Carboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the formula -C(=O)OH, usually written -COOH or -CO2H. Carboxylic acids are Brønsted-Lowry acids — they are proton donors. Salts and anions of carboxylic acids are called carboxylates.
Carboxylic acids are polar, and form hydrogen bonds 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.
Carboxylic acids are widespread in nature and are typically weak acids, meaning that they only partially dissociate into H+ cations and RCOO− anions in 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:
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).
The acidity of a carboxylic acid can also be explained by 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.
In 1H 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.
Vinegar, a dilute solution of acetic acid, is biologically produced from the fermentation of ethanol. It is used in food and beverages, but is not used in industry.
Carboxylic acids may also form from the following reactions:
|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|
|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|
|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|
|12||Lauric acid||Dodecanoic acid||CH3(CH2)10COOH||Coconut oil|
|16||Palmitic acid||Hexadecanoic acid||CH3(CH2)14COOH||Palm oil|
|18||Stearic acid||Octadecanoic acid||CH3(CH2)16COOH||Some waxes, soaps, and oils|
Other carboxylic acids include: