Carboxylic acid
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2008-07-05T22:12:23Z
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[[Image:Carboxylic-acid.svg|thumb|150px|Structure of a carboxylic acid]]
[[Image:Carboxylic-acid-group-3D.png|thumb|150px|The 3D structure of the carboxyl group]]
[[Image:Carboxyl-3D-space-filling-labelled.png|thumb|150px|A space-filling model of the carboxyl group]]
'''Carboxylic acids''' are [[organic acid]]s characterized by the presence of a [[carboxyl group]], which has the [[Chemical formula|formula]] -C(=O)OH, usually written -COOH or -CO<sub>2</sub>H. <ref>[[Compendium of Chemical Terminology]], [http://goldbook.iupac.org/C00852.html carboxylic acids], accessed 15 Jan 2007.</ref> Carboxylic acids are [[Brønsted-Lowry acid-base theory|Brønsted-Lowry acids]] — they are proton donors. [[Salt]]s and [[anion]]s of carboxylic acids are called '''carboxylates'''.
The simplest series of carboxylic acids are the '''alkanoic acids''', R-COOH, where R is a [[hydrogen]] or an [[alkyl]] [[Functional group|group]]. Compounds may also have two or more carboxylic acid groups per molecule.
== Physical properties ==
[[Image:Carboxylic acid dimers.png|thumb|left|Carboxylic acid [[dimer]]s]]
Carboxylic acids are [[polarity (chemistry)|polar]], and form [[hydrogen bond]]s with each other. At high temperatures, in vapor phase, carboxylic acids usually exist as dimeric pairs. Lower carboxylic acids (1 to 4 carbons) are miscible with water, whereas higher carboxylic acids are very much less-soluble due to the increasing hydrophobic nature of the alkyl chain. They tend to be rather soluble in less-polar solvents such as ethers and alcohols.<ref name=M&B>R.T. Morrison, R.N. Boyd. ''Organic Chemistry'', 6th Ed. (1992) ISBN 0-13-643669-2.</ref>
Carboxylic acids are widespread in nature and are typically [[weak acid]]s, meaning that they only partially [[Dissociation (chemistry)|dissociate]] into H<sup>+</sup> [[cation]]s and RCOO<sup>−</sup> [[anion]]s in [[Water (molecule)|aqueous]] solution. For example, at room temperature, only 0.02 % of all [[acetic acid]] molecules are dissociated in water.
Since the carboxylic acids are weak acids, in water, both forms exist in an equilibrium:
:RCOOH ↔ RCOO<sup>−</sup> + H<sup>+</sup>
The acidity of carboxylic acids can be explained by either the stability of the acid or the stability of the [[conjugate base]] using [[inductive effect]]s or [[resonance (chemistry)|resonance]] effects.
===Stability of the acid===
Using inductive effects, the acidity of carboxylic acids can be rationalized by the two [[electronegative]] [[oxygen]] atoms distorting the electron clouds surrounding the O-H bond, weakening it. The weak O-H bond causes the acid molecule to be less stable, and causing the hydrogen atom to be labile, thus it dissociates easily to give the H<sup>+</sup> ion. Since the acid is unstable, the [[chemical equilibrium|equilibrium]] will lie on the right.
Additional electronegative atoms or groups, such as chlorine or hydroxyl, substituted on the R-group have a similar, though lesser effect. The presence of these groups increases the acidity through [[inductive effects]]. For example, [[trichloroacetic acid]] (three -Cl groups) is a stronger acid than [[lactic acid]] (one -OH group), which in turn is stronger than acetic acid (no electronegative constituent).
===Stability of the conjugate base===
[[Image:Resonance stabilization of carboxylic acids.png|right|thumb|Resonance stabilization of carboxylic acids]]
The acidity of a carboxylic acid can also be explained by [[resonance (chemistry)|resonance]] effects. The result of the dissociation of a carboxylic acid is a resonance stabilized product in which the negative charge is shared (delocalized) between the two oxygen atoms. Each of the carbon-oxygen bonds has what is called a partial double-bond characteristic. Since the conjugate base is stabilized, the above equilibrium lies on the right.
==Spectroscopy==
Carboxylic acids are most readily identified as such by [[infrared spectroscopy]]. They exhibit a sharp C=O stretch between 1680 and 1725 cm<sup>−1</sup>, and the characteristic O-H stretch of the carboxyl group appears as a broad peak in the 2500 to 3000 cm<sup>−1</sup> region.<ref name=M&B/>
In <sup>1</sup>H NMR spectrometry, the hydroxyl hydrogen appears in the 10-13 ppm region, though it is often either broadened or not observed due to exchange with any traces of water.
