Carboxylic acid

Structure of a carboxylic acid

Carboxylate ion

3D structure of a carboxylic acid

A carboxylic acid /ˌkɑːrbɒkˈsɪlɪk/ is an organic compound that contains a carboxyl group (C(O)OH).^[1] The general formula of a carboxylic acid is R–C(O)OH, with R referring to the rest of the (possibly quite large) molecule. Carboxylic acids occur widely and include the amino acids (which make up proteins) and acetic acid (which is part of vinegar and occurs in metabolism).

Salts and esters of carboxylic acids are called carboxylates. When a carboxyl group is deprotonated, its conjugate base forms a carboxylate anion. Carboxylate ions are resonance-stabilized, and this increased stability makes carboxylic acids more acidic than alcohols. Carboxylic acids can be seen as reduced or alkylated forms of the Lewis acid carbon dioxide; under some circumstances they can be decarboxylated to yield carbon dioxide.

Example carboxylic acids and nomenclature

Carboxylic acids are commonly named as indicated in the table below. Although rarely used, IUPAC-recommended names also exist. For example, butyric acid (C₃H₇CO₂H) is, according to IUPAC guidelines, also known as butanoic acid.^[2]

To more easily understand much of the below discussion of reactions involving carboxylic acids it can be helpful to notice that the carboxyl group itself is a "hydroxylated carbonyl group" meaning that two of the carbon atom's four bonds are to an oxygen atom, the carbon atom's third bond is to a second oxygen atom (whose other bond is to a hydrogen atom), and the carbon atom's fourth bond attaches to R. (A carbon atom double bonded to an oxygen atom is a carbonyl group and two of the carbon atom's bonds remain available for bonding. A hydrogen atom bonded to an oxygen atom is a hydroxyl group with the oxygen atom's second bond available for bonding.)

The carboxylate anion R–COO⁻ is usually named with the suffix -ate, so acetic acid, for example, becomes acetate ion. In IUPAC nomenclature, carboxylic acids have an -oic acid suffix (e.g., octadecanoic acid). For trivial names, the suffix is usually -ic acid (e.g., stearic acid).

Straight-chain, saturated carboxylic acids

Carbon atoms	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
4	Butyric acid	Butanoic acid	CH3(CH2)2COOH	Butter
5	Valeric acid	Pentanoic acid	CH3(CH2)3COOH	Valerian
6	Caproic acid	Hexanoic acid	CH3(CH2)4COOH	Goat fat
7	Enanthic acid	Heptanoic acid	CH3(CH2)5COOH
8	Caprylic acid	Octanoic acid	CH3(CH2)6COOH	Coconuts and breast milk
9	Pelargonic acid	Nonanoic acid	CH3(CH2)7COOH	Pelargonium
10	Capric acid	Decanoic acid	CH3(CH2)8COOH	Coconut and Palm kernel oil
11	Undecylic acid	Undecanoic acid	CH3(CH2)9COOH
12	Lauric acid	Dodecanoic acid	CH3(CH2)10COOH	Coconut oil and hand wash soaps
13	Tridecylic acid	Tridecanoic acid	CH3(CH2)11COOH
14	Myristic acid	Tetradecanoic acid	CH3(CH2)12COOH	Nutmeg
15	Pentadecanoic acid		CH3(CH2)13COOH
16	Palmitic acid	Hexadecanoic acid	CH3(CH2)14COOH	Palm oil
17	Margaric acid	Heptadecanoic acid	CH3(CH2)15COOH
18	Stearic acid	Octadecanoic acid	CH3(CH2)16COOH	Chocolate, waxes, soaps, and oils
19	Nonadecylic 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=CHCOOH, 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, e.g. acetoacetic acid and pyruvic acid
Aromatic carboxylic acids	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 example citric acid – found in citrus fruits and isocitric acid
Alpha hydroxy acids	containing a hydroxy group example glyceric acid, glycolic acid and lactic acid (2-hydroxypropanoic acid) – found in sour milk tartaric acid – found in wine
Divinylether fatty acids	containing a doubly unsaturated carbon chain attached via an ether bond to a fatty acid, found in some plants

Carboxyl radical

The radical ^•COOH (CAS# 2564-86-5) has only a separate fleeting existence.^[3] The acid dissociation constant of ^•COOH has been measured using electron paramagnetic resonance spectroscopy.^[4] The carboxyl group tends to dimerise to form oxalic acid.

