Carboxylic acid

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Structure of a carboxylic acid
Carboxylate ion
The 3D structure of the carboxyl group

A carboxylic acid /ˌkɑrbɒkˈsɪlɪk/ is an organic compound that contains a carboxyl group (CO2H).1 The general formula of a carboxylic acid is R-CO2H with R referring to the rest of the (possibly quite large) molecule. Carboxylic acids occur widely, and include the amino acids and acetic acid (active ingredient in vinegar).

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 2 of the Carbon atom's 4 bonds are to an Oxygen atom, the Carbon atom's 3rd bond is to a second Oxygen atom (whose other bond is to a Hydrogen atom), and the Carbon atom's 4th bond attaches to R. [A Carbon atom double bonded to an Oxygen atom is a carbonyl group and 2 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). In common nomenclature, 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
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
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
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

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 acid6 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) 0.5
Oxalic acid (HO2CCO2H) 1.27
Benzoic acid (C6H5CO2H) 4.2

Deprotonation of carboxylic acids gives carboxylate anions, which is resonance stabilized because the negative charge is delocalized between the two oxygen atoms increasing its stability. Each of the carbon-oxygen bonds in carboxylate anion has partial double-bond character.

Odor

Carboxylic acids often have strong odors, especially the volatile derivatives. Most common are acetic acid (vinegar) and butyric acid (butter). On the other hand, 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 (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.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 so 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 and 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.

RLi + CO2 → RCO2Li
RCO2Li + HCl → RCO2H + LiCl

Less-common reactions

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

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 → CH3COONa+ + 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-dimethylchloromethylenammonium chloride 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

See also

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 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 C-13 Hyperfine Constant". Journal of Physical Chemistry 94 (4): 1372–1376. doi:10.1021/j100367a033. 
  5. ^ a b R.T. Morrison, R.N. Boyd. Organic Chemistry, 6th Ed. (1992) 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. ^ Wilhelm Riemenschneider “Carboxylic Acids, Aliphatic” in Ullmann's Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH, Weinheim. doi: 10.1002/14356007.a05_235.
  8. ^ Tamotsu Fujisawa and Toshio Sato, Organic Syntheses, Coll. Vol. 8, p.498 (1993); Vol. 66, p.121 (1988)
  9. ^ Organic Syntheses, Coll. Vol. 3, p.234 (1955); Vol. 24, p.38 (1944) Link
  10. ^ Organic Syntheses, Coll. Vol. 3, p.237 (1955); Vol. 24, p.41 (1944) Link.

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