==Sources==
Lower straight-chain [[aliphatic]] carboxylic acids, as well as those of even carbon number up to C<sub>18</sub>, are commercially available. For example, acetic acid is produced by [[methanol]] [[carbonylation]] with [[carbon monoxide]], whereas long chain carboxylic acids are obtained by the hydrolysis of [[triglyceride]]s obtained from plant or animal oils.
Vinegar, a dilute solution of acetic acid, is biologically produced from the [[Fermentation (biochemistry)|fermentation]] of ethanol. It is used in food and beverages, but is not used in industry.
== Synthesis ==
* Carboxylic acids can be produced by [[organic oxidation|oxidation]] of [[alcohol|primary alcohols]] and [[aldehyde]]s with strong oxidants such as [[Potassium Dichromate]], [[Jones reagent]], [[potassium permanganate]], or [[sodium chlorite]].
* They may also be produced by the oxidative cleavage of [[olefin]]s by [[ozonolysis]], [[potassium permanganate]], or [[potassium dichromate]]. In particular, any alkyl group on a benzene ring will be fully oxidized to a carboxylic acid, regardless of its chain length. This is the basis for the industrial synthesis of [[benzoic acid]] from [[toluene]].
*Carboxylic acids can also be obtained by the hydrolysis of [[nitrile]]s, [[ester]]s, or [[amide]]s, with the addition of acid or base.
*They can also be prepared from the action of a [[Grignard reagent]] on [[carbon dioxide]], though this method is not used in industry.
Carboxylic acids may also form from the following reactions:
* Disproportionation of an [[aldehyde]] in the [[Cannizzaro reaction]]
* Rearrangement of diketones in the [[benzilic acid rearrangement]]
* Halogenation followed by hydrolysis of methyl ketones in the [[haloform reaction]]
* Hydroformylation of an alkene followed by hydrolysis in the [[Koch reaction]]
* Less-common reactions involving the generation of benzoic acids are the [[von Richter reaction]] from nitrobenzenes and the [[Kolbe-Schmitt reaction]] from [[phenol]]s.
==Reactions==
*Carboxylic acids react with [[Base (chemistry)|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:
:CH<sub>3</sub>COOH + NaHCO<sub>3</sub> → CH<sub>3</sub>COONa + CO<sub>2</sub> + H<sub>2</sub>O
*Carboxylic acids also react with [[alcohol]]s and [[amine]]s to give [[ester]]s and [[amide]]s. Like other alcohols and [[phenol]]s, the hydroxyl group on carboxylic acids may be replaced with a chlorine atom using [[thionyl chloride]] to give [[acyl chloride]]s.
*As with all carbonyl compounds, the protons on the [[alpha-carbon|α-carbon]] are labile due to keto-enol tautomerization. Thus the α-carbon is easily halogenated in the [[Hell-Volhard-Zelinsky halogenation]].
*The [[Arndt-Eistert synthesis]] inserts an α-methylene group into a carboxylic acid.
*The [[Curtius rearrangement]] converts carboxylic acids to [[isocyanate]]s.
*The [[Schmidt reaction]] converts carboxylic acids to [[amine]]s.
*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''' (1912), the alpha-methylene group in an aliphatic carboxylic acid is removed in a sequence of reaction steps, effectively a chain-shortening <ref>[[Organic Syntheses]], Coll. Vol. 3, p.234 (1955); Vol. 24, p.38 (1944) [http://www.orgsynth.org/orgsyn/pdfs/CV3P0234.pdf Link] </ref> <ref>[[Organic Syntheses]], Coll. Vol. 3, p.237 (1955); Vol. 24, p.41 (1944) [http://www.orgsynth.org/orgsyn/pdfs/CV3P0237.pdf Link].</ref>.
* The addition of a carboxyl group to a compound is known as '''carboxylation'''; the removal of one is '''decarboxylation'''. [[Enzyme]]s that catalyze these reactions are known as [[carboxylase]]s ([[EC number|EC]] 6.4.1) and [[decarboxylase]]s (EC 4.1.1).
== Nomenclature and examples ==
The [[carboxylate|carboxylate anion]] R-COO<sup>–</sup> 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]]).
