A Medley of Potpourri: Formic acid

Formic acid, systematically named methanoic acid, is the simplest carboxylic acid. The chemical formula is CH2O2. The chemical composition is H COOH. It is an important intermediate in chemical synthesis and occurs naturally, most notably in some ants. The word "formic" 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 called formates. Industrially, formic acid is produced from methanol.

Properties

Cyclic dimer of formic acid; dashed green lines represent hydrogen bonds

Formic acid is a colorless liquid having a pungent, penetrating odor at room temperature, not unlike the related acetic acid. 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, which can exist in either of two polymorphs, consists of an effectively endless network of hydrogen-bonded formic acid molecules. Formic acid also forms a low-boiling azeotrope with water (22.4%) and liquid formic acid also tends to supercool.

Natural occurrence

In nature, formic acid is found in most ants and in stingless bees of the genus Oxytrigona. The wood ants from the genus Formica can spray formic acid on their prey or to defend the nest. It is also found in the trichomes of stinging nettle (Urtica dioica). Formic acid is a naturally occurring component of the atmosphere primarily due to forest emissions.

Production

In 2009, the worldwide capacity for producing formic acid was 720,000 tonnes per year, roughly equally divided between Europe (350,000, mainly in Germany) and Asia (370,000, mainly in China) while production was below 1000 tonnes per year 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, Eastman Chemical Company, LC Industrial, and Feicheng Acid Chemicals, with the largest production facilities in Ludwigshafen (200,000 tonnes per year, BASF, Germany), Oulu (105,000, Eastman, Finland), Nakhon Pathom (n/a, LC Industrial), and Feicheng (100,000, Feicheng, China). 2010 prices ranged from around €650/tonne (equivalent to around $800/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 result is methyl formate, 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 + (NH₄)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 excess water used in direct hydrolysis. In one of these processes, used by BASF, the formic acid is removed from the water by 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 by the OxFA process. A Keggin-type polyoxometalate (H₅PV₂Mo₁₀O₄₀) 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 intermediate. If the reaction mixture is heated to higher temperatures, allyl alcohol results. The net reaction is thus:

C₂O₄H₂ → CO₂H₂ + CO₂

Another illustrative method involves the reaction between lead formate and hydrogen sulfide, driven by the formation of lead sulfide.

Pb(HCOO)₂ + H₂S → 2HCOOH + PbS

Biosynthesis

Formic acid is named after ants which have high concentrations of the compound in their venom. In ants formic acid is derived from serine through a 5,10-Methenyltetrahydrofolate intermediate. The conjugate base of formic acid, formate, also occurs widely in nature. 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, 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. 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 (other) animal feed constituted 30% of the global consumption in 2009.

Formic acid is also significantly used in the production of leather, including tanning (23% of the global consumption in 2009), and in dyeing and finishing textiles (9% of the global consumption in 2009) because of its acidic nature. Use as a coagulant in the production of rubber consumed 6% of the global production in 2009.

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 and perfumes.

Beekeepers use formic acid as a miticide against the tracheal mite (Acarapis woodi) and the Varroa destructor mite and Varroa jacobsoni mite.

Formic acid application has been reported to be an effective treatment for warts.

Formic acid can be used as a fuel (it can be used directly in formic acid fuel cells and indirectly in hydrogen fuel cells).

It is possible to use formic acid as an intermediary to produce isobutanol from CO2 using microbes

Formic acid is often used as a component of mobile phase in reversed-phase high-performance liquid chromatography (RP-HPLC) analysis and separation techniques for the separation of hydrophobic macromolecules, such as peptides, proteins and more complex structures including intact viruses. Especially when paired with mass spectrometry detection, formic acid offers several advantages over the more traditionally used phosphoric acid.

Chemical reactions

Formic acid is about ten times stronger than acetic acid. It is used as a volatile pH modifier in HPLC and capillary electrophoresis.

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.

As mentioned below, formic acid readily decomposes with concentrated sulfuric acid to form carbon monoxide.

CH₂O₂ + H₂SO₄ → H₂SO₄ + H₂O + CO

Reactions

Formic acid shares most of the chemical properties of other carboxylic acids. Because of its high acidity, 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 (CO) and water (dehydration). 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.

CH₂O₂ → H₂ + CO₂

Soluble ruthenium catalysts are also effective. Carbon monoxide free hydrogen has been generated in a very wide pressure range (1–600 bar). Formic acid has been considered as a means of 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 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 (14.7 g/L). 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.

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 (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 similar to the process 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 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 (hence its use as a food additive), with an LD₅₀ of 1.8 g/kg (tested orally on mice). The concentrated acid is 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 in 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 containing vessel. 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
2%–10%	Irritant (Xi)	R36/38
10%–90%	Corrosive (C)	R34
>90%	Corrosive (C)	R35

Formic acid in 85% concentration is 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 (PEL) of formic acid vapor in the work environment is 5 parts per million parts of air (ppm).

A Medley of Potpourri

Search This Blog

Monday, July 1, 2019

Formic acid