Formic acid

Template:Chembox new 'Formic acid' (systematically called methanoic acid) is the simplest carboxylic acid. Its formula is HCOOH or CH₂O₂. It is an important intermediate in chemical synthesis and occurs naturally, most notably in the venom of bee and ant stings.

In nature, it is found in the stings and bites of many insects of the order Hymenoptera, mainly ants. It is also a significant combustion product resulting from alternative fueled vehicles burning methanol (and ethanol, if contaminated with water) when mixed with gasoline. Its name comes from the Latin word for ant, formica, referring to its early isolation by the distillation of ant bodies. A chemical compound such as a salt from the neutralization of formic acid with a base, or an ester derived from formic acid, is referred to as formate (or methanoate). The formate ion has the formula HCOO⁻.

Properties

Formic acid is miscible with water and most polar organic solvents, and somewhat soluble in hydrocarbons. In hydrocarbons and in the vapor phase, it actually consists of hydrogen-bonded dimers rather than individual molecules. In the gas phase, this hydrogen-bonding results in severe deviations from the ideal gas law. Liquid and solid formic acid consists of an effectively endless network of hydrogen-bonded formic acid molecules.

Reactions

Formic acid shares most of the chemical properties of other carboxylic acids, although under normal conditions it will not form either an acyl chloride or an acid anhydride. Until very recently, all attempts to form either of these derivatives have resulted in carbon monoxide instead. In 1979, George Olah published a communication describe four distinct routes to this compound. One involved the reaction of formyl fluoride with sodium formate; the other three methods involved the use of dehydrating agents. However, the formic anhydride was only stable in ethereal solution, decomposing on attempts at distillation.

Heat can also cause formic acid to decompose to carbon monoxide and water. Formic acid shares some of the reducing properties of aldehydes.

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 takes place instead, and formic acid adds to the alkene to produce a larger carboxylic acid.

Most simple formate salts are water-soluble. It is also a bi-functional compound.

Production

A significant amount of formic acid is produced as a byproduct in the manufacture of other chemicals, especially acetic acid. This production is insufficient to meet the present demand for formic acid, and some formic acid must be produced for its own sake.

When methanol and carbon monoxide are combined in the presence of a strong base, the formic acid derivative methyl formate results, according to the chemical equation

CH₃OH + CO → HCOOCH₃

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:

HCOOCH₃ + H₂O → HCOOH + CH₃OH

Direct hydrolysis of methyl formate requires a large excess of water to proceed efficiently, and some producers perform it by an indirect route by first reacting the methyl formate with ammonia to produce formamide, and then hydrolyzing the formamide with sulfuric acid to produce formic acid:

HCOOCH₃ + NH₃ → HCONH₂ + CH₃OH

HCONH₂ + H₂O + ½H₂SO₄ → HCOOH + ½(NH₄)₂SO₄

This technique has problems of its own, particularly disposing of the ammonium sulfate byproduct, so some manufacturers have recently developed energy efficient means of 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.

In the laboratory formic acid can be obtained by heating oxalic acid in anhydrous glycerol and extraction by steam distillation. Another preparation (which must be performed under a fume hood) is the acid hydrolysis of ethyl isonitrile using HCl solution.

C₂H₅NC + 2H₂O → C₂H₅NH₂ + HCOOH

The isonitrile being obtained by reacting ethyl amine with chloroform (note that the fume hood is required because of the overpoweringly objectionable odor of the isonitrile).

Uses

The principal use of formic acid is as a preservative and antibacterial agent in livestock feed. When sprayed on fresh hay or other silage, it 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 salmonella bacteria. Other uses:

• It is used to process organic latex (sap) into raw rubber.
• Beekeepers use formic acid as a miticide against the Tracheal (Acarapis woodi) mite and the Varroa mite.
• It is of minor importance in the textile industry and for the tanning of leather.
• Some formate esters are artificial flavorings or perfumes.
• It is the active ingredient in some brands of household limescale remover.
• It is used in laboratories as a solvent modifier for HPLC separations of proteins and peptides, especially when the sample is being prepared for mass spectrometry analysis.
• It is used by clinical pathology laboratories to disinfect prion activity in brain samples

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 is also used as a source of hydrogen in transfer hydrogenation.

In the laboratory formic acid is also used as source for carbon monoxide, which is set free by the addition of sulfuric acid. Formic acid is also a source for a formyl group for example in the formylation of methylaniline to N-methylformanilide in toluene.

Fuel cells that use modified formic acid are promising.

History

As early as the 15th century, some alchemists and naturalists were aware that ant hills gave off an acidic vapor. 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. 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.

In the chemical industry, formic acid was long considered a chemical compound of only minor industrial interest. 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

The principal danger from formic acid is from skin or eye contact with liquid formic acid or with the concentrated vapors. Any of these exposure routes can cause severe chemical burns, and eye exposure can result in permanent eye damage. Inhaled vapors may similarly cause irritation or burns in the respiratory tract. Since carbon monoxide may also be present in formic acid vapors, care should be taken wherever large quantities of formic acid fumes are present. The US OSHA Permissible Exposure Level (PEL) of formic acid vapor in the work environment is 5 parts per million parts of air (ppm).

Formic acid is readily metabolized and eliminated by 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 animal experiments have demonstrated it to be a mutagen, and chronic exposure may cause liver or kidney damage. Another possibility with chronic exposure is development of a skin allergy that manifests upon re-exposure to the chemical.

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
2%–10%	Irritant (Xi)	Template:R36/38
10%–90%	Corrosive (C)	Template:R34
>90%	Corrosive (C)	Template:R35

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.

External links

• Carbon monoxide as reagent in the formylation of aromatic compounds.
• European Chemicals Bureau.
• International Chemical Safety Card 0485.
• NIOSH Pocket Guide to Chemical Hazards.
• ChemSub Online (Formic acid).