Physical properties

Solubility

Carboxylic acid dimers

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 forms the functional group carboxyl. Carboxylic acids usually exist as dimeric pairs in nonpolar media due to their tendency to “self-associate.” Smaller carboxylic acids (1 to 5 carbons) are soluble in water, whereas higher carboxylic acids are less soluble due to the increasing hydrophobic nature of the alkyl chain. These longer chain acids tend to be rather soluble in less-polar solvents such as ethers and alcohols.^[5]

Boiling points

Carboxylic acids tend to have higher boiling points than water, not only because of their increased surface area, but because of their tendency to form stabilised dimers. Carboxylic acids tend to evaporate or boil as these dimers. For boiling to occur, either the dimer bonds must be broken or the entire dimer arrangement must be vaporised, both of which increase the enthalpy of vaporization requirements significantly.

Acidity

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 H⁺ cations and RCOO⁻ anions in neutral aqueous solution. For example, at room temperature, in a 1-molar solution of acetic acid, only 0.4% of the acid molecules are dissociated. Electronegative substituents give stronger acids.

Carboxylic acid[6]	pKa
Formic acid (HCOOH)	3.75
Acetic acid (CH3COOH)	4.76
Chloroacetic acid (CH2ClCO2H)	2.86
Dichloroacetic acid (CHCl2CO2H)	1.29
Trichloroacetic acid (CCl3CO2H)	0.65
Trifluoroacetic acid (CF3CO2H)	0.23
Oxalic acid (HO2CCO2H)	1.27
Benzoic acid (C6H5CO2H)	4.2

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.

Odour

Carboxylic acids often have strong odors, especially the volatile derivatives. Most common are acetic acid (vinegar) and butyric acid (human vomit). Conversely esters of carboxylic acids tend to have pleasant odors and many are used in perfume.

Characterization

Carboxylic acids are readily identified as such by infrared spectroscopy. They exhibit a sharp band associated with vibration of the C–O vibration bond (ν_C=O) between 1680 and 1725 cm⁻¹. A characteristic ν_O–H band appears as a broad peak in the 2500 to 3000 cm⁻¹ region.^[5] 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 pervasive 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 (beverages), ethylenediaminetetraacetic acid (chelating agent), fatty acids (coatings), maleic acid (polymers), propionic acid (food preservative), terephthalic acid (polymers).

Synthesis

Industrial routes

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

• Oxidation of aldehydes 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, the method is nonselective but too inexpensive to be useful. Allylic and benzylic compounds undergo more selective oxidations. Alkyl groups on a benzene ring 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.^[7]
• Base-catalyzed dehydrogenation of alcohols.
• Carbonylation is versatile method when coupled to the addition of water. This method is effective for alkenes that generate secondary and tertiary carbocations, e.g. isobutylene to pivalic acid. In the Koch reaction, the addition of water and carbon monoxide to alkenes is catalyzed by strong acids. Acetic acid and formic acid are produced by the carbonylation of methanol, conducted with iodide and alkoxide promoters, respectively, and often with high pressures of carbon monoxide, usually involving additional hydrolytic steps. Hydrocarboxylations involve the simultaneous addition of water and CO. Such reactions are sometimes called "Reppe chemistry":

HCCH + CO + H₂O → CH₂=CHCO₂H

• Some long-chain carboxylic acids are obtained by the hydrolysis of triglycerides obtained from plant or animal oils; these methods are related to soap making.
• fermentation of ethanol is used in the production of vinegar.

