Butyric acid

Butyric acid
IUPAC name Butanoic acid
Other names Butyric acid; 1-Propanecarboxylic acid; Propanecarboxylic acid; C4:0 (Lipid numbers)
Identifiers
CAS number	107-92-6 Y
PubChem	264
ChemSpider	259
UN number	2820
MeSH	Butyric+acid
IUPHAR ligand	1059
SMILES O=C(O)CCC
InChI InChI=1/C4H8O2/c1-2-3-4(5)6/h2-3H2,1H3,(H,5,6) Key: FERIUCNNQQJTOY-UHFFFAOYAP
Properties
Molecular formula	C4H8O2
Molar mass	88.11 g mol−1
Appearance	Colorless liquid
Density	0.9595 g/mL
Melting point	−7.9 °C, 265 K, 18 °F
Boiling point	163.5 °C, 437 K, 326 °F
Solubility in water	miscible
Acidity (pKa)	4.82
Refractive index (nD)	1.3980 (19 °C)
Viscosity	0.1529 cP
Hazards
MSDS	External MSDS
EU classification	Xn C
R-phrases	R20 R21 R22 R34 R36 R37 R38
Flash point	72 °C
Autoignition temperature	452 °C
Related compounds
Other anions	butyrate
Related carboxylic acids	propionic acid acrylic acid succinic acid malic acid tartaric acid crotonic acid fumaric acid pentanoic acid
Related compounds	butanol butyraldehyde methyl butyrate
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Butyric acid (from Greek βούτυρος = butter), also known under the systematic name butanoic acid, is a carboxylic acid with the structural formula CH₃CH₂CH₂-COOH. Salts and esters of butyric acid are known as butyrates or butanoates. Butyric acid is found in butter, parmesan cheese, vomit, and as a product of anaerobic fermentation (including in the colon and as body odor). It has an unpleasant smell and acrid taste, with a sweetish aftertaste (similar to ether). It can be detected by mammals with good scent detection abilities (such as dogs) at 10 ppb, whereas humans can detect it in concentrations above 10 ppm.

Chemistry

Butyric acid is a fatty acid occurring in the form of esters in animal fats and plant oils. The triglyceride of butyric acid makes up 3% to 4% of butter. When butter goes rancid, butyric acid is liberated from the glyceride by hydrolysis leading to the unpleasant odor. It is an important member of the fatty acid sub-group called short-chain fatty acids. Butyric acid is a weak acid with a pKa of 4.82, similar to acetic acid which has pKa 4.76.^[1] The similar strength of these acids results from their common -CH₂COOH terminal structure.^[2] Pure butyric acid is 10.9 molar.

The acid is an oily colorless liquid that is easily soluble in water, ethanol, and ether, and can be separated from an aqueous phase by saturation with salts such as calcium chloride. Potassium dichromate and sulfuric acid oxidize it to carbon dioxide and acetic acid, while alkaline potassium permanganate oxidizes it to carbon dioxide. The calcium salt, Ca(C₄H₇O₂)₂·H₂O, is less soluble in hot water than in cold.

Butyric acid has a structural isomer called isobutyric acid (2-methylpropanoic acid).

Production

It is industrially prepared by the fermentation of sugar or starch, brought about by the addition of putrefying cheese, with calcium carbonate added to neutralize the acids formed in the process. The butyric fermentation of starch is aided by the direct addition of Bacillus subtilis. Salts and esters of the acid are called butanoates.

Butyric acid or fermentation butyric acid is also found as a hexyl ester (hexyl butanoate) in the oil of Heracleum giganteum (a type of hogweed) and as an octyl ester (octyl butanoate) in parsnip (Pastinaca sativa); it has also been noticed in the fluors of the flesh and in perspiration.

Uses

Butyric acid is used in the preparation of various butanoate esters. Low-molecular-weight esters of butyric acid, such as methyl butanoate, have mostly pleasant aromas or tastes. As a consequence, they find use as food and perfume additives.

