Formic acid
Formic acid, systematically named methanoic acid, is the simplest carboxylic acid. It has the chemical formula HCOOH and structure. This acid is an important intermediate in chemical synthesis and occurs naturally, most notably in some ants. Esters, salts, and the anion derived from formic acid are called formates. Industrially, formic acid is produced from methanol.
Natural occurrence
Formic acid, which has a pungent, penetrating odor, is found naturally in insects, weeds, fruits and vegetables, and forest emissions. It appears in most ants and in stingless bees of the genus Oxytrigona. Wood ants from the genus Formica can spray formic acid on their prey or to defend the nest. The puss moth caterpillar sprays it as well when threatened by predators. It is also found in the trichomes of stinging nettle. Apart from that, this acid is incorporated in many fruits such as pineapple, apple and kiwi, and in many vegetables, namely onion, eggplant, and in extremely low concentrations, cucumber. Formic acid is a naturally occurring component of the atmosphere primarily due to forest emissions.History
As early as the 15th century, some alchemists and naturalists were aware that ant hills give off an acidic vapor. The first person to describe the isolation of this substance was 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 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, significant quantities became available as a byproduct of acetic acid production. It now finds increasing use as a preservative and antibacterial in livestock feed.
Properties
Formic acid is a colorless liquid having a pungent, penetrating odor at room temperature, comparable to the related acetic acid. Formic acid is about 10 times stronger of an acid than acetic acid; its dissociation constant is 3.745, compared to the pKa of 4.756 for 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 forms a high-boiling azeotrope with water. Liquid formic acid tends to supercool.
Chemical reactions
Decomposition
Formic acid readily decomposes by dehydration in the presence of concentrated sulfuric acid to form 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 for producing carbon monoxide-free hydrogen.
Reactant
Formic acid shares most of the chemical properties of other carboxylic acids. Because of its high acidity, solutions in alcohols form esters spontaneously; in Fischer esterifications of formic acid, it self-catalyzes the reaction and no additional acid catalyst is needed. Formic acid shares some of the reducing properties of aldehydes, reducing solutions of metal oxides to their respective metal.Formic acid is a source for a formyl group for example in the formylation of N-methylaniline to N-methylformanilide in toluene.
In synthetic organic chemistry, formic acid is often used as a source of hydride ion, as in the Eschweiler–Clarke reaction:
Image:Eschweiler-Clarke Reaction.svg|center|300px|The Eschweiler–Clark reaction
It is used as a source of hydrogen in transfer hydrogenation, as in the Leuckart reaction to make amines and for hydrogenation of ketones.
Addition to alkenes
Formic acid is unique 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−O−H, can be obtained by dehydration of formic acid with N,''''-dicyclohexylcarbodiimide in ether at low temperature.Production
In 2009, the worldwide capacity for producing formic acid was per year, roughly equally divided between Europe and Asia while production was below per year in all other continents. It is commercially available in solutions of various concentrations between 85 and 99 w/w %., the largest producers are BASF, Eastman Chemical Company, 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.Regenerating CO2 to make useful products that displace incumbent fossil fuel-based pathways is a more impactful process than CO2 sequestration.
Both formic acid and CO are one-carbon molecules. It is a hydrogen-rich liquid that can be transported and easily donates its hydrogen to enable a variety of condensation and esterification reactions to make a wide variety of derivative molecules. CO, while more difficult to transport as a gas, is also one of the primary constituents of syngas useful in synthesizing a wide variety of molecules.
CO2 electrolysis is distinct from photosynthesis and offers a promising alternative to accelerate decarbonization. By converting CO2 into products using clean electricity, CO2 emissions are reduced in two ways - first and most simply by the amount of CO2 that is regenerated, but the second way is less obvious but even more consequential by avoiding the CO2 emissions otherwise generated by making these same products from fossil fuels. This is known as carbon displacement or abatement.
CO2 electrolysis holds promise for reducing atmospheric CO2 levels and providing a sustainable method for producing chemicals, materials, and fuels. Its efficiency and scalability are active areas of research, but now also commercialization, aiming to make it a viable commercial technology for both carbon management and molecule production.
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: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:
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:
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 and obsolete 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 has declined in importance, so 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 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 followed 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:Another illustrative method involves the reaction between lead formate and hydrogen sulfide, driven by the formation of lead sulfide.
Electrochemical production
Formate is formed by the electrochemical reduction of CO2 at a lead cathode at pH 8.6:or
If the feed is and oxygen is evolved at the anode, the total reaction is: