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.
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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]
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.
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) | 3.77 |
| 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.
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.
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.
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).
Industrial routes to carboxylic acids generally differ from those used on smaller scale because they require specialized equipment.
Preparative methods for small scale reactions for research or for production of fine chemicals often employ expensive consumable reagents.
Many reactions afford carboxylic acids but are used only in specific cases or are mainly of academic interest:
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:
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]
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).
| 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 |
| 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 |
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.
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