Butyric Acid
Properties
| State | Liquid (oily) |
| Color | Colorless |
| Solubility | Miscible with water, ethanol, and diethyl ether |
| Melting Point | -5.1°C |
| Boiling Point | 163.8°C |
About Butyric Acid
Butyric acid is the four-carbon straight-chain fatty acid and the molecule responsible for some of the most aversive smells in chemistry — rancid butter, parmesan, vomit, stale sweat, the inside of a billy-goat barn. Michel Chevreul isolated it from rancid butter in 1818, named it from the Latin butyrum, and gave the entire fatty-acid class its naming convention. In fresh butter, butyrate is locked up as the C4 ester of glycerol (tributyrin makes up 3–4% of butter fat); pancreatic and bacterial lipases cleave that ester, and free butyric acid floods out at ppb-level olfactory thresholds the human nose detects almost instantly. The smell is both a feature and a liability: parfumiers use butyric esters at heavy dilution because the same C4 backbone that smells appalling on its own gives extraordinary fruit notes when esterified — methyl butyrate reads as pineapple, ethyl butyrate as banana-pear, isoamyl butyrate as apricot-jam. Inside the human gut, butyrate has become one of the most-studied microbial metabolites of the 2010s and 2020s. Anaerobic Firmicutes (Faecalibacterium prausnitzii, Roseburia, Eubacterium rectale) ferment dietary fiber to butyrate at concentrations of 10–30 mM in the colonic lumen, and colonocytes oxidize it as their primary fuel — about 70% of their energy comes from luminal butyrate rather than blood glucose. The histone-deacetylase inhibition that butyrate exerts at millimolar concentrations is the leading hypothesis for its anti-inflammatory and anti-tumorigenic effects in the colon.
Where you'll encounter it
If you've ever forgotten a stick of butter on the counter for a few days and then walked back into the kitchen, the smell is butyric acid being liberated from tributyrin by bacterial lipases. In a flavor and fragrance lab, butyrate esters are made by Fischer esterification of butyric acid with the appropriate alcohol — small-scale, water-azeotrope, often with concentrated sulfuric acid as catalyst — and the workup includes aggressive ventilation because trace unreacted acid will overwhelm the sweet ester smell. In gastroenterology research, sodium butyrate solutions and tributyrin (the prodrug, which doesn't smell until it's hydrolyzed in the gut) are dosed in colitis and colon-cancer studies; the prodrug approach exists almost entirely to spare patients and labmates the smell.
Common Uses
- Fischer esterification feedstock for fruit-flavor esters (methyl/ethyl/amyl butyrate)
- Cellulose acetate butyrate (CAB) production for tool handles and automotive trim
- Tributyrin and sodium butyrate for poultry and swine gut-health additives
- HDAC-inhibitor research reagent in colitis and colorectal cancer studies
- Carp and catfish bait attractant; trace component in cheese-flavor compounds
Safety Information
Corrosive liquid. GHS H226 (flammable, flash point 72 °C), H314 (severe skin burns and eye damage). The odor threshold is around 0.001 ppm — far below any irritation level — so the smell warns long before exposure is dangerous, which is fortunate because the smell penetrates clothing, hair, and skin and persists for days. Always handle in a fume hood; use butyl rubber or neoprene gloves and splash goggles. A skin spill is washed off with copious soap and water then a baking-soda paste; clothes contaminated with even a few drops are usually a write-off.
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.