Science Fair Project Encyclopedia
|Formula weight||30.03 g/mol|
|Synonyms||methanal, methyl aldehyde, methylene oxide|
|Melting point||156.2 K (−116.9 °C)|
|Boiling point||254.1 K (-19.0 °C)|
|Triple point||155.1 K (-118.0 °C)
|Liquid density||1130 kg/m3|
|Acute effects||Toxic. May cause irritation to mucous membranes, difficulty breathing, lowered body temperature, lethargy, coma, and death.|
|Chronic effects||May cause cancer of the nose and mucous membranes.|
|Flash point||-53 °C|
|Autoignition temperature||430 °C|
|MSDS||Hazardous Chemical Database|
The chemical compound formaldehyde (also known by IUPAC nomenclature as methanal), is a gas with a strong pungent smell. It is the simplest aldehyde. Its chemical formula is H2CO. It has a boiling point of -21°C (262 K). Formaldehyde was first synthesized by the Russian chemist Aleksandr Butlerov in 1859 but was conclusively identified by Hoffman in 1867.
Formaldehyde readily results from the incomplete combustion of carbon-containing materials. It may be found in the smoke from forest fires, in automobile exhaust, and in tobacco smoke. In the atmosphere, formaldehyde is produced by the action of sunlight and oxygen on atmospheric methane and other hydrocarbons. Small amounts of formaldehyde are produced as a metabolic byproduct in most organisms, including humans.
Although formaldehyde is a gas at room temperature, it is readily soluble in water, and it is most commonly sold as a 37% solution in water called by trade names such as formalin or formol. In water, formaldehyde polymerizes, and formalin actually contains very little formaldehyde in the form of H2CO monomer. Usually, these solutions contain a few percent methanol to limit the extent of polymerization.
Formaldehyde exhibits most of the general chemical properties of the aldehydes, except that is generally more reactive than other aldehydes. Formaldehyde is a potent electrophile. It can participate in electrophilic aromatic substitution reactions with aromatic compounds and can undergo electrophilic addition reactions with alkenes. In the presence of basic catalysts, formaldehyde undergoes a Cannizaro reaction to produce formic acid and methanol.
Formaldehyde reversibly polymerizes to produce its cyclic trimer, 1,3,5-trioxane or the linear polymer polyoxymethylene. Formation of these substances makes formaldehyde's gas behavior differ substantially from the ideal gas law, especially at high pressure or low temperature.
Formaldehyde is readily oxidized by atmospheric oxygen to form formic acid. Formaldehyde solutions must be kept tightly sealed to prevent this from happening in storage.
Industrially, formaldehyde is produced by the catalytic oxidation of methanol. The most commonly used catalysts are silver metal or a mixture of an iron oxide with molybdenum and vanadium. In the more commonly used iron oxide system (Formox process), methanol and oxygen react at 400°C to produce formaldehyde according to the chemical equation
The silver-based catalyst is usually operated at a higher temperature, about 650°C. On it, two chemical reactions simultaneously produce formaldehyde: the one shown above, and the dehydrogenation reaction
Further oxidation of the formaldehyde product during its production usually gives formic acid that is found in formaldehyde solution, found in ppm values.
On a smaller scale, formalin can be produced using a whole range of other methods including conversion from ethanol instead of the normally-fed methanol feedstock. Such methods however are of less commercial importance.
Most formaldehyde, however, is used in the production of polymers and other chemicals. When combined with phenol, urea, or melamine, formaldehyde produces a hard thermoset resin. These resins are commonly used in permanent adhesives, such as those used in plywood or carpeting. They are also foamed to make insulation, or cast into molded products. Production of formaldehyde resins accounts for more than half of formaldehyde consumption.
Formaldehyde is also used to make numerous other chemicals. Many of these are polyfunctional alcohols such as pentaerythritol, which is used to make paints and explosives. Other formaldehyde derivatives include diphenylmethane diisocyanate, an important component in polyurethane paints and foams, and hexamethylene tetramine, which is used in phenol-formaldehyde resins and to make the explosive RDX.
Formaldehyde cross links amino groups.
Because formaldehyde resins are used in many construction materials, including plywood, carpet, and spray-on insulating foams, and because these resins slowly give off formaldehyde over time, formaldehyde is one of the more common indoor air pollutants. At concentrations above 0.1 mg/kg in air, inhaled formaldehyde can irritate the eyes and mucous membranes, resulting in watery eyes, headache, a burning sensation in the throat, and difficulty breathing.
Large formaldehyde exposures, for example from drinking formaldehyde solutions, are potentially lethal. Formaldehyde is converted to formic acid in the body, leading to a rise in blood acidity, rapid, shallow breathing, hypothermia, and coma or death. People who have ingested formaldehyde require immediate medical attention.
In the body, formaldehyde can cause proteins to irreversibly bind to DNA. Laboratory animals exposed to large doses of inhaled formaldehyde over their lifetimes have developed more cancers of the nose and throat than are usual, as have workers in particle-board sawmills. However, some studies suggest that smaller concentrations of formaldehyde like those encountered in most buildings have no carcinogenic effects. Formaldehyde is classifed as a probable human carcinogen.
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