer synthesis
Fatty acids medium to long-chain saturated and unsaturated monocarboxylic acids, with even number of carbons; examples: docosahexaenoic acid and eicosapentaenoic acid (nutritional supplements)
Amino acids the building-blocks of proteins
Keto acids acids of biochemical significance that contain a ketone group; examples: acetoacetic acid and pyruvic acid
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 acids containing two carboxyl groups; examples: adipic acid the monomer used to produce nylon and aldaric acid – a family of sugar acids
Tricarboxylic acids containing three carboxyl groups; examples: citric acid – found in citrus fruits and isocitric acid
Alpha hydroxy acids 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 acids containing a hydroxy group in the second position
Omega hydroxy acids 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


Carboxylic acids are polar. Because they are both hydrogen-bond acceptors (the carbonyl −C(=O)−) and hydrogen-bond donors (the hydroxyl −OH), they also participate in hydrogen bonding. 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.[3] 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.

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 bonds. 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.

Carboxylic acid dimers


Carboxylic acids are Brønsted–Lowry acids because they are proton (H+) donors. They are the most common type of organic acid.

Carboxylic acids are typically weak acids, meaning that they only partially dissociate into [H3O]+ cations and R−CO2 anions in neutral aqueous solution. For example, at room temperature, in a 1-molar solution of acetic acid, only 0.001% of the acid are dissociated (i.e. 10−5 moles out of 1 mol). Electron-withdrawing substituents, such as -CF3 group, give stronger acids (the pKa of acetic acid is 4.76 whereas trifluoroacetic acid, with a trifluoromethyl substituent, has a pKa of 0.23). Electron-donating substituents give weaker acids (the pKa of formic acid is 3.75 whereas acetic acid, with a methyl substituent, has a pKa of 4.76)

Carboxylic acid[4] pKa
Formic acid (HCO2H) 3.75
Chloroformic acid (ClCO2H) 0.27[5]
Acetic acid (CH3CO2H) 4.76
Glycine (NH2CH2CO2H) 2.34
Fluoroacetic acid (FCH2CO2H) 2.586
Difluoroacetic acid (F2CHCO2H) 1.33
Trifluoroacetic acid (CF3CO2H) 0.23
Chloroacetic acid (ClCH2CO2H) 2.86
Dichloroacetic acid (Cl2CHCO2H) 1.29
Trichloroacetic acid (CCl3CO2H) 0.65
Benzoic acid (C6H5−CO2H) 4.2
2-Nitrobenzoic acid (ortho-C6H4(NO2)CO2H) 2.16
Oxalic acid (HO−C(=O)−C(=O)−OH) (first dissociation) 1.27
Hydrogen oxalate (HO−C(=O)−CO2) (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 -1/2 negative charges on the 2 oxygen atoms.


Carboxylic acids often have strong sour odours. Esters of carboxylic acids tend to have fruity, pleasant odours, and many are used in perfume.


Carboxylic acids are readily identified as such by infrared spectroscopy. They exhibit a sharp band associated with vibration of the C=O carbonyl bond (νC=O) between 1680 and 1725 cm−1. A characteristic νO–H band appears as a broad peak in the 2500 to 3000 cm−1 region.[3][6] By 1H 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.

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 aminocarboxylic acids are the main components of proteins.

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 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 acids (coatings), maleic acid (polymers), propionic acid (food preservative), terephthalic acid (polymers). Important carboxylate salts are soaps.


Industrial routes

In general, industrial routes to carboxylic acids differ from those used on a smaller scale because they require specialized equipment.

HC≡CH + CO + H2O → CH2=CH−CO2H

Laboratory methods

Preparative methods for small scale reactions for research or for production of fine chemicals often employ expensive consumable reagents.

RLi + CO2 → RCO2Li+
RCO2Li+ + HCl → RCO2H + LiCl
R−C(=O)−Ar + H2O → R−CO2H + ArH

Less-common reactions

Many reactions produce carboxylic acids but are used only in specific cases or are mainly of academic interest.


Carboxylic acid organic reactions

The most widely practiced reactions convert carboxylic acids into esters, amides, carboxylate salts, acid chlorides, and alcohols. Carboxylic acids react with 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:

CH3COOH + NaHCO3 → CH3COONa+ + CO2 + H2O

Carboxylic acids also react with alcohols to give esters. This process is widely used, e.g. in the production of polyesters. Likewise, carboxylic acids are converted into amides, 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 acids into peptides is a significant biochemical process that requires ATP.

The hydroxyl group on carboxylic acids may be replaced with a chlorine atom using thionyl chloride to give acyl chlorides. In nature, carboxylic acids are converted to thioesters.


Like esters, most carboxylic acids can be reduced to alcohols by hydrogenation, or using hydride transferring agents such as lithium aluminium hydride. Strong alkyl transferring agents, such as organolithium compounds but not Grignard reagents, will reduce carboxylic acids to ketones along with transfer of the alkyl group.

Vilsmaier reagent (N,N-Dimethyl(chloromethylene)ammonium chloride [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.[9]

Specialized reactions

Carboxyl radical

The carboxyl radical, •COOH, only exists briefly.[10] The acid dissociation constant of •COOH has been measured using electron paramagnetic resonance spectroscopy.[11] The carboxyl group tends to dimerise to form oxalic acid.

See also


  1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "carboxylic acids". doi:10.1351/goldbook.C00852
  2. ^ Recommendations 1979. Organic Chemistry IUPAC Nomenclature. Rules C-4 Carboxylic Acids and Their Derivatives.
  3. ^ a b Morrison, R.T.; Boyd, R.N. (1992). Organic Chemistry (6th ed.). Prentice Hall. ISBN 0-13-643669-2.
  4. ^ Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). CRC Press. pp. 5–94 to 5–98. ISBN 978-1439855119.
  5. ^ "Chlorocarbonic acid". Human Metabolome Database.
  6. ^ Smith, Brian. "The C=O Bond, Part VIII: Review". Spectroscopy. Retrieved 12 February 2024.
  7. ^ Riemenschneider, Wilhelm (2002). "Carboxylic Acids, Aliphatic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a05_235. ISBN 3527306730..
  8. ^ Perry C. Reeves (1977). "Carboxylation of Aromatic Compounds: Ferrocenecarboxylic Acid". Org. Synth. 56: 28. doi:10.15227/orgsyn.056.0028.
  9. ^ Fujisawa, Tamotsu; Sato, Toshio. "Reduction of carboxylic acids to aldehydes: 6-Ooxdecanal". Organic Syntheses. 66: 121. doi:10.15227/orgsyn.066.0121; Collected Volumes, vol. 8, p. 498.
  10. ^ Milligan, D. E.; Jacox, M. E. (1971). "Infrared Spectrum and Structure of Intermediates in Reaction of OH with CO". Journal of Chemical Physics. 54 (3): 927–942. Bibcode:1971JChPh..54..927M. doi:10.1063/1.1675022.
  11. ^ The value is pKa = −0.2 ± 0.1. Jeevarajan, A. S.; Carmichael, I.; Fessenden, R. W. (1990). "ESR Measurement of the pKa of Carboxyl Radical and Ab Initio Calculation of the Carbon-13 Hyperfine Constant". Journal of Physical Chemistry. 94 (4): 1372–1376. doi:10.1021/j100367a033.