carboxylic acid

Structure of a carboxylic acid

Carboxylate ion

The 3D structure of the carboxyl group

Carboxylic acids ( /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.

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.

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 (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.^[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.

• 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, generally 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

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. 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

• 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, the alpha-methylene group in an aliphatic carboxylic acid is removed in a sequence of reaction steps, effectively a chain-shortening.^[6][7] The inverse procedure is the Arndt-Eistert synthesis, where an acid is converted into acyl halide and reacts with diazomethane to give the highest homolog.
• Many acids undergo 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.

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).

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

Wikimedia Commons has media related to: Carboxylic acids

• Acid anhydride
• Acid chloride
• Amide
• Ester

References

External links

Look up carboxyl in Wiktionary, the free dictionary.

• Carboxylic acids synthesis - Collection of links pertaining to synthesis of Carboxylic acid
• Carboxylic acids pH and titration - freeware for calculations, data analysis, simulation, and distribution diagram generation