Formaldehyde ( , ) (systematic name methanal) is a pungent colorless gas mainly used in the production of industrial resins, such as for particle board, coatings, plastics, pulp, paper, synthetic fibers, and textiles. A ~5% solution in water of formaldehyde is used as a disinfectant and fumigant in industrial, agricultural, and healthcare settings, and a ~37% solution is used to preserve tissue samples in labs. In 2024, the global production of formaldehyde was estimated at 26 million tons per year, and is a precursor to many other materials and chemical compounds.
It is an organic compound with the chemical formula and structure . The gas spontaneously polymerizes into paraformaldehyde. It is stored as a ~37% aqueous solution known as formalin, which consists mainly of the hydrate . It is the simplest of the aldehydes ().
Formaldehyde also occurs naturally. It is derived from the degradation of serine, dimethylglycine, and lipids. Demethylases act by converting N-methyl groups to formaldehyde.
Formaldehyde is classified as a group 1 carcinogen and can cause respiratory and skin irritation upon exposure.
Forms Formaldehyde is more complex than most carbon compounds in that it takes several forms at normal temperatures. These compounds can often be used interchangeably and can be interconverted. Molecular formaldehyde. A colorless gas with a characteristic sweetish-pungent irritating odor. It is stable at about 150 °C, but polymerizes when condensed to a liquid. 1,3,5-Trioxane, with the formula . It is a white solid that dissolves without degrading in organic solvents. It is a trimer of molecular formaldehyde. Paraformaldehyde, with the formula . It is a white solid that is insoluble in most solvents. It slowly releases monomeric formaldehyde at room temperature. Methanediol, with the formula . This compound also exists in equilibrium with various oligomers (short polymers), depending on the concentration and temperature. A saturated water solution, of about 40% formaldehyde by volume or 37% by mass, is called "100% formalin".
A small amount of stabilizer, such as methanol, is usually added to suppress oxidation and polymerization. A typical commercial-grade formalin may contain 10–12% methanol as well as metallic impurities.
"Formaldehyde" was first used as a generic trademark in 1893 following a previous trade name, "formalin".
Structure and bonding The formaldehyde molecule is Y-shaped and its molecular symmetry belongs to the C2v point group. The precise molecular geometry of gaseous formaldehyde has been determined by gas electron diffraction and microwave spectroscopy. The bond lengths are for the carbon–oxygen bond and around for the carbon–hydrogen bond, while the H–C–H bond angle is 117°, close to the 120° angle found in an ideal trigonal planar molecule. Some excited electronic states of formaldehyde are pyramidal rather than planar as in the ground state.
Occurrence Atmospheric Sources Formaldehyde (HCHO) enters the atmosphere via primary emission and secondary formation. Primary sources of atmospheric HCHO include direct biogenic emissions from vegetation and biomass burning, and anthropogenic emissions from vehicles, industrial activities, and coal combustion. Most atmospheric HCHO is secondary, produced by radical-driven oxidation of methane () and non-methane volatile organic compounds (VOCs) rather than direct emissions. Secondary production from oxidation dominates in remote and marine environments. Oxidation of biogenic and anthropogenic VOCs is the main source of HCHO in continental regions. Vegetation emits large amounts of isoprene, which is a key natural precursor of tropospheric formaldehyde over land.
Role in atmospheric chemistry As a short-lived but very reactive carbonyl compound, HCHO is important in the oxidation chemistry of the troposphere. It is a key source of odd-hydrogen radicals () that help to sustain the atmosphere's oxidative capacity. It is also an important ozone precursor.
HCHO removal in the troposphere occurs primarily via photodissociation, through two main pathways: molecular and radical-forming channels, the threshold wavelengths for which are given in parentheses:
Molecular: (λ < ) Radical: (λ < 330 nm)
The H and HCO radicals react with the atmospheric to form hydroperoxyl () radicals, giving a net radical channel of:
Also, in the presence of NOx and sunlight, HCHO contributes to tropospheric ozone formation, which is a key component of photochemical smog.
Remote monitoring Spectrometers on earth-observation satellites allow monitoring of formaldehyde in the troposphere. Using ultraviolet–visible spectroscopy, instruments detect absorption features in sunlight reflected by the Earth and use differential optical absorption spectroscopy to calculate vertical column densities. Near-global maps of tropospheric HCHO with daily coverage and kilometer-scale spatial resolution come from the OMI instrument on Aura, the GOME-2 instrument on MetOp, and the newer TROPOMI sensor on Sentinel-5P. Regional maps with hourly coverage over North America are available from the TEMPO instrument on Intelsat 40e.
The close coupling between isoprene and formaldehyde has been widely used in top-down approaches that derive regional and global isoprene emissions from satellite measurements of tropospheric formaldehyde columns. These studies provide important constraints on the magnitude and spatial pattern of biogenic isoprene emissions.
Processes in the upper atmosphere contribute more than 80% of the total formaldehyde in the environment. Formaldehyde is an intermediate in the oxidation (or combustion) of methane and other carbon compounds from sources such as forest fires, automobile exhaust, and tobacco smoke. When produced in the atmosphere by the action of sunlight and oxygen on atmospheric methane and other hydrocarbons, it becomes part of smog. Formaldehyde has also been detected in outer space.
