Formaldehyde
\nThe
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
HCHO, sometimes rendered as H
2CO.\nIt has a boiling point of -21°
C (262 K). Formaldehyde was discovered by the
Russian chemist
Aleksandr Butlerov in
1859.
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.
Properties
\nAlthough 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 HCHO 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.
Production
\nIndustrially, formaldehyde is produced by the catalytic oxidation of methanol. The most commonly catalysts are silver metal and a mixture of an iron oxide with molybdenum. In the more commonly used iron oxide system, methanol and oxygen react at 400°C to produce formaldehyde according to the chemical equation
- CH3OH + ½ O2 → HCHO + H2O
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
- CH3OH → HCHO + H2
Uses
\nFormaldehyde kills most bacteria, and so a solution of formaldehyde in water is commonly used as a disinfectant or to preserve biological specimens. It is also used as a
preservative in vaccinations.
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.
Health effects
\nBecause 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 ppm 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.
Category:Aldehydes
