Formaldehyde

Formaldehyde is the chemical compound with the formula H₂CO. The simplest aldehyde, it was first synthesized by the Russian chemist Aleksandr Butlerov, but was conclusively identified by August Wilhelm von Hofmann.^[1] Formaldehyde exists in several forms aside from H₂CO: the cyclic trimer trioxane and the polymer paraformaldehyde. In water, it exists as the hydrate H₂C(OH)₂.

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. In atmosphere, formaldehyde is produced by the action of sunlight and oxygen on atmospheric methane and other hydrocarbons; thus, it becomes part of smog. Small amounts of formaldehyde are produced as a metabolic byproduct in most organisms, including humans.

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Properties

Although formaldehyde is a gas at room temperature, it is readily soluble in water. It is usually sold as a saturated aqueous solution with concentration of around 37% formaldehyde, stabilized with 10%-15% methanol. The commercial name is either Formalin or Formol. In water, formaldehyde converts to mostly the hydrate CH₂(OH)₂ or methanediol. A small percentage of methanol is usually added to these solutions to limit the extent of polymerization. Formalin reversibly polymerizes to produce its cyclic trimer, 1,3,5-trioxane or the linear polymer polyoxymethylene. The latter one can be heated to obtain methanol-free formaldehyde. Because of the formation of these derivatives, formaldehyde gas deviates strongly from the ideal gas law, especially at high pressure or low temperature.

Production

In industry, formaldehyde is produced 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. On it, two chemical reactions simultaneously produce formaldehyde: the one 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.

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 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. In the presence of basic catalysts, formaldehyde undergoes a Cannizaro reaction to produce formic acid and methanol.

Condensation with acetaldehyde affords pentaerythritol.^[2] Condensation with phenols gives phenol-formaldehyde resins. WIth 4-substituted phenols, one obtains calixarenes.^[3]

With hydrogen sulfide, it forms trithiane.^[4]

3 CH₂O + 3 H₂S → (CH₂S)₃ + 3 H₂O

Biology

Formaldehyde (and its oligomers and hydrates) are rarely encountered in living organisms, but are often used in biology when fixing tissues. Methanogenesis, which can start from many C1 sources, proceeds via the equivalent of formaldehyde, but this one-carbon species is masked as a methylene group carried by methanopterin.

As a disinfectant and biocide

An aqueous solution of formaldehyde can be useful as a disinfectant, as it kills most bacteria and fungi (including their spores). It is also used as a preservative in vaccinations. In medicine, formaldehyde solutions are applied topically to dry the skin, such as in the treatment of warts. Many aquarists use formaldehyde as a treatment for the parasite ichthyophthirius.

Formaldehyde preserves or fixes tissue or cells by irreversibly cross-linking primary amino groups in proteins with other nearby nitrogen atoms in protein or DNA through a -CH₂- linkage.

Formaldehyde is also used as a detergent in RNA gel electrophoresis, preventing RNA from forming secondary structures.

Formaldehyde is converted to formic acid in the body, leading to a rise in blood acidity (acidosis).

Industry

Formaldehyde is a common building block for the synthesis of more complex compounds and materials.

Most formaldehyde 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. It is used as the wet-strength resin added to sanitary paper products such as (listed in increasing concentrations injected into the paper machine headstock chest) facial tissue, table napkins, and roll towels. They are also foamed to make insulation, or cast into moulded products. Production of formaldehyde resins accounts for more than half of formaldehyde consumption.

Many of these are polyfunctional alcohols such as pentaerythritol, which is used to make paints and explosives. Other formaldehyde derivatives include methylene diphenyl diisocyanate, an important component in polyurethane paints and foams, and hexamine, which is used in phenol-formaldehyde resins as well as the explosive RDX.

Formaldehyde is still used in low concentrations for process C-41 (color negative film) stabilizer in the final wash step, as well as in the process E-6 pre-bleach step, to obviate the need for it in the final wash.

Formaldehyde is used to produce glues used in the manufacture of particleboard, plywood, veneers, and other wood products, as well as spray-on insulating foams.^{[citation needed]}

Formaldehyde, along with 18 M (concentrated) sulfuric acid (the entire solution often called the Marquis reagent)^[5] is used as an MDMA "testing kit" by such groups as Dancesafe as well as MDMA consumers. The solution alone cannot verify the presence of MDMA, but reacts with many other chemicals that the MDMA tablet itself may be adulterated with. The reaction itself produces colors that correlate with these components.

