Formic acid (also called methanoic acid) is the simplest carboxylic acid. Its chemical formula is HCOOH or HCO2H. It is an important intermediate in chemical synthesis and occurs naturally, most notably in the venom of bee and ant stings. In fact, its name comes from the Latin word for ant, formica, referring to its early isolation by the distillation of ant bodies. Esters, salts, and the anion derived from formic acid are referred to as formates.
Formic acid is a colorless liquid having a highly pungent, penetrating odor<ref>OSHA description.</ref> at room temperature. It is miscible with water and most polar organic solvents, and is somewhat soluble in hydrocarbons. In hydrocarbons and in the vapor phase, it consists of hydrogen-bonded dimers rather than individual molecules.<ref name=Ullmann_2009>Werner Reutemann and Heinz Kieczka “Formic Acid” in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. Template:DOI</ref><ref name=Balabin_2009>Template:Cite journal</ref> Owing to its tendency to hydrogen-bond, gaseous formic acid does not obey the ideal gas law.<ref name=Balabin_2009/> Solid formic acid (two polymorphs) consists of an effectively endless network of hydrogen-bonded formic acid molecules. This relatively complicated compound also forms a low-boiling azeotrope with water (22.4%) and liquid formic acid also tends to supercool.
- See also Insect defenses
In nature, it is found in the stings and bites of many insects of the order Hymenoptera, mainly ants. Formic acid is a naturally occurring component of the atmosphere due primarily to forest emissions. <ref>New study confirms that nature is responsible for 90% of the Earth's atmospheric acidity – wattsupwiththat.com</ref>
In 2009, the worldwide capacity for producing this compound was 720,000 tonnes/annum, with production capacity roughly equally divided between Europe (350,000, mainly in Germany) and Asia (370,000, mainly in China), while production was below 1000 tonnes/annum in all other continents.<ref name=CEH>Template:Cite web</ref> It is commercially available in solutions of various concentrations between 85 and 99 w/w %.<ref name = Ullmann_2009/> Template:As of, the largest producers are BASF, Kemira and Feicheng Acid Chemicals, with the largest production facilities in Ludwigshafen (200,000 tonnes/annum, BASF, Germany), Oulu (105,000, Kemira, Finland) and Feicheng (100,000, Feicheng, China). 2010 Prices ranged from circa € 650/tonne in Western Europe and $ 1250/tonne in the United States.<ref name=CEH/>
From methyl formate and formamide
- CH3OH + CO → HCO2CH3
In industry, this reaction is performed in the liquid phase at elevated pressure. Typical reaction conditions are 80 °C and 40 atm. The most widely-used base is sodium methoxide. Hydrolysis of the methyl formate produces formic acid:
- HCO2CH3 + H2O → HCO2H + CH3OH
Efficient hydrolysis of methyl formate requires a large excess of water. Some routes proceed indirectly by first treating the methyl formate with ammonia to give formamide, which is then hydrolyzed with sulfuric acid:
- HCO2CH3 + NH3 → HC(O)NH2 + CH3OH
- 2 HC(O)NH2 + 2 H2O + H2SO4 → 2HCO2H + (NH4)2SO4
This approach suffers from the need to dispose of the ammonium sulfate byproduct. This problem has led some manufacturers to develop energy efficient means for separating formic acid from the large excess amount of water used in direct hydrolysis. In one of these processes (used by BASF) the formic acid is removed from the water via liquid-liquid extraction with an organic base.
By-product of acetic acid production
A significant amount of formic acid is produced as a byproduct in the manufacture of other chemicals. At one time, acetic acid was produced on a large scale by oxidation of alkanes, via a process that cogenerates significant formic acid. This oxidative route to acetic acid is declining in importance, so that the aforementioned dedicated routes to formic acid have become more important.
