Formic 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 ant
venom. 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 anions derived from formic
acid are referred to as formates.
Properties
Formic acid is a colorless liquid having a highly pungent, penetrating odor 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. Owing to its tendency to hydrogen-bond, gaseous formic acid does
not obey the ideal gas law. Solid formic acid consists of an effectively endless network
of hydrogen-bonded formic acid molecules. This relatively complicated compound also
forms a low-boiling azeotrope with water and liquid formic acid also tends to supercool.
Natural occurrence
In nature, it is found in the venom of ants. Formic acid is a naturally occurring component
of the atmosphere due primarily to forest emissions.
Production In 2009, the worldwide capacity for producing
formic acid was 720,000 tonnes/annum, roughly equally divided between Europe and Asia while
production was below 1000 tonnes/annum in all other continents. It is commercially available
in solutions of various concentrations between 85 and 99 w/w %. As of 2009, the largest
producers are BASF, Kemira, LC Industrial and Feicheng Acid Chemicals, with the largest
production facilities in Ludwigshafen, Oulu, Nakhon Pathom and Feicheng. 2010 prices ranged
from around €650/tonne in Western Europe to $1250/tonne in the United States.
From methyl formate and formamide When methanol and carbon monoxide are combined
in the presence of a strong base, the acid derivative methyl formate results, according
to the chemical equation: 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 + 2H2O + H2SO4 → 2HCO2H +2SO4
A disadvantage of this approach is the need to dispose of the ammonium sulfate byproduct.
This problem has led some manufacturers to develop energy-efficient methods of separating
formic acid from the large excess amount of water used in direct hydrolysis. In one of
these processes the formic acid is removed from the water via liquid-liquid extraction
with an organic base. Niche chemical routes
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, by 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 to formic
acid has long been studied. This reaction can be conducted homogeneously.
Oxidation of biomass Formic acid can also be obtained by aqueous
catalytic partial oxidation of wet biomass. A Keggin-type polyoxometalate is used as the
homogeneous catalyst to convert sugars, wood, waste paper or cyanobacteria to formic acid
and CO2 as the sole byproduct. Yields of up to 53% formic acid can be achieved.
Laboratory methods In the laboratory, formic acid can be obtained
by heating oxalic acid in glycerol and extraction by steam distillation. 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
Biosynthesis Formic acid occurs widely in nature as its
conjugate base formate. This anion is produced by reduction of carbon dioxide, catalyzed
by the enzyme formate dehydrogenase. 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. Uses
A major use of formic acid is as a preservative and antibacterial agent in livestock feed.
In Europe, it is applied on silage 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. 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. In the poultry industry, it is sometimes added to feed to
kill E. coli bacteria. Use as preservative for silage and animal feed constituted 30%
of the global consumption in 2009. Formic acid is also significantly used in
the production of leather, including tanning, and in dyeing and finishing of textile because
of its acidic nature. Use as a coagulant in the production of rubber constituted in 2009
6% of the global consumption. Formic acid is also used in place of mineral
acids for various cleaning products, 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 and the Varroa mite. The use of formic acid in fuel
cells is also under investigation. Laboratory use
Formic acid is a source for a formyl group for example in the formylation of methylaniline
to N-methylformanilide in toluene. 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.
Like acetic acid and trifluoroacetic acid, formic acid is commonly used as a volatile
pH modifier in HPLC and capillary electrophoresis. As mentioned below, formic acid may serve
as a convenient source of carbon monoxide by being readily decomposed by sulfuric acid.
Medical use Formic acid has been shown to be an effective
treatment against warts and is marketed for that purpose by Meda AB under the trade name
Vårtfri in Sweden and Endwarts internationally. Reactions
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. Decomposition
Heat and especially acids cause formic acid to decompose to carbon monoxide and water.
Treatment of formic acid with sulfuric acid is a convenient laboratory source of CO.
In the presence of platinum, it decomposes with a release of hydrogen and carbon dioxide.
Soluble ruthenium catalysts are also effective. Carbon monoxide free hydrogen has been generated
in a very wide pressure range. Formic acid has even been considered as a material for
hydrogen storage. 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 three and a half times 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 or ethanol. 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.
History 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 was the English naturalist John Ray, in 1671.
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.
Safety Formic acid has low toxicity, with an LD50
of 1.8 g/kg. The concentrated acid is, however, corrosive to the skin.
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. Some chronic effects
of formic acid exposure have been documented. Some experiments on bacterial species have
demonstrated it to be a mutagen. Chronic exposure to humans may cause kidney damage. Another
possible 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:
Formic acid in 85% concentration is not flammable, and diluted formic acid is on the U.S. Food
and Drug Administration list of food additives. The principal danger from formic acid is from
skin or eye contact with the concentrated liquid or vapors. The U.S. OSHA Permissible
Exposure Level of formic acid vapor in the work environment is 5 parts per million parts
of air. See also
Orthoformic acid References
External links 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 Use in Beekeeping: Handbook and Manual of Treatments.