Due to its powerful odor, it has also been used as a fishing bait additive.^[3] Many of the commercially available flavours used in carp (Cyprinus carpio) baits use butyric acid as their ester base; however, it is not clear whether fish are attracted by the butyric acid itself or the additional substances added to it. Butyric acid was, however, one of the few organic acids shown to be palatable for both tench and bitterling.^[4]

The substance has also been used as a noxious, nausea-inducing repellent by anti-whaling protesters, against Japanese whaling crews,^[5] as well as by anti-abortion protestors to disrupt and harass clinics.^[6]

Biological functionality

Butanoate fermentation

Butanoate is produced as end-product of a fermentation process solely performed by obligate anaerobic bacteria. Fermented Kombucha "tea" includes butyric acid as a result of the fermentation. This fermentation pathway was discovered by Louis Pasteur in 1861. Examples of butanoate-producing species of bacteria:

• Clostridium acetobutylicum
• Clostridium butyricum
• Clostridium kluyveri
• Clostridium pasteurianum
• Fusobacterium nucleatum
• Butyrivibrio fibrisolvens
• Eubacterium limosum

The pathway starts with the glycolytic cleavage of glucose to two molecules of pyruvate, as happens in most organisms. Pyruvate is then oxidized into acetyl coenzyme A using a unique mechanism that involves an enzyme system called pyruvate-ferredoxin oxidoreductase. Two molecules of carbon dioxide (CO₂) and two molecules of elemental hydrogen (H₂) are formed as wastes products from the cell. Then,

ATP is produced, as can be seen, in the last step of the fermentation. Three molecules of ATP are produced for each glucose molecule, a relatively high yield. The balanced equation for this fermentation is

C₆H₁₂O₆ → C₄H₈O₂ + 2CO₂ + 2H₂.

Acetone and butanol fermentation

Several species form acetone and butanol in an alternative pathway, which starts as butyrate fermentation. Some of these species are

• Clostridium acetobutylicum, the most prominent acetone and butanol producer, used also in industry,
• Clostridium beijerinckii,
• Clostridium tetanomorphum,
• Clostridium aurantibutyricum.

These bacteria begin with butanoate fermentation as described above, but, when the pH drops below 5, they switch into butanol and acetone production in order to prevent further lowering of the pH. Two molecules of butanol are formed for each molecule of acetone.

The change in the pathway occurs after acetoacetyl CoA formation. This intermediate then takes two possible pathways:

• acetoacetyl CoA → acetoacetate → acetone, or
• acetoacetyl CoA → butyryl CoA → butanal → butanol.

Butyric acid function/activity

Highly-fermentable fiber residues, like resistant starch, oat bran, and pectin are transformed by colonic bacteria into short-chain fatty acids including butyrate. One study found that resistant starch consistently produces more butyrate than other types of dietary fiber.^[7]

The role of butyrate changes depending on its role in cancer or normal cells. This is known as the "butyrate paradox". Butyrate inhibits colonic tumor cells but promotes healthy colonic epithelial cells,^[8] but the signaling mechanism is not well understood.^[9] A review suggested that the chemopreventive benefits of butanoate depend in part on amount, time of exposure with respect to the tumorigenic process, and the type of fat in the diet.^[10] Low carbohydrate diets like the Atkins diet are known to reduce the amount of butyrate produced in the colon.

Butyric acid can act as an HDAC inhibitor, inhibiting the function of histone deacetylase enzymes, thereby favoring an acetylated state of histones in the cell. Acetylated histones have a lower affinity for DNA than non-acetylated histones, due to the neutralization of electrostatic charge interactions. In general, it is thought that transcription factors will be unable to access regions where histones are tightly associated with DNA (i.e. non-acetylated, e.g., heterochromatin). Therefore, it is thought that butyric acid enhances the transcriptional activity at promoters, which are typically silenced or downregulated due to histone deacetylase activity.

Two HDAC inhibitors, sodium butyrate (NaB) and trichostatin A (TSA), increase lifespan in experimental animals.^[11]

See also

• Indole-3-butyric acid
• Acids in wine

References

 This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed (1911). Encyclopædia Britannica (Eleventh ed.). Cambridge University Press. 

External links

• International Chemical Safety Card 1334
• 2004 review of the scientific evidence on butanoate/butyrate vs. colon cancer