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Propionic acid
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Propionic acid (systematically named propanoic acid) is a naturally-occurring carboxylic acid with chemical formula CH3CH2COOH. In the pure state, it is a colorless liquid with a pungent odor. The anion CH3CH2COO− as well as the salts and esters of propionic acid are known as propionates (or propanoates).
ionic acid was first described in 1844 by Johann Gottlieb, who found it among the degradation products of sugar.
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Encyclopedia
Propionic acid (systematically named propanoic acid) is a naturally-occurring carboxylic acid with chemical formula CH3CH2COOH. In the pure state, it is a colorless liquid with a pungent odor. The anion CH3CH2COO− as well as the salts and esters of propionic acid are known as propionates (or propanoates).
History
Propionic acid was first described in 1844 by Johann Gottlieb, who found it among the degradation products of sugar. Over the next few years, other chemists produced propionic acid in various other ways, none of them realizing they were producing the same substance. In 1847, the French chemist Jean-Baptiste Dumas established that all the acids were the same compound, which he called propionic acid, from the Greek words protos = "first" and pion = "fat," because it was the smallest H(CH2)nCOOH acid that exhibited the properties of the other fatty acids, such as producing an oily layer when salted out of water and having a soapy potassium salt.
Properties
Propionic acid has physical properties intermediate between those of the smaller carboxylic acids, formic and acetic acid, and the larger fatty acids. It is miscible with water, but can be removed from water by adding salt. As with acetic and formic acids, it consists of hydrogen bonded pairs of molecules both as the liquid and vapor.
Propionic acid displays the general properties of carboxylic acids: it can form amide, ester, anhydride, and chloride derivatives. It can undergo alpha-halogenation with bromine in the presence of PBr3 as catalyst (the HVZ reaction) to form CH3CHBrCOOH.
Production
In industry, propionic acid is main produced by the hydrocarboxylation of ethylene using nickel carbonyl as the catalyst:
- H2C=CH2 + H2O + CO ? CH3CH2CO2H
It is also produced by the aerobic oxidation of propionaldehyde. In the presence of cobalt or manganese ns, this reaction proceeds rapidly at temperatures as mild as 40–50 °C:
- CH3CH2CHO + ½ O2 ? CH3CH2COOH.
Large amounts of propionic acid were once produced as a byproduct of acetic acid manufacture. Currently the world's largest producer of propionic acid is BASF, with approximately 80 kt/a production capacity.
Propionic acid is produced biologically as its coenzyme A ester, propionyl-CoA, from the metabolic breakdown of fatty acids containing odd numbers of carbon atoms, and also it the breakdown of some amino acids. Bacteria of the genus Propionibacterium produce propionic acid as the end product of their anaerobic metabolism. This class of bacteria is commonly found in the stomachs of ruminants and the sweat glands of humans, and their activity is partially responsible for the odor of both Swiss cheese and sweat.
Uses
Propionic acid inhibits the growth of mold and some bacteria at the levels between 0.1 and 1% by weight. As a result, most propionic acid produced is consumed as a preservative for both animal feed and food for human consumption. For animal feed, it is used either directly or as its ammonium salt. This application accounts for appoximately half of the world production of propionic acid. Another major application is as a preservative in baked goods, which use the sodium and calcium salts.
Propionic acid is also useful as an intermediate in the production of other chemicals, especially polymers. Cellulose-acetate-propionate is a useful thermoplastic. Vinyl propionate is also used. In more specialized applications, it is also used to make pesticides and pharmaceuticals. The esters of propionic acid have fruit-like odors and are sometimes used as solvents or artificial flavorings.
Metabolism
The metabolism of propionic acid begins with its conversion to propionyl coenzyme A (propionyl-CoA), the usual first step in the metabolism of carboxylic acids. Since propionic acid has three carbons, propionyl-CoA cannot directly enter either beta oxidation nor the citric acid cycles. In most vertebrates, propionyl-CoA is carboxylated to D-methylmalonyl-CoA, which is isomerised to L-methylmalonyl-CoA. A vitamin B12-dependent enzyme catalyzes rearrangement of L-methylmalonyl-CoA to succinyl-CoA, which is an intermediate of the citric acid cycle and can be readily incorporated there.
In propionic acidemia, propionate acts as a metabolic toxin in liver cells by accumulating in mitochondria as propionyl-CoA and its derivative, methylcitrate, two tricarboxylic acid cycle inhibitors. Propionate is metabolized oxidatively by glia, which suggests astrocytic vulnerability in propionic acidemia when intramitochondrial propionyl-CoA may accumulate. Propionic acidemia may alter both neuronal and glial gene expression by affecting histone acetylation. Propionic acid in rodents produces reversible behavior reminiscent of autism, e.g. hyperactivity, dystonia.
Human occurrence
The human skin is host to a species of bacteria known as Propionibacterium acnes, which is named after its ability to produce propionic acid. This bacterium lives mainly in the sebaceous glands of the skin and is one of the principal causes of acne.
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