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[edit]

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 atomsCommon nameIUPAC nameChemical formulaCommon location or use
1Formic acidMethanoic acidHCOOHInsect stings
2Acetic acidEthanoic acidCH3COOHVinegar
3Propionic acidPropanoic acidCH3CH2COOHPreservative for stored grains
4Butyric acidButanoic acidCH3(CH2)2COOHButter
5Valeric acidPentanoic acidCH3(CH2)3COOHValerian
6Caproic acidHexanoic acidCH3(CH2)4COOHGoat fat
7Enanthic acidHeptanoic acidCH3(CH2)5COOH
8Caprylic acidOctanoic acidCH3(CH2)6COOHCoconuts and breast milk
9Pelargonic acidNonanoic acidCH3(CH2)7COOHPelargonium
10Capric acidDecanoic acidCH3(CH2)8COOH
11Undecylic acidUndecanoic acidCH3(CH2)9COOH
12Lauric acidDodecanoic acidCH3(CH2)10COOHCoconut oil and hand wash soaps.
13Tridecylic acidTridecanoic acidCH3(CH2)11COOH
14Myristic acidTetradecanoic acidCH3(CH2)12COOHNutmeg
15Pentadecanoic acidCH3(CH2)13COOH
16Palmitic acidHexadecanoic acidCH3(CH2)14COOHPalm oil
17Margaric acidHeptadecanoic acidCH3(CH2)15COOH
18Stearic acidOctadecanoic acidCH3(CH2)16COOHChocolate, waxes, soaps, and oils
20Arachidic acidIcosanoic acidCH3(CH2)18COOHPeanut oil
Other carboxylic acids
Compound classMembers
unsaturated monocarboxylic acidsacrylic acid (2-propenoic acid) – CH2=CHCOOH, used in polymer synthesis
Fatty acidsmedium to long-chain saturated and unsaturated monocarboxylic acids, with even number of carbons examples docosahexaenoic acid and eicosapentaenoic acid (nutritional supplements)
Amino acidsthe building-blocks of proteins
Keto acidsacids of biochemical significance that contain a ketone group, e.g. acetoacetic acid and pyruvic acid
Aromatic carboxylic acidsbenzoic 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 acidscontaining two carboxyl groups examples adipic acid the monomer used to produce nylon and aldaric acid – a family of sugar acids
Tricarboxylic acidscontaining three carboxyl groups example citric acid – found in citrus fruits and isocitric acid
Alpha hydroxy acidscontaining 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[edit]

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[edit]

Solubility[edit]

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[edit]

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[edit]

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)0.5
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[edit]

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[edit]

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[edit]

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[edit]

Industrial routes[edit]

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

HCCH + CO + H2O → CH2=CHCO2H

Laboratory methods[edit]

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[edit]

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

Reactions[edit]

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[edit]

See also[edit]

References[edit]

  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.

External links[edit]