Hydrogenation of carbon dioxide
The catalytic hydrogenation of CO2 has long been studied. This reaction can be conducted homogeneously.<ref>P. G. Jessop, in Handbook of Homogeneous Hydrogenation (Eds.: J. G. de Vries, C. J. Elsevier), Wiley-VCH, Weinheim, Germany, 2007, pp. 489–511.</ref><ref>Template:Cite journal</ref>
In the laboratory, formic acid can be obtained by heating oxalic acid in glycerol and extraction by steam distillation.<ref name="chattaway">Template:Cite doi Available at HathiTrust.</ref> Glycerol acts as a catalyst, as the reaction proceeds through a glyceryl oxalate intermediary. If the reaction mixture is heated to higher temperatures, allyl alcohol results. The net reaction is thus:
- C2O4H2 → CO2H2 + CO2
Another preparation (which must be performed under a fume hood) is the acid hydrolysis of ethyl isonitrile (C2H5NC) using HCl solution.<ref name="cohen">Cohen, Julius B.: Practical Organic Chemistry MacMillan 1930</ref>
- C2H5NC + 2 H2O → C2H5NH2 + HCO2H
A major use of formic acid is as a preservative and antibacterial agent in livestock feed. In Europe, it is applied on silage (including fresh hay) to promote the fermentation of lactic acid and to suppress the formation of butyric acid; it also allows fermentation to occur quickly, and at a lower temperature, reducing the loss of nutritional value.<ref name = Ullmann_2009/> Formic acid arrests certain decay processes and causes the feed to retain its nutritive value longer, and so it is widely used to preserve winter feed for cattle.Template:Citation needed In the poultry industry, it is sometimes added to feed to kill E. coli bacteria.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Use as preservative for silage and (other) animal feed constituted 30% of the global consumption in 2009.<ref name=CEH/>
Formic acid is also significantly used in the production of leather -incl. tanning- (23% of the global consumption in 2009<ref name=CEH/>) and in dyeing and finishing of textile (9% of the global consumption in 2009<ref name=CEH/>) because of its acidic nature. Use as a coagulant in the production of rubber<ref name = Ullmann_2009/> constituted in 2009 6% of the global consumption.<ref name=CEH/>
Formic acid is also used in place of mineral acids for various cleaning products,<ref name =Ullmann_2009/> such as limescale remover and toilet bowl cleaner. Some formate esters are artificial flavorings or perfumes. Beekeepers use formic acid as a miticide against the tracheal mite (Acarapis woodi) and the Varroa mite.<ref>http://www.biobees.com/library/pesticides_GM_threats/miticides_varroa_acarapis.pdf</ref> The use of formic acid in fuel cells is also under investigation.<ref>Template:Cite journal</ref>
Formic acid is a source for a formyl group for example in the formylation of methylaniline to N-methylformanilide in toluene.<ref>Template:OrgSynth</ref> In synthetic organic chemistry, formic acid is often used as a source of hydride ion. The Eschweiler-Clarke reaction and the Leuckart-Wallach reaction are examples of this application. It, or more commonly its azeotrope with triethylamine, is also used as a source of hydrogen in transfer hydrogenation.
As mentioned below, formic acid may serve as a convenient source of carbon monoxide by being readily decomposed by sulfuric acid.
Formic acid shares most of the chemical properties of other carboxylic acids. Reflecting its high acidity, its solutions in alcohols form esters spontaneously. Formic acid shares some of the reducing properties of aldehydes, reducing solutions of gold, silver, and platinum to the metals.
Heat and especially acids cause formic acid to decompose to carbon monoxide (CO) and water (dehydration). Treatment of formic acid with sulfuric acid is a convenient laboratory source of CO.<ref>Template:OrgSynth</ref><ref>Template:OrgSynth</ref>
In the presence of platinum, it decomposes with a release of hydrogen and carbon dioxide. Soluble ruthenium catalysts are also effective.<ref name="Fellay2008">Template:Cite journal</ref><ref>G. Laurenczy, C. Fellay, P. J. Dyson, Hydrogen production from formic acid. PCT Int. Appl. (2008), 36pp. CODEN: PIXXD2 WO 2008047312 A1 20080424 AN 2008:502691</ref> Carbon monoxide free hydrogen has been generated in a very wide pressure range (1-600 bar).<ref name="Fellay2008" /> Formic acid has even been considered as a material for hydrogen storage.<ref>Template:Cite journal</ref> The co-product of this decomposition, carbon dioxide, can be rehydrogenated back to formic acid in a second step. Formic acid contains 53 g L−1 hydrogen at room temperature and atmospheric pressure, which is twice as much as compressed hydrogen gas can attain at 350 bar pressure. Pure formic acid is a liquid with a flash point of + 69 °C, much higher than that of gasoline (– 40 °C) or ethanol (+ 13 °C).
