Formaldehyde

Formaldehyde is an organic compound with the chemical formula and structure. It is a pungent, colorless gas that spontaneously polymerizes into paraformaldehyde. It is stored as a ~37% aqueous solution, which consists mainly of the hydrate CH₂₂. It is the simplest of the aldehydes. As a precursor to many other materials and chemical compounds, in 2024 the global production of formaldehyde was estimated at 26 million tons per year. It is mainly used in the production of industrial resins, e.g., for particle board and coatings, plastics, pulp, paper, synthetic fibers, and textiles. A ~5% solution of formaldehyde is used as a disinfectant and fumigant in industrial, agricultural, and healthcare settings, and formalin is used to preserve tissue samples.
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 ₃. It is a white solid that dissolves without degrading in organic solvents. It is a trimer of molecular formaldehyde.
Paraformaldehyde, with the formula HO_nH. It is a white solid that is insoluble in most solvents. It slowly releases monomeric formaldehyde at room temperature.
Methanediol, with the formula CH₂₂. This compound also exists in equilibrium with various oligomers, 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 C_2v point group. The precise molecular geometry of gaseous formaldehyde has been determined by gas electron diffraction and microwave spectroscopy. The bond lengths are 1.21 Å for the carbon–oxygen bond and around 1.11 Å 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

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.
The H and HCO radicals react with the atmospheric O2 to form hydroperoxyl radicals, giving a net radical channel of:
Also, in the presence of NO_x and sunlight, HCHO contributes to tropospheric ozone formation, which is a key component of photochemical smog.

Remote monitoring

s 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 of methane and other carbon compounds 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, H₂CO, and dust have also been observed inside the comae of comets C/2012 F6 and C/2012 S1.

Synthesis and industrial production

Laboratory synthesis

Formaldehyde was discovered in 1859 by the Russian chemist Aleksandr Butlerov when he tried to synthesize methanediol from iodomethane and silver oxalate. In his paper, Butlerov called formaldehyde "dioxymethylen" 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. 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, iron oxide, iron molybdenum oxides 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 and/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, but its amount must also be tightly controlled to avoid self-poisoning.

Serine hydroxymethyltransferase can decompose serine into formaldehyde and glycine, according to this reaction: HOCH₂CHCO₂H → CH₂O + H₂CCO₂H.
Methylotrophic microbes convert methanol into formaldehyde and energy via methanol dehydrogenase: CH₃OH → CH₂O + 2e⁻ + 2H⁺
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 CH₂O is a gas and is rarely encountered in the laboratory. Aqueous formaldehyde, unlike some other small aldehydes 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 HO_nH. 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 lowers that tendency.
Gaseous formaldehyde polymerizes at active sites on vessel walls, but the mechanism of the reaction is unknown. Small amounts of hydrogen chloride, boron trifluoride, or stannic chloride present in gaseous formaldehyde provide the catalytic effect and make the polymerization rapid.