The textile industry uses formaldehyde-based resins as finishers to make fabrics crease-resistant.^{[citation needed]}

Embalming

Formaldehyde-based solutions are used in embalming to disinfect and temporarily preserve human remains. It is the ability of formaldehyde to fix the tissue that produces the tell-tale firmness of flesh in an embalmed body. Whereas other heavier aldehydes also produce a similar firming action, none approaches the completeness of formaldehyde. Several European countries restrict the use of formaldehyde, including the import of formaldehyde-treated products and embalming, and the European Union is considering a complete ban on formaldehyde usage (including embalming), subject to a review of List 4B of the Technical Annex to the Report from the Commission to the European Parliament and the Council on the Evaluation of the Active Substances of Plant Protection Products by the European Commission Services. Countries with a strong tradition of embalming corpses, such as Ireland and other colder-weather countries, have raised concerns. The European Union has decided on the date September 22, 2007 to ban Formaldehyde use throughout Europe, because of its carcinogenic properties.^[6]

Safety

Occupational exposure to formaldehyde by inhalation is mainly from three types of sources: thermal or chemical decomposition of formaldehyde-based resins, formaldehyde emission from aqueous solutions (for example, embalming fluids), and the production of formaldehyde resulting from the combustion of a variety of organic compounds (for example, exhaust gases). Formaldehyde can be toxic, allergenic, and carcinogenic.^[7] Because formaldehyde resins are used in many construction materials, formaldehyde is one of the more common indoor air pollutants.^{[citation needed]} At concentrations above 0.1 ppm in air, formaldehyde can irritate the eyes and mucous membranes, resulting in watery eyes. If inhaled, formaldehyde at this concentration may cause headaches, a burning sensation in the throat, and difficulty breathing, as well as triggering or aggravating asthma symptoms.^[8] Formaldehyde is classified as a probable human carcinogen by the U.S. Environmental Protection Agency, which provides sufficient evidence that formaldehyde causes nasopharyngeal cancer in humans by the International Agency for Research on Cancer.^[9] The United States Environmental Protection Agency USEPA allows no more than 0.016 ppm formaldehyde in the air in new buildings constructed for that agency^[10] Formaldehyde can cause allergies, and is part of the standard patch test series. People with formaldehyde allergy are advised to avoid formaldehyde-releasing chemicals as well (e.g. Quaternium-15, imidazolidinyl urea, and diazolidinyl urea).^[11]

References

1. ^ J Read, Text-Book of Organic Chemistry, G Bell & Sons, London, 1935
2. ^ H. B. J. Schurink (1941). "Pentaerythritol". Organic Syntheses, Collected Volume 1: 425.http://www.orgsyn.org/orgsyn/pdfs/CV1P0425.pdf
3. ^ Gutsche, C. D.; Iqbal, M. "p-tert-Butylcalix[4]arene" Organic Syntheses, Collected Volume 8, p.75 (1993); Vol. 68, p.234 (1990). http://www.orgsyn.org/orgsyn/pdfs/CV8P0075.pdf
4. ^ Bost, R. W.; Constable, E. W. "sym-Trithiane" Organic Syntheses, Collected Volume 2, p.610 (1943). http://www.orgsyn.org/orgsyn/pdfs/CV2P0610.pdf
5. ^ http://www.dancesafe.org/documents/druginfo/testkits.php
6. ^ http://www.webwire.com/ViewPressRel.asp?aId=41468
7. ^ IARC Press Release June 2004, http://www.iarc.fr/ENG/Press_Releases/archives/pr153a.html
8. ^ Symptoms of Low-Level Formaldehyde Exposures, Health Canada, http://www.hc-sc.gc.ca/iyh-vsv/environ/formaldehyde_e.html
9. ^ http://monographs.iarc.fr/ENG/Monographs/vol88/volume88.pdf "Formaldehyde".
10. ^ Testing for Indoor Air Quality, Baseline IAQ, and Materials, http://www.epa.gov/rtp/new-bldg/environmental/s_01445.htm
11. ^ Allergy to formaldehyde at DermNetNZ, http://dermnetnz.org/dermatitis/formaldehyde-allergy.html