Propionic acid

Propionic acid
General
Systematic name	propanoic acid
Other names	ethanecarboxylic acid
Molecular formula	C₃H₆O₂
SMILES	CCC(=O)O
Molar mass	74.08 g/mol
Appearance	colourless liquid
CAS number	^*
Properties
Density and phase	0.99 g/cm³, liquid
Solubility in water	miscible
Melting point	-21 °C (252 K)
Boiling point	141 °C (414 K)
Acidity (pK_a)	4.88
Viscosity	10 mPa·s
Dipole moment	? D
Hazards
MSDS	External MSDS
Main hazards	Corrosive
NFPA 704
Flash point	54°C
R/S statement	R: 34 S: 1/2, 23, 36, 45
RTECS number	UE5950000
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	sodium propionate
Related carboxylic acids	acetic acid butyric acid
Related compounds	1-propanol propionaldehyde methyl propionate
Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Chemical infobox

Propionic acid (systematically named propanoic acid) is a naturally occurring carboxylic acid with chemical formula C H₃CH₂COOH. In the pure state, it is a colorless, corrosive 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 it can be removed from water by adding salt. Like acetic and formic acids, its vapor grossly violates the ideal gas law because it does not consist of individual propionic acid molecules, but instead of hydrogen bonded pairs of molecules. It also undergoes this pairing in the liquid state.

Chemically, propionic acid displays the general properties of carboxylic acids, and like most other 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

Industrially, propionic acid is usually made from the air oxidation of propionaldehyde. In the presence of cobalt or manganese ions, this reaction proceeds rapidly even at mild termperatures. Usually, the industrial process is carried out at 40-50°C, and is represented by the chemical equation

CH₃CH₂CHO + ½ O₂ → CH₃CH₂COOH

Large amounts of propionic acid were once produced as a byproduct of acetic acid manufacture, but changes in the way acetic acid is made have made this a very minor source of propionic acid today. Current world's largest producer is BASF, with approx. 80 ktpa production capacity.

Propionic acid is produced biologically 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. These bacteria are commonly found in the stomachs of ruminants, 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. Accordingly, most propionic acid produced is used 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. In human foods, especially bread and other baked goods, it is used as its sodium or calcium salt. Similar usage occurs in some of the older anti-fungal foot powders.

Propionic acid is also useful as a chemical intermediate. It can be used to modify synthetic cellulose fibers. It is also used to make pesticides and pharmaceuticals. The esters of propionic acid are sometimes used as solvents or artificial flavorings.

Safety

The chief danger from propionic acid is chemical burns that can result from contact with the concentrated liquid. In studies on laboratory animals, the only adverse health effect associated with long-term exposure to small amounts of propionic acid has been ulceration of the esophagus and stomach from consuming a corrosive substance. No toxic, mutagenic, carcinogenic, or reproductive effects have ever been observed. In the body propionic acid is readily metabolized so it does not bioaccumulate.

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 can enter neither beta oxidation nor the citric acid cycle

In most vertebrates propionyl-CoA is carboxylated to D-methylmalonyl-CoA, isomerised to L-methylmalonyl-CoA, and rearranged to yield succinyl-CoA via a vitamin B12-dependent enzyme. Succinyl-CoA is an intermediate of the citric acid cycle and can be readily incorporated there.

External links

Propionates | Carboxylic acids

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