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| Names | |||
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| Preferred IUPAC name Propanoic acid | |||
| Other names Carboxyethane Ethanecarboxylic acid Ethylformic acid Metacetonic acid Methylacetic acid C3:0 (Lipid numbers) | |||
| Identifiers | |||
3D model (JSmol) |
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| ECHA InfoCard | 100.001.070 | ||
| EC Number |
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| E number | E280(preservatives) | ||
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| Properties | |||
| C3H6O2 | |||
| Molar mass | 74.079 g·mol−1 | ||
| Appearance | Colorless, oily liquid[1] | ||
| Odor | Pungent, rancid, unpleasant[1] | ||
| Density | 0.98797 g/cm3[2] | ||
| Melting point | −20.5 °C (−4.9 °F; 252.7 K)[8] | ||
| Boiling point | 141.15 °C (286.07 °F; 414.30 K)[8] | ||
| Sublimes at −48 °C ΔsublH | |||
| 8.19 g/g (−28.3 °C) 34.97 g/g (−23.9 °C) Miscible (≥ −19.3 °C)[4] | |||
| Solubility | Miscible inEtOH,ether,CHCl 3[5] | ||
| logP | 0.33[6] | ||
| Vapor pressure | 0.32 kPa (20 °C)[7] 0.47 kPa (25 °C)[6] 9.62 kPa (100 °C)[3] | ||
Henry's law constant (kH) | 4.45·10−4 L·atm/mol[6] | ||
| Acidity (pKa) | 4.88[6] | ||
| −43.50·10−6 cm3/mol | |||
Refractive index (nD) | 1.3843[2] | ||
| Viscosity | 1.175 cP (15 °C)[2] 1.02 cP (25 °C) 0.668 cP (60 °C) 0.495 cP (90 °C)[6] | ||
| Structure | |||
| Monoclinic (−95 °C)[9] | |||
| P21/c[9] | |||
a = 4.04 Å,b = 9.06 Å,c = 11 Å[9] α = 90°, β = 91.25°, γ = 90° | |||
| 0.63D (22 °C)[2] | |||
| Thermochemistry | |||
| 152.8 J/mol·K[5][3] | |||
Std molar entropy(S⦵298) | 191 J/mol·K[3] | ||
Std enthalpy of formation(ΔfH⦵298) | −510.8 kJ/mol[3] | ||
Std enthalpy of combustion(ΔcH⦵298) | 1527.3 kJ/mol[2][3] | ||
| Hazards | |||
| Occupational safety and health (OHS/OSH): | |||
Main hazards | Corrosive | ||
| GHS labelling:[7] | |||
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| Danger | |||
| H314[7] | |||
| P280,P305+P351+P338,P310[7] | |||
| NFPA 704 (fire diamond) | |||
| Flash point | 54 °C (129 °F; 327 K)[7] | ||
| 512 °C (954 °F; 785 K) | |||
| Lethal dose or concentration (LD, LC): | |||
LD50 (median dose) | 1370 mg/kg (mouse, oral)[5] | ||
| NIOSH (US health exposure limits): | |||
PEL (Permissible) | none[1] | ||
REL (Recommended) | TWA 10 ppm (30 mg/m3) ST 15 ppm (45 mg/m3)[1] | ||
IDLH (Immediate danger) | N.D.[1] | ||
| Related compounds | |||
RelatedCarboxylic acids | Acetic acid Lactic acid 3-Hydroxypropionic acid Tartronic acid Acrylic acid Butyric acid | ||
Related compounds | 1-Propanol Propionaldehyde Sodium propionate Propionic anhydride | ||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Propionic acid (/proʊpiˈɒnɪk/, from theGreek words πρῶτος :prōtos, meaning "first", and πίων :píōn, meaning "fat"; also known aspropanoic acid) is a naturally occurringcarboxylic acid withchemical formulaCH
3CH
2CO
2H. It is an oily liquid with a pungent and unpleasant smell. TheanionCH
3CH
2CO−
2 as well as thesalts andesters of propionic acid are known aspropionates orpropanoates.
About half of the world production of propionic acid is consumed as apreservative 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 byJohann Gottlieb, who found it among the degradation products of sugar.[10] 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 chemistJean-Baptiste Dumas established all the acids to be the same compound, which he called propionic acid, from theGreek words πρῶτος (prōtos), meaningfirst, and πίων (piōn), meaningfat, because it is the smallestH(CH
2)
nCOOH acid that exhibits the properties of the otherfatty acids, such as producing an oily layer when salted out of water and having a soapypotassiumsalt.[11]
Propionic acid has physical properties intermediate between those of the smaller carboxylic acids,formic andacetic acids, and the largerfatty acids. It is miscible with water, but can be removed from water by adding salt. As with acetic and formic acids, it consists ofhydrogen bonded pairs of molecules in both the liquid and the vapor.
Propionic acid displays the general properties of carboxylic acids: it can formamide,ester,anhydride, andchloride derivatives. It undergoes theHell–Volhard–Zelinsky reaction that involves α-halogenation of a carboxylic acid withbromine,catalysed byphosphorus tribromide, in this case to form2-bromopropanoic acid,CH
3CHBrCOOH.[12] This product has been used to prepare aracemic mixture ofalanine byammonolysis.[13][14]
In industry, propionic acid is mainly produced by thehydrocarboxylation ofethylene usingnickel carbonyl as the catalyst:[15]
.svg)
It is also produced by the aerobicoxidation ofpropionaldehyde. In the presence ofcobalt ormanganese salts (manganese propionate is most commonly used), this reaction proceeds rapidly at temperatures as mild as 40–50 °C:
.svg)
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 isBASF, with approximately 150 kt/a production capacity.
