Formic Acid

Description

Formic acid is a colourless, fuming liquid that is miscible with water. In the vapor phase, it consists of hydrogen bonded dimers  rather than individual molecules. In the gas phase, significant deviations from the ideal gas law arise as a result of this hydrogen bonding. In its liquid and solid state, formic acid can be thought of as an effectively infinite network of hydrogen bonded molecules.

Being the first in the carboxylic acid series, formic acid shares most of the same chemical properties, and so it will react with alkalis to form water soluble formate salts. But formic acid is unique among the carboxylic acids in that it reacts with alkenes to form formate esters. In the presence of sulfuric and hydrofluoric acids, a variant of the Gatterman-Koch reaction takes place instead, and formic acid adds to the alkene to produce a larger carboxylic acid.

Formic or methanoic acid (CAS: 64-18-6) is a colorless organic acid with the formula HCOOH. At ambient temperatures, it has a strong, penetrating odor, like acetic acid. Used in chemical synthesis as an intermediate, this simple carboxylic acid is miscible in water and most organic solvents and is somewhat hydrocarbon-soluble.

Formic acid occurs naturally in the venom of some ants and bees. Formate, the conjugate base of formic acid, also occurs naturally in bodily fluids following methanol poisoning.

Formic acid is a source of hydride ion in synthetic organic chemistry, as in the Eschweiler-Clarke and Leuckart-Wallach reactions. It is also a useful component of the mobile phase in reversed-phase high-performance liquid chromatography (RP-HPLC) for peptides, proteins, and intact viruses.

Although formic acid is easily metabolized and eliminated, it can have toxic effects. Methanol metabolism produces formic acid and formaldehyde, which are responsible for the optic nerve damage that results from methanol poisoning. Chronic exposure in humans may cause kidney damage and skin allergies in some people.

In diluted forms, formic acid and formate esters are used as artificial flavorings and perfume additives. In higher concentrations, formic acid is flammable, and skin and eye contact with concentrated liquid or vapors is dangerous.

In agriculture, formic acid is used as a preservative and antibacterial agent and by beekeepers to kill mites. It is useful in leather tanning, textile dyeing and finishing, and in rubber production. Formic acid replaces mineral acids in limescale removers and other cleaning products.

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 infinite network of hydrogen-bonded formic acid molecules.

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. It has now been shown that the anhydride may be produced by reaction of formyl fluoride with sodium formate at −78°C, and the chloride by passing HCl into a solution of 1-formimidazole in monochloromethane at −60°C. 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.

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 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.

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 metylaniline to N-methylformanilide in toluene.

Fuel cells that use modified formic acid are promising.