Addition to alkenes
Formic acid is unique among the carboxylic acids in its ability to participate in addition reactions with alkenes. Formic acids and alkenes readily react to form formate esters. In the presence of certain acids, including sulfuric and hydrofluoric acids, however, a variant of the Koch reaction occurs instead, and formic acid adds to the alkene to produce a larger carboxylic acid.
Formic acid anhydride
An unstable formic anhydride, H(C=O)-O-(C=O)H, can be obtained by dehydration of formic acid with N,N'-Dicyclohexylcarbodiimide in ether at low temperature.<ref name=gwu>G. Wu, S. Shlykov, F. S. Van Alseny, H. J. Geise, E. Sluyts, B. J. Van der Veken (1995), Formic Anhydride in the Gas Phase, Studied by Electron Diffraction and Microwave and Infrared Spectroscopy, Supplemented with Ab-Initio Calculations of Geometries and Force Fields. J. Phys. Chem., volume 99, issue 21, pages 8589–8598 Template:Doi</ref>
Some alchemists and naturalists were aware that ant hills give off an acidic vapor as early as the 15th century. The first person to describe the isolation of this substance (by the distillation of large numbers of ants) was the English naturalist John Ray, in 1671.<ref>Template:Cite journal</ref><ref>Template:Cite book</ref> Ants secrete the formic acid for attack and defense purposes. Formic acid was first synthesized from hydrocyanic acid by the French chemist Joseph Gay-Lussac. In 1855, another French chemist, Marcellin Berthelot, developed a synthesis from carbon monoxide that is similar to that used today.
Formic acid was long considered a chemical compound of only minor interest in the chemical industry. In the late 1960s, however, significant quantities of it became available as a byproduct of acetic acid production. It now finds increasing use as a preservative and antibacterial in livestock feed.
Formic acid has low toxicity (hence its use as a food additive), with an LD50 of 1.8 g/kg (oral, mice). The concentrated acid is, however, corrosive to the skin.<ref name=Ullmann_2009/>
Formic acid is readily metabolized and eliminated by the body. Nonetheless, it has specific toxic effects; the formic acid and formaldehyde produced as metabolites of methanol are responsible for the optic nerve damage, causing blindness seen in methanol poisoning.<ref>Template:Cite web</ref> Some chronic effects of formic acid exposure have been documented. Some experiments on bacterial species have demonstrated it to be a mutagen.<ref name="osha.gov">Template:Cite web</ref> Chronic exposure to humans may cause kidney damage.<ref name="osha.gov"/> Another effect of chronic exposure is development of a skin allergy that manifests upon re-exposure to the chemical.
Concentrated formic acid slowly decomposes to carbon monoxide and water, leading to pressure buildup in the container it is kept in. For this reason, 98% formic acid is shipped in plastic bottles with self-venting caps.
The hazards of solutions of formic acid depend on the concentration. The following table lists the EU classification of formic acid solutions:
|Concentration (weight percent)||Classification||R-Phrases|
An assay for formic acid in body fluids, designed for determination of formate after methanol poisoning, is based on the reaction of formate with bacterial formate dehydrogenase.<ref>Template:Cite journal</ref>
Formic acid in 85% concentration is not flammable, and diluted formic acid is on the US Food and Drug Administration list of food additives.<ref>US Code of Federal Regulations: 21 CFR 186.1316, 21 CFR 172.515</ref> The principal danger from formic acid is from skin or eye contact with the concentrated liquid or vapors. The US OSHA Permissible Exposure Level (PEL) of formic acid vapor in the work environment is 5 parts per million parts of air (ppm).
- Carbon monoxide as reagent in the formylation of aromatic compounds.
- International Chemical Safety Card 0485.
- NIOSH Pocket Guide to Chemical Hazards.
- ChemSub Online (Formic acid).
- Ants shooting formic acid (video).
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