Formaldehyde is a chemical compound with the formula H₂CO. It is the simplest aldehyde—an organic compound containing a terminal carbonyl group: it consists of exactly one carbonyl. It was first synthesized by the Russian chemist Aleksandr Butlerov (1828-1886), but was conclusively identified by August Wilhelm von Hofmann. Formaldehyde exists in several forms aside from H₂CO: the cyclic trimer trioxane and the polymer paraformaldehyde. It exists in water as the hydrate H₂C(OH)₂. Aqueous solutions of formaldehyde are referred to as formalin. "100%" formalin consists of a saturated solution of formaldehyde (roughly 40% by mass) in water, with a small amount of stabilizer, usually methanol to limit oxidation and polymerization.

Formaldehyde is an intermediate in the oxidation (or combustion) of methane as well as other carbon compounds, e.g. forest fires, in automobile exhaust, and in 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. Small amounts of formaldehyde are produced as a metabolic byproduct in most organisms, including humans.

Production

Formaldehyde is produced industrially by the catalytic oxidation of methanol. The most common catalysts are silver metal or a mixture of an iron oxide with molybdenum and vanadium. In the more commonly used FORMOX process methanol and oxygen react at ca 250-400 °C in presence of iron oxide in combination with molybdenum and/or vanadium to produce formaldehyde according to the chemical equation

2 CH₃OH + O₂ → 2 H₂CO + 2 H₂O

The silver-based catalyst is usually operated at a higher temperature, about 650 °C. Two chemical reactions on it simultaneously produce formaldehyde: that shown above and the dehydrogenation reaction

CH₃OH → H₂CO + H₂

Formaldehyde is readily oxidized by atmospheric oxygen to form formic acid. Formic acid is found in ppm levels in commercial formaldehyde.

Formalin can be produced on a smaller scale using a whole range of other methods including conversion from ethanol instead of the normally-fed methanol feedstock. Such methods are of less commercial importance.

Organic chemistry

Formaldehyde is a central building block in the synthesis of many other compounds. It exhibits most of the chemical properties of other aldehydes but is more reactive. Formaldehyde is a good electrophile, participating in electrophilic aromatic substitution reactions with aromatic compounds, and can undergo electrophilic addition reactions with alkenes. Formaldehyde undergoes a Cannizzaro reaction in the presence of basic catalysts to produce formic acid and methanol.

Condensation with acetaldehyde affords pentaerythritol. Condensation with phenols gives phenol-formaldehyde resins. With 4-substituted phenols one obtains calixarenes.