Propionic acid

Ball-and-stick Stick Space-filling

Drag to rotate; pinch or Ctrl + scroll to zoom. Arrow keys rotate, + − zoom, R resets.

11 atoms, 10 bonds.

IUPAC name: propanoic acid
Molar mass: 74.0790(92) g/mol
Atoms: 11
SMILES: CCC(=O)O
InChIKey: XBDQKXXYIPTUBI-UHFFFAOYSA-N
PubChem CID: 1032

Made of

Structural formula of propionic acid. Each unlabelled corner is a carbon atom, and the two hydrogens on carbon are not drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Propionic acid answers to several names. Propanoic acid is the name used by IUPAC, the International Union of Pure and Applied Chemistry. NIOSH, the US National Institute for Occupational Safety and Health, also records metacetonic acid, methyl acetic acid and ethylformic acid. The molecule pairs an ethyl group, C₂H₅–, with a carboxyl group, –COOH, the cluster of atoms that marks the organic acids. ChEBI, the Chemical Entities of Biological Interest database, files it as a short-chain saturated fatty acid. Its molar mass is 74.08 g/mol.

Undiluted, it is an oily liquid without colour, and it mixes with water in all proportions. Like most organic acids it is a weak acid. The Hazardous Substances Data Bank (HSDB) gives its pK_a as 4.88, a measure the phosphoric acid page explains.

Structure and bonding

Propionic acid’s three carbon atoms form a short chain, and only the last bonds to oxygen: one by a double bond, one by a single bond. Five hydrogen atoms cover the first two carbons, and a sixth sits on the single-bonded oxygen. In Ball-and-stick, the C=O bond is 1.22 Å long, shorter than the 1.36 Å of the C–O bond beside it. Both lengths belong to PubChem’s computed conformer, a 3D shape calculated rather than measured.

Seen edge-on, that last carbon and its three neighbours lie in one plane. Its angles are 123.8°, 126.2° and 110.0°: they total 360° but are far from equal. At the two chain carbons the angles run from 107.8° to 112.0°, and the Stick view, reduced to bare bonds, shows the chain bending there. In Space-filling, the five chain hydrogens round off the ethyl end.

The viewer draws the neutral acid, uncharged. In water, some molecules lose the –OH hydrogen and become propionate ions, which the model does not show. Why that hydrogen is the acidic one is taken up on the formic acid page.

Where it turns up

Cheese contains propionic acid naturally, and the compound is also an approved food preservative, according to Haz-Map, an occupational health database. The US Food and Drug Administration (FDA) lists three technical effects for it in food: antimicrobial agent, flavouring agent and pH control agent. It also appears among cosmetic preservatives and fragrance ingredients. Among other organisms, it has been reported in Ipomoea leptophylla and Angelica gigas.

On farms it works against microbes. The US Environmental Protection Agency (EPA) covers its use as a bactericide and fungicide in livestock drinking water, in poultry litter, and in the stores that hold silage and grain. HSDB lists it as a grain preservative and reports it fungicidal against a toxin-forming strain of the mould Aspergillus parasiticus.

Why it matters

Industry uses it as an intermediate, a starting point for other chemicals. HSDB lists calcium and sodium propionates, cellulose propionate plastics, plasticisers, pharmaceuticals and three herbicides: dalapon, erbon and propanil. In 1983, 33 per cent went to the propionates, 26 per cent to cellulose acetate propionate and 16 per cent to herbicides. Patents for making it went to du Pont in 1939 and Kodak in 1942, and a 1962 patent covered fermenting wood-pulp waste with Propionibacterium bacteria.

In the body, propionate forms as propionyl-coenzyme A, the three-carbon piece left over when fatty acids with an odd number of carbons are oxidised. Oxidising the side chain of cholesterol yields it too. Carboxylation, the addition of a carboxyl group, turns it into methylmalonyl-coenzyme A, and further conversion gives succinic acid, which feeds into the citric acid cycle. Unlike acetic acid, it can end up in carbohydrate as well as lipid. Propionic acidaemia is one of the organic acidurias, disorders in which short-chain acyl-coenzyme A compounds build up inside the mitochondria.

Properties

Properties of propionic acid Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 1032.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₃H₆O₂
Molar mass	74.0790(92) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	3
Hydrogen atoms	6
Oxygen atoms	2
Bonds	10
Single bonds	9
Double bonds	1
PubChem molecular weight	74.08 g/mol	g/mol	PubChem

Safety

Undiluted propionic acid is corrosive. Under the EU’s harmonised classification it carries the signal word Danger for severe burns to the skin and eyes. The International Chemical Safety Card rates it very corrosive to eyes, skin and the respiratory tract, and NIOSH notes that it also corrodes steel. For people who handle the concentrated liquid at work, the PubChem Laboratory Chemical Safety Summary for propionic acid and the NIOSH Pocket Guide card are the sheets to consult.

Questions

Is propionic acid safe in food?

The US Food and Drug Administration affirms it as generally recognised as safe, or GRAS, for adding directly to human food, and gives it the same status as a preservative in animal feeds and animal drugs. JECFA, the food-additive committee of the Food and Agriculture Organization together with the World Health Organization, lists it both as a flavouring agent and as a preservative. At its 1997 meeting the committee found no safety concern with its use as a flavouring.

How many atoms are in a propionic acid molecule?

Eleven: three carbon, six hydrogen and two oxygen atoms, giving the molecular formula C₃H₆O₂, also written C₂H₅COOH to set the acid group apart. PubChem's model joins them with ten bonds. Two are carbon–carbon single bonds and five join carbon to hydrogen. The other three belong to the acid group: a C=O double bond, a C–O single bond, and the O–H bond whose hydrogen the acid can lose in water.

Is propionic acid an antifungal?

ChEBI assigns it the role of an antifungal drug, and on farms it serves as both a fungicide and a bactericide. For medicines sold without a prescription, the US Food and Drug Administration found the data inadequate to establish that topical antifungal products containing it, those applied to the skin, are generally recognised as safe and effective. As a pesticide it appears on List D of the US Environmental Protection Agency, the pesticides of less concern.

What does propionic acid smell like?

The US workplace-safety institute NIOSH describes the smell of the liquid as pungent, disagreeable and rancid. The Hazardous Substances Data Bank records its taste as sour and mildly cheese-like. FEMA, the Flavor and Extract Manufacturers Association, gives its flavour profile as fat, fruit, pungent, silage and soy. The undiluted liquid itself is corrosive.

Made of the same elements

C₃H₆O Acetone 10 atoms, 9 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.

All 51 compounds made of carbon, hydrogen and oxygen

C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
HCOOH Formic acid 5 atoms, 4 bonds.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
CH₂O Formaldehyde 4 atoms, 3 bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
C₁₄H₁₄O₃ Naproxen 31 atoms, 32 bonds.
C₁₅H₁₆O₂ Bisphenol A 33 atoms, 34 bonds.
C₁₃H₁₈O₂ Ibuprofen 33 atoms, 33 bonds.
C₁₀H₂₀O Menthol 31 atoms, 31 single bonds.
C₂₀H₁₄O₄ Phenolphthalein 38 atoms, 41 bonds.
C₁₈H₂₄O₂ Estradiol 44 atoms, 47 bonds.
C₁₂H₂₂O₁₁ Lactose 45 atoms, 46 single bonds.
C₁₂H₂₂O₁₁ Sucrose 45 atoms, 46 single bonds.
C₁₉H₂₈O₂ Testosterone 49 atoms, 52 bonds.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.