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 or aldehydes with strong oxidants such as potassium dichromate, Jones reagent, potassium permanganate, or sodium chlorite. The method is amenable to laboratory conditions compared to the industrial use of air, which is "greener", since it yields less inorganic side products such as chromium or manganese oxides.
• Oxidative cleavage of olefins by ozonolysis, potassium permanganate, or potassium dichromate.
• Carboxylic acids can also be obtained by the hydrolysis of nitriles, esters, or amides, in general with acid- or base-catalysis.
• Carbonation of a Grignard and organolithium reagents:

RLi + CO₂ → RCO₂Li

RCO₂Li + HCl → RCO₂H + LiCl

• Halogenation followed by hydrolysis of methyl ketones in the haloform reaction
• The Kolbe–Schmitt reaction provides a route to salicylic acid, precursor to aspirin.

Less-common reactions

Many reactions afford 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 phenols.

Reactions

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. Thus, acetic acid found in vinegar reacts with sodium bicarbonate (baking soda) to form sodium acetate, carbon dioxide, and water:

CH₃COOH + NaHCO₃ → CH₃COO⁻Na⁺ + CO₂ + H₂O

Carboxylic acids also react with alcohols to give esters. This process is heavily used 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 major 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.

Carboxylic acid can be reduced to the alcohol by hydrogenation or using stoichiometric hydride reducing agents such as lithium aluminium hydride.

N,N-Dimethyl(chloromethylene)ammonium chloride (ClHC=N⁺(CH₃)₂Cl⁻) is a highly chemoselective agent for carboxylic acid reduction. It selectively activate the carboxylic acid and is known to tolerate active functionalities such as ketone as well as the moderate ester, olefin, nitrile, and halide moeties.^[8]

Specialized reactions

• As with all carbonyl compounds, the protons on the α-carbon are labile due to keto–enol tautomerization. Thus, the α-carbon is easily halogenated in the Hell–Volhard–Zelinsky halogenation.
• The Schmidt reaction converts carboxylic acids to amines.
• 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, an carboxylic acid on an aliphatic chain having a simple the 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. Enzymes that catalyze these reactions are known as carboxylases (EC 6.4.1) and decarboxylases (EC 4.1.1).
• Carboxylic acids are reduced to aldehydes via the ester and 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.
• The Kolbe electrolysis is an electrolytic, decarboxylative dimerization reaction. In other words, it gets rid of the carboxyl groups of two acid molecules, and joins the remaining fragments together.

See also

• Acid anhydride
• Acid chloride
• Amide
• Ester
• List of carboxylic acids
• Dicarboxylic acid
• Pseudoacid
• Thiocarboxy

References

1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "carboxylic acids".
2. Recommendations 1979. Organic Chemistry IUPAC Nomenclature. Rules C-4 Carboxylic Acids and Their Derivatives.
3. 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.
4. The value is pK_a = −0.2 ± 0.1. Jeevarajan, A. S.; Carmichael, I.; Fessenden, R. W. (1990). "ESR Measurement of the pK_a of Carboxyl Radical and Ab Initio Calculation of the C-13 Hyperfine Constant". Journal of Physical Chemistry 94 (4): 1372–1376. doi:10.1021/j100367a033.
5. Morrison, R.T.; Boyd, R.N. (1992). Organic Chemistry (6th ed.). ISBN 0-13-643669-2.
6. Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). CRC Press. pp. 5–94 to 5–98. ISBN 1439855110.
7. Riemenschneider, Wilhelm (2002). "Carboxylic Acids, Aliphatic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a05_235..
8. Fujisawa, Tamotsu; Sato, Toshio. "Reduction of carboxylic acids to aldehydes: 6-Ooxdecanal". Org. Synth. 66: 121. doi:10.15227/orgsyn.066.0121.; Coll. Vol. 8, p. 498

External links

• Carboxylic acids pH and titration – freeware for calculations, data analysis, simulation, and distribution diagram generation