Environmental exposure Formaldehyde and its adducts are ubiquitous in nature. Food may contain formaldehyde at levels 1–100 mg/kg. Formaldehyde, formed in the metabolism of the amino acids serine and threonine, is found in the bloodstream of humans and other primates at concentrations of approximately 50 micromolar. Even in animals that were deliberately exposed to formaldehyde, most formaldehyde-DNA adducts found in non-respiratory tissues derive from endogenously produced formaldehyde.
Formaldehyde does not accumulate in the environment. It is broken down within a few hours by sunlight or by bacteria in soil or water. Humans metabolize formaldehyde quickly, converting it to formic acid. It nonetheless presents significant health concerns as a contaminant.
Interstellar formaldehyde
Formaldehyde appears to be a useful probe in astrochemistry due to prominence of the 110←111 and 211←212 K-doublet transitions. It was the first polyatomic organic molecule detected in the interstellar medium. Since its initial detection in 1969, it has been observed in many regions of the galaxy. Because of the widespread interest in interstellar formaldehyde, it has been extensively studied, yielding new extragalactic sources. A proposed mechanism for the formation is the hydrogenation of CO ice:
HCN, HNC, , and dust have also been observed inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).
Synthesis and industrial production Laboratory synthesis Formaldehyde was discovered in 1859 by the Russian chemist Aleksandr Butlerov (1828–1886) when he tried to synthesize methanediol ("methylene glycol") from iodomethane and silver oxalate. In his paper, Butlerov called formaldehyde "dioxymethylen" (methylene dioxide) because his empirical formula for it was incorrect, as atomic weights were not precisely determined until the Karlsruhe Congress.
August Wilhelm von Hofmann first identified the compound as an aldehyde. He announced its production by passing methanol vapor in air over hot platinum wire.See: A. W. Hofmann (14 October 1867) "Zur Kenntnis des Methylaldehyds" ([Contributions] to our knowledge of methylaldehyde), Monatsbericht der Königlich Preussischen Akademie der Wissenschaften zu Berlin (Monthly Report of the Royal Prussian Academy of Sciences in Berlin), vol. 8, pages 665–669. Reprinted in: A.W. Hofmann, (1868) "Zur Kenntnis des Methylaldehyds", Annalen der Chemie und Pharmacie (Annals of Chemistry and Pharmacy), vol. 145, no. 3, pages 357–361. A.W. Hofmann (1868) "Zur Kenntnis des Methylaldehyds", Journal für praktische Chemie (Journal for Practical Chemistry), vol. 103, no. 1, pages 246–250. However, it was not until 1869 that Hofmann determined the correct empirical formula of formaldehyde. See: A.W. Hofmann (5 April 1869) "Beiträge zur Kenntnis des Methylaldehyds", Monatsbericht der Königlich Preussischen Akademie der Wissenschaften zu Berlin, vol. ?, pages 362–372. Reprinted in:
A.W. Hofmann (1869) "Beiträge zur Kenntnis des Methylaldehyds," Berichte der Deutschen Chemischen Gesellschaft (Reports of the German Chemical Society), vol. 2, pages 152–159. With modifications, Hofmann's method remains the basis of the modern industrial route.
Solution routes to formaldehyde also entail oxidation of methanol or iodomethane.
Industry Formaldehyde is produced industrially by the catalytic oxidation of methanol. The most common catalysts are silver metal (i.e. the FASIL process), iron(III) oxide, iron molybdenum oxides (e.g. iron(III) molybdate) with a molybdenum-enriched surface, or vanadium oxides. In the commonly used formox process, methanol and oxygen react at c. 250–400 °C in presence of iron oxide in combination with molybdenum or vanadium to produce formaldehyde according to the chemical equation
The silver-based catalyst usually operates at a higher temperature, about 650 °C. Two chemical reactions on it simultaneously produce formaldehyde: that shown above and the dehydrogenation reaction
In principle, formaldehyde could be generated by oxidation of methane, but this route is not industrially viable because the methanol is more easily oxidized than methane.
Biochemistry Formaldehyde is produced via several enzyme-catalyzed routes. Living beings, including humans, produce formaldehyde as part of their metabolism. Formaldehyde is key to several bodily functions (e.g. epigenetics), but its amount must also be tightly controlled to avoid self-poisoning.
Serine hydroxymethyltransferase can decompose serine into formaldehyde and glycine, according to the reaction . Methylotrophic microbes convert methanol into formaldehyde and energy via methanol dehydrogenase: Other routes to formaldehyde include oxidative demethylations, semicarbazide-sensitive amine oxidases, dimethylglycine dehydrogenases, lipid peroxidases, P450 oxidases, and N-methyl group demethylases.
Formaldehyde is catabolized by alcohol dehydrogenase ADH5 and aldehyde dehydrogenase ALDH2.
Organic chemistry Formaldehyde is a building block in the synthesis of many other compounds of specialized and industrial significance. It exhibits most of the chemical properties of other aldehydes but is more reactive.
Polymerization and hydration Monomeric is a gas and is rarely encountered in the laboratory. Aqueous formaldehyde, unlike some other small aldehydes (which need specific conditions to oligomerize through aldol condensation) oligomerizes spontaneously at a common state. The trimer 1,3,5-trioxane, , is a typical oligomer. Many cyclic oligomers of other sizes have been isolated. Similarly, formaldehyde hydrates to give the geminal diol methanediol, which condenses further to form hydroxy-terminated oligomers . The polymer is called paraformaldehyde. The higher concentration of formaldehyde—the more equilibrium shifts towards polymerization. Diluting with water or increasing the solution temperature, as well as adding alcohols (such as methanol or ethanol) lowers that tendency.