The formic acid or methanoic acid is the simplest and smallest of all organic acids compound. It is also known as methanoic acid and its molecular formula is HCOOH, having just one hydrogen atom bonded to the carbon atom. Its name derives from the word Formica , which is Latin for ant.
Naturalists of the 15th century found that certain types of insects (the formicids), such as ants, termites, bees and beetles, secrete this compound responsible for their painful stings. Likewise, these insects use formic acid as an attack, defense and chemical signaling mechanism.
They have poisonous glands that excrete this and other acids (for example, acetic acid) as a spray to the outside. Formic acid is stronger than acetic acid (CH 3 COOH); therefore, dissolved in equal amounts in water, formic acid produces solutions with lower pH values.
The English naturalist John Ray succeeded in isolating formic acid in 1671, distilling it from large numbers of ants.
On the other hand, the first successful synthesis of this compound was carried out by the French chemist and physicist Joseph Gay-Lussac, using hydrocyanic acid (HCN) as a reagent.
Where is formic acid found?
Formic acid can be present at terrestrial levels, as a component of biomass or in the atmosphere , involved in a wide spectrum of chemical reactions; It can even be found under the ground, inside the oil or in the gaseous phase on its surface.
In terms of biomass, insects and plants are the main generators of this acid. When fossil fuels are burned they produce gaseous formic acid; consequently, vehicle engines release formic acid into the atmosphere.
However, the Earth is home to an exorbitant number of ants, and among all of these they are capable of producing in a year thousands of times the amount of formic acid generated by human industry. Likewise, forest fires represent gaseous sources of formic acid.
Higher up in the complex atmospheric matrix, photochemical processes that synthesize formic acid occur.
At this point, many volatile organic compounds (VOCs) are degraded under the effects of ultraviolet radiation, or are oxidized by OH free radical mechanisms. The rich and complex atmospheric chemistry is by far the predominant source of formic acid on the planet.
Structure of formic acid
The upper image illustrates the structure of a formic acid gas phase dimer. The white spheres correspond to hydrogen atoms, the red spheres to oxygen atoms, and the black spheres to carbon atoms.
Two groups can be seen in these molecules: hydroxyl (–OH) and formyl (–CH = O), both capable of forming hydrogen bonds.
These interactions are of the O – H — O type, the hydroxyl groups being the donors of the H and the formyl groups the donors of the O.
However, H attached to the carbon atom lacks this ability. These interactions are very strong and, due to the electron-poor H atom, the hydrogen in the OH group is more acidic; therefore, this hydrogen further stabilizes the bridges.
As a result of the above, formic acid exists as a dimer and not as an individual molecule.
As the temperature drops, the dimer orients its hydrogen bonds to generate the most stable structure possible together with the other dimers, thus creating infinite α and β chains of formic acid.
Another nomenclature is the “cis” and “trans” conformers. In this case, “cis” is used to designate groups oriented in the same direction, and “trans” for those groups in opposite directions.
For example, in the α chain the formyl groups “point” to the same side (the left), in contrast to the β chain, where these formyl groups point to opposite sides (upper image).
This crystalline structure depends on the physical variables that act on it, such as pressure and temperature. Thus, the chains are convertible; that is, under different conditions a “cis” chain can be transformed into a “trans” chain, and vice versa.
If the pressures increase to drastic levels, the chains are compressed enough to be considered a crystalline polymer of formic acid.
Properties of formic acid
The following are the physical and chemical properties of formic acid:
- Formic acid is a colorless liquid at room temperature with a strong, pungent odor. It has a molecular weight of 46g / mol, melts at 8.4ºC and has a boiling point of 100.8ºC, higher than that of water.
- It is miscible in water and in polar organic solvents, such as ether, acetone, methanol, and ethanol.
- On the other hand, in aromatic solvents (such as benzene and toluene) it is slightly soluble, because formic acid barely has one carbon atom in its structure.
- It has a pKa of 3.77, more acidic than acetic acid, which can be explained because the methyl group provides electronic density to the carbon atom oxidized by the two oxygens. This results in a slight decrease in the acidity of the proton (CH 3 COOH, HCOOH).
- Deprotonated the acid, it becomes the HCOO – format anion , which can delocalize the negative charge between the two oxygen atoms. Consequently, it is a stable anion and explains the high acidity of formic acid.
Formic acid can dehydrate to carbon monoxide (CO) and water. In the presence of platinum catalysts, it can also decompose into molecular hydrogen and carbon dioxide:
HCOOH (l) → H 2 (g) + CO 2 (g)
This property allows formic acid to be considered a safe way to store hydrogen.
Uses / applications of formic acid
The food and agriculture industry
Despite how harmful formic acid can be, it is used in adequate concentrations as a preservative in food due to its antibacterial action. For the same reason it is used in agriculture, where it also has a pesticidal action.
It also has a preservative action on pastures, which helps prevent intestinal gas in breeding animals.
The textile and footwear industry
It is used in the textile industry in the dyeing and refining of textiles, being perhaps the most frequent use of this acid.
Formic acid is used in leather processing due to its degreasing action and in the hair removal of this material.
Road safety on the roads
In addition to the indicated industrial uses, formic acid derivatives (formats) are used in Switzerland and Austria on roads during winter, in order to reduce the risk of accidents. This treatment is more efficient than the use of common salt.