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carboxylic acid

 
American Heritage Dictionary:

car·box·yl·ic acid

(kär'bŏk-sĭl'ĭk) pronunciation
n.
An organic acid that contains one or more carboxyl groups.


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Britannica Concise Encyclopedia:

carboxylic acid

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Any organic compound with the general chemical formula -COOH in which a carbon (C) atom is bonded to an oxygen (O) atom by a double bond to make a carbonyl group (-C=O; see functional group) and to a hydroxyl group (-OH) by a single bond (see bonding). The fourth bond on the carbon links it to a hydrogen (H) atom (for formic acid), a methyl (-CH3) group (for acetic acid), or another natural or synthetic monovalent group. Carboxylic acids occur widely in nature. In fatty acids, the fourth group is a hydrocarbon chain. In aromatic acids (see aromatic compound), it is a ring-structured hydrocarbon. In amino acids, it contains a nitrogen atom. Carboxylic acids participate in chemical reactions as acids, usually fairly weak. Many carboxylic acids (acetic acid, citric acid, lactic acid) are intermediates in metabolism and can be found in natural products; others (e.g., salicylic acid) are used as solvents and to prepare many chemical compounds. Important carboxylic-acid derivatives include esters, anhydrides, amides, halides (see halogen), and salts (see soap).

For more information on carboxylic acid, visit Britannica.com.

One of a large family of organic substances widely distributed in nature, and characterized by the presence of one or more carboxyl groups (—COOH). These groups typically yield protons in aqueous solution. In the type formula, R(CXY)nCOOH, symbols R, X, and Y can be hydrogen, saturated or unsaturated groups, carboxyl, alicyclic, or aromatic groups, halogens, or other substituents, and n may vary from zero (formic acid, HCOOH) to more than 100, provided that the normal carbon covalence of four is maintained.

Physical and chemical properties of carboxylic acids are represented, grossly, by the resultant of the various chemical groupings present in the molecule. A short-chain aliphatic acid, wherein the carboxyl is dominant, is a pungent, corrosive, water-soluble liquid of abnormally high boiling point (because of molecular association), with specific gravity close to 1 (higher for formic and acetic acids). With increasing molecular weight, the hydrocarbon grouping overbalances the carboxyl; sharpness of odor diminishes, boiling and melting points rise, the specific gravity falls toward that of the parent hydrocarbon, and the water solubility decreases. Thus the typical high-molecular-weight saturated acid is a bland, waxlike solid.

Acids are used in large quantities in the production of esters, acid halides, acid amides, and acid anhydrides. They find wide use in the manufacture of soaps and detergents, in thickening lubricating greases (stearate soaps), in modifying rigidity in plastics, in compounding buffing bricks and abrasives, and in the manufacture of crayons, dictaphone cylinders, and phonograph records. The solvent action of acids finds use in manufacture of carbon paper, inks, and in the compounding of synthetic and natural rubber. Because of the stability of saturated fatty acids toward oxidation, these are often used as solvents for carrying out oxidation reactions upon sensitive compounds.



any organic acid containing one or more carboxyl ( — COOH) groups.

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Saunders Veterinary Dictionary:

carboxylic acid

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An organic compound containing the carboxy group (−COOH), which is weakly ionized in solution forming a carboxylate ion (−COO−).

Wikipedia on Answers.com:

Carboxylic acid

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

Carboxylic acids (play /ˌkɑrbɒkˈsɪlɪk/) are organic acids characterized by the presence of at least one carboxyl group.[1] The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group. A carboxyl group (or carboxy) is a functional group consisting of a carbonyl (RR'C=O) and a hydroxyl (R-O-H), which has the formula -C(=O)OH, usually written as -COOH or -CO2H.[2]

Carboxylic acids are Brønsted-Lowry acids because they are proton (H+) donors. They are the most common type of organic acid. Among the simplest examples are formic acid H-COOH, that occurs in ants, and acetic acid CH3-COOH, that gives vinegar its sour taste. Acids with two or more carboxyl groups are called dicarboxylic, tricarboxylic, etc. The simplest dicarboxylic example is oxalic acid (COOH)2, which is just two connected carboxyls. Mellitic acid is an example of a hexacarboxylic acid. Other important natural examples are citric acid (in lemons) and tartaric acid (in tamarinds).

Salts and esters of carboxylic acids are called carboxylates. When a carboxyl group is deprotonated, its conjugate base, a carboxylate anion is formed. 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.

Contents

Physical properties

Solubility

Carboxylic acid dimers

Carboxylic acids are polar. Because they are both hydrogen-bond acceptors (the carbonyl) and hydrogen-bond donors (the hydroxyl), 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 with 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.[3]

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 enthalpy of vaporisation requirements significantly.

Acidity

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, only 0.4% of all acetic acid molecules are dissociated. Electronegative substituents give stronger acids.

Carboxylic acid pKa
Formic acid (HCO2H) 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.

Odour

Carboxylic acids often have strong odours, especially the volatile derivatives. Most common are acetic acid (vinegar) and butanoic acid (rancid butter). On the other hand, esters of carboxylic acids tend to have pleasant odours and many are used in perfumes.

Characterization

Carboxylic acids are most 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.[3] 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

Industrial routes to carboxylic acids generally 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.[4]
  • 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

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. Similarly 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.[5]

Specialized reactions

Nomenclature and examples

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.[8]

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-chained, 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 Rancid 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.[9] The acid dissociation constant of ·COOH has been measured using electron paramagnetic resonance spectrocopy.[10] The carboxyl group tends to dimerise to form oxalic acid.

See also

References

  1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "carboxylic acids".
  2. ^ March, Jerry (1992), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (4th ed.), New York: Wiley, ISBN 0-471-60180-2 
  3. ^ a b R.T. Morrison, R.N. Boyd. Organic Chemistry, 6th Ed. (1992) ISBN 0-13-643669-2.
  4. ^ Wilhelm Riemenschneider “Carboxylic Acids, Aliphatic” in Ullmann's Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH, Weinheim. doi: 10.1002/14356007.a05_235.
  5. ^ Tamotsu Fujisawa and Toshio Sato, Organic Syntheses, Coll. Vol. 8, p.498 (1993); Vol. 66, p.121 (1988)
  6. ^ Organic Syntheses, Coll. Vol. 3, p.234 (1955); Vol. 24, p.38 (1944) Link
  7. ^ Organic Syntheses, Coll. Vol. 3, p.237 (1955); Vol. 24, p.41 (1944) Link.
  8. ^ Organic Chemistry IUPAC Nomenclature. Rules C-4 Carboxylic Acids and Their Derivatives. http://www.acdlabs.com/iupac/nomenclature/79/r79_24.htm
  9. ^ 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. doi:10.1063/1.1675022. 
  10. ^ 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. 

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American Heritage Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.  Read more
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McGraw-Hill Science & Technology Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.  Read more
 Oxford Dictionary of Biochemistry. Oxford University Press. Oxford Dictionary of Biochemistry and Molecular Biology © 1997, 2000, 2006 All rights reserved.  Read more
Saunders Veterinary Dictionary. Saunders Comprehensive Veterinary Dictionary 3rd Edition. Copyright © 2007 by D.C. Blood, V.P. Studdert and C.C. Gay, Elsevier. All rights reserved.  Read more
Wikipedia on Answers.com. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article Carboxylic acid Read more

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