Biotechnological production of propionic acid mainly usesPropionibacterium strains.[16] However, large scale production of propionic acid byPropionibacteria faces challenges such as severe inhibition of end-products during cell growth and the formation of by-products (acetic acid andsuccinic acid).[17] One approach to improve productivity and yield during fermentation is through the use of cell immobilization techniques, which also promotes easy recovery, reuse of the cell biomass and enhances microorganisms' stress tolerance.[18] In 2018, 3D printing technology was used for the first time to create a matrix for cell immobilization in fermentation. Propionic acid production byPropionibacterium acidipropionici immobilized on 3D-printed nylon beads was chosen as a model study. It was shown that those 3D-printed beads were able to promote high density cell attachment and propionic acid production, which could be adapted to other fermentation bioprocesses.[19] Other cell immobilization matrices have been tested, such as recycled-glass Poraver and fibrous-bed bioreactor.[20][21]
Alternative methods of production have been trialled, by genetically engineering strains ofEscherichia coli to incorporate the necessary pathway, the Wood-Werkman cycle.[22]
Propionic acid inhibits the growth ofmold and some bacteria at levels between 0.1 and 1% by weight. As a result, some propionic acid produced is consumed as apreservative for both animal feed and food for human consumption. For animal feed, it is used either directly or as itsammonium salt. This application accounts for about half of the world production of propionic acid. The antibioticmonensin is added to cattle feed to favorpropionibacteria over acetic acid producers in therumen; this produces less carbon dioxide and feed conversion is better. Another major application is as a preservative in baked goods, which use thesodium andcalcium salts.[15] As afood additive, it is approved for use in the EU,[23] US,[24] Australia and New Zealand.[25]
Propionic acid is also useful as an intermediate in the production of other chemicals, especially polymers.Cellulose-acetate-propionate is a usefulthermoplastic.Vinyl propionate is also used. In more specialized applications, it is also used to makepesticides andpharmaceuticals. Theesters of propionic acid have fruit-like odors and are sometimes used assolvents or artificial flavorings.[15]
Inbiogas plants, propionic acid is a common intermediate product, which is formed by fermentation with propionic acid bacteria. Its degradation in anaerobic environments (e.g. biogas plants) requires the activity of complex microbial communities.[26]
In production of theJarlsberg cheese, a propionic acid bacterium is used to give both taste and holes.[27]
Propionic acid is produced biologically as its coenzyme A ester,propionyl-CoA, from themetabolic breakdown of fatty acids containingodd numbers ofcarbon atoms, and also from the breakdown of someamino acids. Bacteria of the genusPropionibacterium produce propionic acid as the end-product of theiranaerobic metabolism. This class of bacteria is commonly found in the stomachs ofruminants and thesweat glands of humans, and their activity is partially responsible for the odor ofEmmental cheese,American "Swiss cheese" andsweat.
The metabolism of propionic acid begins with its conversion to propionylcoenzyme A, the usual first step in the metabolism ofcarboxylic acids. Since propionic acid has three carbons, propionyl-CoA cannot directly enter eitherbeta oxidation or thecitric acid cycles. In mostvertebrates, propionyl-CoA iscarboxylated toD-methylmalonyl-CoA, which isisomerised toL-methylmalonyl-CoA. Avitamin B12-dependent enzyme catalyzes rearrangement ofL-methylmalonyl-CoA tosuccinyl-CoA, which is an intermediate of the citric acid cycle and can be readily incorporated there.[28]
Propionic acid serves as a substrate forhepaticgluconeogenesis via conversion to succinyl-CoA.[29][30] Additionally,exogenous propionic acid administration results in moreendogenous glucose production than can be accounted for by gluconeogenic conversion alone.[31] Exogenous propionic acid mayupregulate endogenous glucose production via increases innorepinephrine andglucagon, suggesting that chronic ingestion of propionic acid may have adverse metabolic consequences.[32]
Inpropionic acidemia, a rare inherited genetic disorder, 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. Propanoate is metabolized oxidatively byglia, 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.[33][34] When propionic acid is infused directly into rodents' brains, it produces reversible behavior (e.g.,hyperactivity,dystonia, social impairment,perseveration) and brain changes (e.g., innate neuroinflammation, glutathione depletion) that may be used as a means to modelautism in rats.[33]
The human skin is host of several species ofPropionibacteria. The most notable one is theCutibacterium acnes (formerly known asPropionibacterium acnes), which lives mainly in thesebaceous glands of the skin and is one of the principal causes ofacne.[35] Propionate is observed to be among the most commonshort-chain fatty acids produced in thelarge intestine of humans bygut microbiota in response to indigestible carbohydrates (dietary fiber) in the diet.[36][37] The role of the gut microbiota and their metabolites, including propionate, in mediating brain function has been reviewed.[38]
A study in mice suggests that propionate is produced by the bacteria of the genusBacteroides in the gut, and that it offers some protection againstSalmonella there.[39] Another study finds that fatty acid propionate can calm the immune cells that drive up blood pressure, thereby protecting the body from damaging effects of high blood pressure.[40]
The Bacteria speciesCoprothermobacter platensis produces propionate when fermenting gelatin.[41]Prevotella brevis andPrevotella ruminicola also generate propionate when fermenting glucose.[42]
Thepropionate/ˈproʊpiəneɪt/, orpropanoate,ion isC
2H
5COO−
, theconjugate base of propionic acid. It is the form found in biological systems atphysiological pH. A propionic, or propanoic, compound is acarboxylate salt orester of propionic acid. In these compounds, propionate is often written in shorthand, asCH
3CH
2CO
2 or simplyEtCO
2.
Propionates should not be confused with propenoates (commonly known asacrylates), the ions/salts/esters of propenoic acid (also known as 2-propenoic acid oracrylic acid).
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