Propionic Acid
Properties
| State | Liquid (oily) |
| Color | Colorless |
| Solubility | Miscible with water, ethanol, and diethyl ether |
| Melting Point | -21°C |
| Boiling Point | 141°C |
About Propionic Acid
Propionic acid is the three-carbon carboxylic acid (CH3CH2COOH, molar mass 74.079 g/mol) that smells like a combination of vinegar and locker-room sweat — sharp, oily, and unmistakable below the parts-per-million range. The smell is biological: Propionibacterium freudenreichii ferments lactate into propionate plus CO2 during Swiss cheese aging, and that's what gives Emmental and Gruyère their nutty bite and the eyes (the holes) their gas pockets. The largest commercial source isn't fermentation but the Reppe carbonylation route — ethylene plus CO and water over a nickel or rhodium catalyst at 200-300 °C and 100-300 bar — which BASF and Eastman use to feed an annual market around 500,000 tonnes. Most of that volume goes into mold inhibition: calcium and sodium propionate (E282 and E281) are the standard antifungals in commercial bread, baked goods, and ammoniated grain storage for livestock feed. Propionic acid is also the headwater for 2-arylpropionic acid drugs — ibuprofen, naproxen, ketoprofen — where the 2-aryl-substituted propionate scaffold defines an entire class of NSAID analgesics.
Where you'll encounter it
If you've ever sliced a loaf of supermarket bread that stays mold-free in a plastic bag for two weeks, calcium propionate at 0.1-0.3% by flour weight is doing that work — propanoate ion crosses the fungal cell membrane and acidifies the cytoplasm enough to halt Aspergillus and Penicillium growth without changing how the bread tastes. In a livestock feed mill, ammoniated propionic acid (Luprosil or similar trade names) is sprayed onto wet corn at harvest to prevent silo mold and aflatoxin production until the grain dries down. In a pharmaceutical synthesis lab, propionic acid is the C3 carboxyl source you reach for when you want to install a methyl-CH2- spacer between an aromatic ring and the carboxylate — the asymmetric Friedel-Crafts acylations that build the ibuprofen scaffold start with propionyl chloride. And anyone who has done biochemistry coursework on odd-chain fatty acid metabolism knows propionyl-CoA as the entry point that needs methylmalonyl-CoA mutase and vitamin B12 to feed into the citric acid cycle.
Common Uses
- Calcium and sodium propionate (E281/E282) as antifungal preservatives in commercial bread and baked goods
- Ammoniated propionic acid sprayed on wet harvest corn to prevent silo mold and aflatoxin
- C3 carboxyl source for ibuprofen, naproxen, and ketoprofen synthesis via Friedel-Crafts acylation
- Plasticizer feedstock for cellulose acetate propionate films in laminated glass interlayers
- Herbicide-ester precursor for 2,4-D propylene glycol butyl ether ester formulations
- Fragrance fixative for synthetic apple, rum, and pineapple ester components
Safety Information
GHS classification: Flammable liquid Category 3 (H226, flash point 54 °C), Skin corrosion Category 1B (H314 — severe burns and eye damage). OSHA has no specific PEL; ACGIH TLV is 10 ppm 8-hour TWA based on respiratory irritation. The pungent vinegar-sweat odor has a detection threshold around 50 ppb, well below the irritation threshold, so workers usually smell exposure long before it becomes hazardous — a useful built-in warning. Concentrated propionic acid burns skin like glacial acetic acid does; nitrile gloves and splash goggles are mandatory above 30% concentration. Calcium propionate at food-additive levels is GRAS and metabolizes through normal short-chain fatty acid pathways at the human dietary intake range.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.