Formic acid

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5 atoms, 4 bonds.

IUPAC name: formic acid
Molar mass: 46.0250(44) g/mol
Atoms: 5
SMILES: C(=O)O
InChIKey: BDAGIHXWWSANSR-UHFFFAOYSA-N
PubChem CID: 284

Made of

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

Formic acid starts the series of carboxylic acids, the acid family acetic acid also belongs to. ChEBI, the Chemical Entities of Biological Interest database, calls it the simplest of them, built on a single carbon atom. Its systematic name is methanoic acid, and NIOSH, the US workplace-safety institute, also lists it as hydrogen carboxylic acid. Undiluted, it is a colourless liquid that fumes in air and has a pungent, penetrating smell, in the description of the Hazardous Substances Data Bank (HSDB).

It mixes with water in all proportions, and ChEBI lists its roles as a solvent, a metabolite and an antibacterial agent. HSDB also calls it a strong reducing agent, a substance that readily passes electrons to another. In water it is a weak acid, and HSDB gives its pK_a, a measure of acid strength explained on the phosphoric acid page, as 3.75 at 20 °C.

Structure and bonding

Four bonds hold formic acid’s five atoms: one carbon, two oxygens and two hydrogens. Turned edge-on, all five lie in a single plane. Besides its two oxygens, the carbon carries a hydrogen of its own: the acid group has no carbon chain behind it, only that one atom. Stick leaves a flat spray of bonds around the carbon, an arrangement explained under molecular shapes. In Space-filling the two hydrogens show only as small bumps on the carbon and on one oxygen.

The model draws the neutral molecule, without charges. In water some molecules give up the hydrogen on the –OH group and become formate ions. On one account, Stephen Lower’s in Chem1, that hydrogen is acidic because the other oxygen, the one with no hydrogen, draws electrons towards itself. Neither the ion nor that pull of electrons is among the atoms and bonds the viewer draws.

Where it turns up

An ant or bee sting delivers formic acid in its venom, ChEBI records, and Chem1 describes it as a chemical weapon supplied to both insects by nature. It was first distilled from ants, Chem1 adds, and is now made synthetically. US Food and Drug Administration rules record it naturally in some insects and, as the free acid, in a number of plants. Formate, the ion some of it becomes in water, is a normal part of the body’s own metabolism, HSDB notes.

Exposed to air, a formaldehyde solution slowly oxidises to formic acid, HSDB adds. Polyethylene breaking down under heat during manufacture can release it, and Haz-Map counts smoking cigarettes among the activities that bring exposure to it. The Consumer Product Information Database finds it in car products, home-maintenance goods and personal care.

Why it matters

In ChEBI’s account its principal use is in livestock feed, which it preserves and protects from bacteria. Beekeeping is another. It is registered as a pesticide with the US Environmental Protection Agency (EPA), and HSDB records its use in bee colonies, controlling tracheal mites and suppressing varroa mites. ATCvet, the World Health Organization’s index of veterinary medicines, files it among organic-acid ectoparasiticides, treatments for parasites living on an animal’s outside, for use on the body’s surface.

Industry makes it from methanol and carbon monoxide by way of methyl formate, and recovers more as a by-product when hydrocarbons are oxidised to acetic acid, HSDB reports. Its other uses run from electroplating and the silvering of mirrors to textile dyeing and finishing, paper and leather. Brewers use it as an antiseptic, and in hydraulic fracturing fluid it keeps the pipe from corroding. Textile dyeing and finishing took 55 per cent of formic acid use in 1965, on HSDB’s figures, and 21 per cent in 1985, when pharmaceuticals took 20 per cent.

Properties

Properties of formic acid Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 284.
Property	Value	Unit	Source
Molecular formula (Hill order)	CH₂O₂
Molar mass	46.0250(44) g/mol	g/mol	IUPAC/CIAAW 2024
Hydrogen atoms	2
Carbon atoms	1
Oxygen atoms	2
Bonds	4
Single bonds	3
Double bonds	1
PubChem molecular weight	46.025 g/mol	g/mol	PubChem

Safety

Concentrated formic acid is both corrosive and flammable. Under the EU’s harmonised classification it carries the signal word Danger: it causes severe skin burns and eye damage, is toxic if inhaled and is harmful if swallowed. Its NIOSH Pocket Guide card, which names the eyes, skin and airways as target organs, and the PubChem Laboratory Chemical Safety Summary are the sheets to consult.

Questions

Is formic acid a flat molecule?

In PubChem's computed model, yes: all five atoms share one plane. The carbon has three neighbours, a hydrogen, a double-bonded oxygen and the oxygen of the OH group, and the model spaces them at 125.2°, 125.1° and 109.7°. Those three angles add up to 360°, the total that only arises when an atom and its three neighbours lie in a single flat layer.

Is formic acid safe in food and cosmetics?

US Food and Drug Administration rules allow it as a synthetic flavouring added directly to food, and as a component of the paper and paperboard used to wrap food. JECFA, the expert committee on food additives that the World Health Organization and the Food and Agriculture Organization convene together, classes it as a flavouring agent and a preservative. The Cosmetic Ingredient Review panel judged it safe in cosmetics as currently used, provided the product is made non-irritating.

What does formic acid do inside the body?

It is taken up through the gut, the lungs and unbroken skin, HSDB reports. Inside cells it blocks cytochrome oxidase, an enzyme of the mitochondria, causing histotoxic hypoxia: tissue that cannot use the oxygen reaching it. Its harm falls mainly on tissues such as the retina and the basal ganglia, nerve-cell clusters deep in the brain. These tolerate poorly any break in the energy supplied by oxidative phosphorylation, the mitochondria's oxygen-driven route to the cell's fuel.

Why is formic acid linked to methanol poisoning?

Formic acid causes the metabolic acidosis, a build-up of acid in the body, that marks methanol poisoning, according to HSDB. Primates turn formic acid into carbon dioxide far more slowly than rats do, and a species' sensitivity to methanol possibly depends on its level of tetrahydrofolate, a form of folate. In people, formic acid brings severe metabolic acidosis and injury to the eyes, ChEBI records.

Made of the same elements

H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
CH₂O Formaldehyde 4 atoms, 3 bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.

All 51 compounds made of hydrogen, carbon and oxygen

C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
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.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 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.
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.