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Propionic acid

Propionic acid (/prpiˈɒnɪk/, from the Greek words πρῶτος : prōtos, meaning "first", and πίων : píōn, meaning "fat"; also known as propanoic acid) is a naturally occurring carboxylic acid with chemical formula CH
3
CH
2
CO
2
H
. It is a liquid with a pungent and unpleasant smell somewhat resembling body odor. The anion CH
3
CH
2
CO
2
as well as the salts and esters of propionic acid are known as propionates or propanoates.

About half of the world production of propionic acid is consumed as a preservative for both animal feed and food for human consumption. It is also useful as an intermediate in the production of other chemicals, especially polymers.

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 by different means, none of them realizing they were producing the same substance. In 1847, French chemist Jean-Baptiste Dumas established all the acids to be the same compound, which he called propionic acid, from the Greek words πρῶτος (prōtos), meaning first, and πίων (piōn), meaning fat, because it is the smallest H(CH
2
)
n
COOH
acid that exhibits the properties of the other fatty acids, such as producing an oily layer when salted out of water and having a soapy potassium salt.

Propionic acid has physical properties intermediate between those of the smaller carboxylic acids, formic and acetic acids, 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 in both the liquid and the vapor.

Propionic acid displays the general properties of carboxylic acids: it can form amide, ester, anhydride, and chloride derivatives. It undergoes the Hell–Volhard–Zelinsky reaction that involves α-halogenation of a carboxylic acid with bromine, catalysed by phosphorus tribromide, in this case to form 2-bromopropanoic acid, CH
3
CHBrCOOH
. This product has been used to prepare a racemic mixture of alanine by ammonolysis.

In industry, propionic acid is mainly produced by the hydrocarboxylation of ethylene using nickel carbonyl as the catalyst:

It is also produced by the aerobic oxidation of propionaldehyde. In the presence of cobalt or manganese salts (manganese propionate is most commonly used), this reaction proceeds rapidly at temperatures as mild as 40–50 °C:

Large amounts of propionic acid were once produced as a byproduct of acetic acid manufacture. At the current time, the world's largest producer of propionic acid is BASF, with approximately 150 kt/a production capacity.

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