Formic Acid

Formic acid, formally known by its preferred IUPAC name methanoic acid, is the simplest and most fundamental member of the carboxylic acid family. With the molecular formula HCOOH, it plays a significant role both in nature and in various industrial chemical processes. Recognised for its pungent, penetrating odour, the acid occurs widely across biological systems and has long been an important intermediate in synthetic chemistry. Its salts, esters and the corresponding anion are collectively termed formates, and commercial production typically proceeds from methanol.

Natural Occurrence

Formic acid is widely distributed in nature and appears in notable quantities in many insects, plants and environmental emissions. It is most famously associated with ants, particularly species in the Formica rufa group, which spray the compound as a defensive and predatory mechanism. Stingless bees of the genus Oxytrigona also possess the ability to secrete formic acid, while the caterpillar Cerura vinula can release it in response to threats. In plants, the acid occurs in the stinging trichomes of Urtica dioica (stinging nettle), contributing to the painful reaction encountered upon contact.
Trace concentrations are also found across a variety of fruits and vegetables. Approximate content includes pineapple (0.21 mg per 100 g), apple (2 mg per 100 g) and kiwi (1 mg per 100 g). Vegetables show similar variability: onion contains about 45 mg per 100 g, aubergine around 134 mg per 100 g, and cucumber extremely low levels at 0.11 mg per 100 g. Beyond biological reservoirs, formic acid is a naturally occurring atmospheric constituent, arising chiefly from emissions in forested regions.

Historical Development

Awareness of acidic vapours from anthills dates back to mediaeval natural history, but the first documented isolation of formic acid was achieved by the English naturalist John Ray in 1671 through the distillation of crushed ants. Over subsequent centuries, laboratory synthesis gradually replaced natural extraction. Joseph Gay-Lussac first prepared formic acid from hydrocyanic acid, while Marcellin Berthelot advanced a method in 1855 involving the reaction of carbon monoxide, a principle underlying modern synthetic routes.
Though initially regarded as a compound of limited industrial value, production expanded considerably during the late twentieth century when formic acid became available in large volumes as a by-product of acetic acid manufacture. It is now applied in various sectors, including as an antibacterial preservative in livestock feed.

Physical and Chemical Properties

Formic acid appears as a colourless liquid at room temperature with an odour reminiscent of acetic acid, though considerably sharper. It is approximately ten times stronger than acetic acid, as reflected in their respective dissociation constants (pKa 3.75 for formic acid compared with 4.76 for acetic acid). The acid is fully miscible with water and most polar organic solvents, and only partially soluble in hydrocarbons.
In the vapour phase and in non-polar liquids, formic acid exists predominantly as hydrogen-bonded dimers rather than isolated molecules. This strong association causes gaseous formic acid to deviate from ideal gas behaviour. Solid formic acid exhibits two known polymorphs, each characterised by extended hydrogen-bonded networks. A notable physical property is its formation of a high-boiling azeotrope with water at 107.3°C containing approximately 77.5% formic acid. Liquid formic acid also has a marked tendency to supercool.

Chemical Reactivity

Formic acid participates in a broad range of chemical reactions typical of carboxylic acids but also displays distinctive behaviour due to its structural simplicity and comparatively high reactivity.
Decomposition ReactionsIn the presence of concentrated sulphuric acid, formic acid dehydrates readily to yield carbon monoxide and water. This reaction provides a convenient laboratory method for producing carbon monoxide. When contacted with platinum catalysts, formic acid decomposes to produce hydrogen and carbon dioxide, and similar transformations using soluble ruthenium catalysts generate hydrogen with minimal carbon monoxide contamination.
General Reactivity and Functional TransformationsAs a carboxylic acid, formic acid readily forms esters through Fischer esterification and is sufficiently acidic to catalyse its own ester-forming reactions in alcoholic media. It also displays mild reducing behaviour similar to aldehydes; for example, it can reduce metal oxides to elemental metals.
Formic acid serves as a formylating agent, contributing formyl groups in organic synthesis, as seen in the preparation of N-methylformanilide from N-methylaniline. In hydride-based reactions, the acid is a key reagent in processes such as the Eschweiler–Clarke reaction and in transfer hydrogenation, including applications in the Leuckart reaction for amine synthesis. Aqueous formic acid or its azeotrope with triethylamine is also employed in the hydrogenation of ketones.
Addition Reactions with AlkenesA distinctive feature of formic acid is its ability to add across carbon–carbon double bonds, forming formate esters. Under strongly acidic conditions, including those involving sulphuric or hydrofluoric acid, a variation of the Koch reaction may occur, producing carboxylic acids of higher carbon number.
Formic Acid AnhydrideA highly unstable anhydride, HCOOCOH, can be produced through low-temperature dehydration of formic acid using N,N-dicyclohexylcarbodiimide in an organic solvent such as ether.

Industrial Production

Global production capacity for formic acid is dominated by facilities in Europe and Asia, with major contributions from Germany and China. Commercial solutions are typically sold at concentrations ranging from 85% to 99% by mass. Important producers include BASF, Eastman Chemical Company, LC Industrial and Feicheng Acid Chemicals. Manufacturing plants are located in regions such as Ludwigshafen (Germany), Oulu (Finland), Nakhon Pathom (Thailand) and Feicheng (China). Prices have shown considerable geographic variation, historically ranging from approximately £800 per tonne in Western Europe to above £1,200 per tonne in the United States.

Role in Carbon Dioxide Conversion and Sustainable Technologies

Formic acid, alongside carbon monoxide, constitutes an important C1 molecule for emerging low-carbon processes. The acid acts as a hydrogen-rich liquid capable of donating hydrogen atoms in numerous condensation and esterification reactions, while carbon monoxide remains a critical component of synthesis gas used for producing a variety of chemicals.
Electrochemical reduction of carbon dioxide provides a promising route for generating formic acid and related molecules using renewable electricity. This approach offers two-fold environmental benefits: direct reduction of atmospheric CO₂ and displacement of emissions that would otherwise arise in conventional fossil-fuel-based manufacturing pathways. Current research emphasises efficiency, scalability and commercial viability, aiming to integrate CO₂ electrolysis technologies into sustainable chemical production and carbon-management systems.

Synthetic Routes from Methyl Formate and Formamide

A major industrial process for formic acid production involves synthetic pathways beginning with methyl formate. Methanol reacts with carbon monoxide under elevated pressure (around 40 atm) and moderate temperature (about 80°C) in the presence of a strong base such as sodium methoxide, yielding methyl formate. Hydrolysis of methyl formate produces formic acid and regenerates methanol, although efficient conversion requires a substantial excess of water.
An alternative multi-step route treats methyl formate with ammonia to produce formamide, which is subsequently hydrolysed using sulphuric acid. While effective, this method generates ammonium sulphate as a by-product, requiring suitable disposal or further processing. As a result, energy-efficient methods for separating formic acid from aqueous mixtures have been developed. One such technique, employed by BASF, utilises liquid–liquid extraction with an organic base to separate high-purity formic acid from dilute solutions.