{|class = "wikitable"
|+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 || CH<sub>3</sub>COOH || [[Vinegar]]
|-
|3 || [[Propionic acid]] || Propanoic acid || CH<sub>3</sub>CH<sub>2</sub>COOH ||
|-
|4 || [[Butyric acid]] || Butanoic acid
| CH<sub>3</sub>(CH<sub>2</sub>)<sub>2</sub>COOH || Rancid butter
|-
|5 || [[Valeric acid]] || Pentanoic acid
| CH<sub>3</sub>(CH<sub>2</sub>)<sub>3</sub>COOH || [[Valerian (herb)|Valerian]]
|-
|6 || Caproic acid || [[Hexanoic acid]]
| CH<sub>3</sub>(CH<sub>2</sub>)<sub>4</sub>COOH ||
|-
|7 || Enanthic acid || [[Heptanoic acid]]
| CH<sub>3</sub>(CH<sub>2</sub>)<sub>5</sub>COOH ||
|-
|8 || [[Caprylic acid]] || Octanoic acid
| CH<sub>3</sub>(CH<sub>2</sub>)<sub>6</sub>COOH || Coconuts and breast milk
|-
|9 || Pelargonic acid || [[Nonanoic acid]]
| CH<sub>3</sub>(CH<sub>2</sub>)<sub>7</sub>COOH || [[Pelargonium]]
|-
|10 || Capric acid || [[Decanoic acid]]
| CH<sub>3</sub>(CH<sub>2</sub>)<sub>8</sub>COOH ||
|-
|12 || [[Lauric acid]] || Dodecanoic acid
| CH<sub>3</sub>(CH<sub>2</sub>)<sub>10</sub>COOH || Coconut oil
|-
|16 || [[Palmitic acid]] || Hexadecanoic acid
| CH<sub>3</sub>(CH<sub>2</sub>)<sub>14</sub>COOH || Palm oil
|-
|18 || [[Stearic acid]] || Octadecanoic acid
| CH<sub>3</sub>(CH<sub>2</sub>)<sub>16</sub>COOH || Some waxes, soaps, and oils
|}
Other carboxylic acids include:
* Short-chain unsaturated monocarboxylic acids
** [[Acrylic acid]] (2-propenoic acid) – CH<sub>2</sub>=CHCOOH, used in polymer synthesis
* [[Fatty acid]]s – medium to long-chain saturated and unsaturated monocarboxylic acids, with even number of carbons
** [[Docosahexaenoic acid]] – nutritional supplement
** [[Eicosapentaenoic acid]] – nutritional supplement
* [[Amino acid]]s – the building blocks of [[protein]]s
* [[Keto acids]] – acids of biochemical significance that contain a [[ketone]] group
** [[Pyruvic acid]]
** [[Acetoacetic acid]]
* [[Aromatic compound|Aromatic]] carboxylic acids
** [[Benzoic acid]] – C<sub>6</sub>H<sub>5</sub>COOH; sodium benzoate, the sodium salt of benzoic acid is used as a food preservative
** [[Salicylic acid]] – found in many skin care products
* [[Dicarboxylic acid]]s – containing two carboxyl groups
** [[Aldaric acid]] – a family of sugar acids
** [[Oxalic acid]] – found in many foods
** [[Malonic acid]]
** [[Malic acid]] – found in apples
** [[Succinic acid]] – a component of the [[citric acid cycle]]
** [[Glutaric acid]]
** [[Adipic acid]] – the monomer used to produce [[nylon]]
* [[Tricarboxylic acid]]s – containing three carboxyl groups
** [[Citric acid]] – found in [[citrus fruit]]s
** [[Isocitric acid]]
** [[Aconitic acid]]
** [[Propane-1,2,3-tricarboxylic acid]] (tricarballylic acid, carballylic acid)
* [[Alpha hydroxy acid]]s – containing a hydroxy group
** [[Lactic acid]] (2-hydroxypropanoic acid) – found in sour milk
==See also==
{{Commonscat|Carboxylic acids}}
* [[Acid anhydride]]
* [[Acid chloride]]
* [[Amide]]
* [[Ester]]
==References==
<div class="references-small"><references/></div>
== External links ==
* Carboxylic acids pH and titration [http://www2.iq.usp.br/docente/gutz/Curtipot_.html - freeware for calculations, data analysis, simulation, and distribution diagram generation]
{{Functional Groups}}
[[Category:Carboxylic acids|*]]
[[Category:Functional groups]]
[[Category:Acids]]
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