Butyric acid

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14 atoms, 13 bonds.

IUPAC name: butanoic acid
Molar mass: 88.106(12) g/mol
Atoms: 14
SMILES: CCCC(=O)O
InChIKey: FERIUCNNQQJTOY-UHFFFAOYSA-N
PubChem CID: 264

Made of

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

Butter and animal fat hold butyric acid as an ester of glycerol, according to MeSH, the subject vocabulary of the US National Library of Medicine. An ester is the product of an organic acid reacting with an alcohol, here glycerol. The US National Cancer Institute’s thesaurus also finds it in esterified form in plant oils. On its own the acid is a colourless liquid whose smell CAMEO Chemicals, a US hazardous-materials database, calls penetrating and unpleasant. The liquid is slightly less dense than water, CAMEO adds, and its vapour is heavier than air.

Its IUPAC (International Union of Pure and Applied Chemistry) name is butanoic acid. Four carbon, eight hydrogen and two oxygen atoms give C₄H₈O₂ at 88.11 g/mol. The Hazardous Substances Data Bank (HSDB) describes an oily liquid and puts its pK_a at 4.82 at 25 °C, a quantity explained under phosphoric acid.

Structure and bonding

Butyric acid is butane with one end methyl group oxidised to a carboxy group, in the description of ChEBI, a database of biologically relevant small molecules. Three carbons carry all seven C–H hydrogens. The fourth ends the chain with one oxygen on a double bond, drawn as two sticks, and a second oxygen holding the only O–H hydrogen. In the model, the four carbons and both oxygens lie close to one plane, the chain bending back and forth in a zigzag. At the acid end, the model’s last carbon and its three neighbours sit flat.

With Labels on, element symbols mark all 14 atoms, the two O symbols together at one end. In the Stick view, the C=O bond stands out as the model’s only doubled stick, at the acid end of the chain. In the Space-filling view the atoms swell into one elongated outline, red oxygens capping one end.

The model draws the neutral acid, its acidic hydrogen still bonded to oxygen. ChEBI calls butyric acid the conjugate acid of butyrate, the ion left when that hydrogen goes.

Where it turns up

HSDB lists butyric acid as a flavouring that gives body to caramel, butterscotch, cheese, butter, fruit and nut flavours, and records a butter-fat taste. FEMA, the Flavor and Extract Manufacturers Association, profiles its flavour as butter, cheese and sour. JECFA, an expert committee on food additives run jointly by the UN’s food and health agencies, likens its strong, rancid smell to butter. The US Food and Drug Administration (FDA) files it among food substances as a flavouring agent, an antioxidant and a pH control agent.

Tanneries use it to strip lime from hides and swell them, according to HSDB. Haz-Map, a US occupational-health database, lists it as an additive to disinfectants, petrol and varnishes. HSDB also names it as a starting material for cellulose derivatives in lacquers and plastics, and for butyrate esters used as perfume and flavour ingredients.

Why it matters

Fermentation of fibre in the human colon forms butyric acid as butyrates. HSDB calls it a very important energy source for the normal lining of the colon and rectum. Its formation there from fibre has been suggested as one factor in why high-fibre diets protect against colon cancer, HSDB notes. Butyric acid may also shield that lining from dysplastic change, an abnormal growth of its cells. In laboratory cultures it pushes malignant cells to differentiate, maturing into more specialised forms. Those separate roles may reflect the differing metabolic activities of normal and tumour tissues, in HSDB’s account.

Butyric acid is readily absorbed from the gut and quickly metabolised by the liver. In liver tissue studied outside the body, it becomes ketone bodies mainly by splitting into two-carbon pieces that then recombine. Direct beta-oxidation is the smaller route. It can also be oxidised further through the Krebs cycle or joined to glutathione, and its breakdown gives acetic acid as well.

Properties

Properties of butyric acid Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 264.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₄H₈O₂
Molar mass	88.106(12) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	4
Hydrogen atoms	8
Oxygen atoms	2
Bonds	13
Single bonds	12
Double bonds	1
PubChem molecular weight	88.11 g/mol	g/mol	PubChem

Safety

Concentrated butyric acid is corrosive. Companies notifying the European Chemicals Agency give it the signal word Danger, and 99 per cent of those reporting a classification cite severe skin burns and eye damage. The International Chemical Safety Card adds corrosion of the respiratory tract, and CAMEO notes that skin absorbs it readily. The PubChem Laboratory Chemical Safety Summary for butyric acid gathers these classifications.

Questions

What has the same chemical formula as butyric acid?

Ethyl acetate is one compound that does. Both are C₄H₈O₂, with a molar mass of 88.11 g/mol, but their atoms join differently. In butyric acid's model the four carbons form one chain ending in –COOH, with one hydrogen on oxygen. Ethyl acetate places an oxygen between two carbons and bonds no hydrogen to oxygen. On this site's Build a formula page, C₄H₈O₂ built from element tiles brings up both.

Is butyric acid a fatty acid?

Yes. ChEBI, the Chemical Entities of Biological Interest database, classes it as a straight-chain saturated fatty acid, and the US National Cancer Institute's thesaurus as a short-chain one with a four-carbon backbone. Saturated means its carbons are joined only by single bonds, and the model draws all three of its C–C bonds that way. MeSH, the US medical subject vocabulary, writes the formula as CH₃CH₂CH₂COOH, the chain spelled out carbon by carbon.

Is butyric acid safe to eat?

As a flavouring, US regulators treat it as safe: the Hazardous Substances Data Bank records it on the US Food and Drug Administration's list of flavourings generally recognised as safe. JECFA, the joint expert committee of the UN's food and health agencies on additives, evaluated it as a flavouring in 1997 and found no safety concern at the intake levels then current. The concentrated liquid, however, is corrosive, as its PubChem safety summary sets out.

How is butyric acid made?

The Hazardous Substances Data Bank (HSDB) records it as a by-product of oxidising butane in the liquid phase to acetic acid. It is also made by oxidising 1-butanol or butyraldehyde, the aldehyde most often with air or oxygen. Fermenting starches and molasses with selective enzymes gives another route, the acid then recovered as its calcium salt. Reports to the US Environmental Protection Agency put the national volume for 2023 at 100 million to under 250 million pounds.

Made of the same elements

CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.

All 51 compounds made of carbon, hydrogen and oxygen

C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
HCOOH Formic acid 5 atoms, 4 bonds.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
CH₂O Formaldehyde 4 atoms, 3 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 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.