How does the molecular shape affect solubility?



Formic acid (according to the IUPAC nomenclatureMethanoic acid called, narrow: formic acid, lat: acidum formicum) is a colorless, caustic and water-soluble liquid that is often used in nature by living things for defense purposes. It is one of the saturated carboxylic acids and, with the structured empirical formula HCOOH, is the simplest carboxylic acid. Formic acid was first isolated from red ants in 1671 by John Ray, from whom it was named. Another common name for methanoic acid, Formylic acid, goes from the Latin word for ant, formica, back.

Structure and nomenclature

Formic acid is the simplest carboxylic acid and the short-chain alkanoic acid; the carboxyl group (-COOH) has a particularly strong impact on its properties. The carbon atom has a formal oxidation state of +2. It can therefore act as a hydride transfer analogous to the carbonyl compounds, hence its reducing effect.

Due to the strongly polar carboxyl group, formic acid is miscible with water in all proportions.

Formic acid, a historically determined name that indicates its occurrence in ants, is the much more commonly used name. The salts of formic acid are called methanoates - here, too, the common name formate is used more often, although it does not correspond to the nomenclature. The formates have the structured empirical formula (HCOO)nMe, where n corresponds to the valence of the metal ion. Examples of formates are sodium formate (HCOONa) and aluminum formate [(HCOO)3Al)].

properties

Formic acid has a strong and pungent odor. The odor threshold is 1 ml / m³. Formic acid can be mixed in any ratio with water, ethanol and glycol. It is also soluble in most other polar organic substances, but only in small amounts in hydrocarbons.

Formic acid is a relatively unstable, colorless, clear and highly volatile liquid. At 8 ° C the formic acid solidifies to a colorless solid. It boils at 100.7 ° C. The melting and boiling points are significantly higher than those of organic compounds with similar molar masses (e.g. methanol), since hydrogen bonds between the individual molecules also have to be broken during melting and boiling. Some of these persist in the gaseous state, which is why formic acid deviates significantly from the behavior of an ideal gas. In the presence of oxygen, it burns to carbon dioxide and water. Formic acid is a powerful reducing agent because the deprotonated acid splits into hydrogen and carbon dioxide.

The formic acid has a density of 1.22 g · cm−3 at 20 ° C. The acid constant (pKa value) is 3.75. It is the strongest unsubstituted monocarboxylic acid. For comparison: hydrochloric acid has a pKa value of −7, sulfuric acid of −3. 12.7 kJ / mol are required to melt formic acid and 22.7 kJ / mol to evaporate. The triple point is 8.3 ° C and 0.0236 bar.

The formic acid vapor is detected with the help of Dräger test tubes. Otherwise, the formic acid is detected via its reducing effect, usually by the fact that it can reduce an ammoniacal silver nitrate solution to silver.

Thermodynamic properties

The standard enthalpy of formation ΔfH0liquid is −424.72 kJ mol−1, ΔfH0gas is −378.6 kJ mol−1.

The standard entropy S0liquid is 128.95 J mol−1· K−1, S.0gas 248.7 J mol−1· K−1.

The heat capacity of the liquid is 99.04 J · mol−1· K−1that of the gas with 45.7 J mol−1· K−1 specified.

Reactions


  • Formic acid breaks down into water and carbon monoxide; with the addition of strongly dehydrating substances (e.g. sulfuric acid).

  • In the presence of air, formic acid burns to carbon dioxide and water.

  • At higher temperatures and in the presence of a catalyst, it breaks down to carbon dioxide and hydrogen.

Formic acid reacts with metals to form metal formates and hydrogen:

Formic acid reacts with alcohols in the presence of a catalyst (usually sulfuric acid) to form water and alkyl formates (methyl esters).


  • Formic acid reacts with methanol to form water and methyl formate.

use

Formic acid was used as a preservative in fish, fruit and vegetable products under the E number E236, but this has been prohibited by law in Germany since 1998. Sodium and calcium formate are also used as preservatives (E237 and E238). In medicine it is used as an anti-rheumatic agent, in the textile and leather industry for pickling and impregnating. Sometimes it is also used as a disinfectant (also in acidic cleaning agents). It kills bacteria well. However, it can also react with other substances. In the chemical industry it is used for neutralization, in rubber production and in general in organic synthesis. In electronics production, formic acid is used as a reducing agent in the soldering process. Beekeepers use them to treat the bees against the Varroa mite, and in private households formic acid is often used to descale washing machines. In genetics, formic acid can be used in conjunction with the enzyme AP endonuclease to create insertion mutants at random, the so-called in vitro mutagenesis. Furthermore, formic acid can be found as a component of tobacco smoke. At airports, it is used to de-icing runways, as salt used for this purpose in other areas would corrode aircraft. In the plastics industry it is used to glue polyamide plastics. Concentrated formic acid is used to clean raw gemstones because it strongly attacks limestone and other impurities, exposing the gemstone without damaging it. This cleaning process should only be used with acid-resistant gemstones.

Historical information

In the early 15th century, some alchemists and naturalists observed that ants secrete an acidic liquid. The English naturalist John Ray was the first to isolate formic acid in 1671 by distilling large numbers of ants. The doctor Christoph Girtanner wrote the following text about the extraction of formic acid in 1795:

The formic acid is obtained from the ants (Formica rufa) by distillation. Ants are distilled over a gentle fire, and the formic acid is obtained in the template. It is about half the weight of the ants. Or you wash the ants in cold water, then lay them on a cloth and pour boiling water over them. If you squeeze the ants gently, the acid becomes stronger. To purify the acid, it is subjected to repeated distillation, and to concentrate it, it is allowed to freeze. Or even better: you collect ants, squeeze them out, without water, and distill the acid from them.

The French chemist Joseph Louis Gay-Lussac was the first to synthesize formic acid from hydrogen cyanide. In 1855 another French chemist, Marcellin Berthelot, invented the synthesis from carbon monoxide, which is still used today. For a long time, formic acid was of little technical importance. In the late 1960s, significant amounts of formic acid were produced as a by-product in the synthesis of acetic acid. Only later was formic acid used on a larger scale. It was no longer just obtained as a by-product, but specifically produced synthetically.

Extraction

The primeval isolation of formic acid from dead ants is of course no longer carried out these days. In the chemical industry, formic acid is usually produced using the process invented by Marcellin Berthelot in 1855. The synthesis is divided into two process steps in which toxic carbon monoxide is used:

Sodium hydroxide reacts with carbon monoxide at around 8 bar and 120 ° C to form sodium formate.
Sodium formate is reacted with sulfuric acid to form formic acid and sodium sulfate.

Formic acid is also produced from methanol with the help of carbon monoxide, among other things. Here, too, two process steps are carried out. Methyl formate is produced as an intermediate. In the end, methanol is recovered, which can be used again as a starting product for this synthesis:

At 80 ° C and 40 atm, methanol reacts with carbon monoxide to form methyl formate.
Formic acid methyl ester reacts with water to form formic acid and methanol.

Because the hydrolysis of methyl formate would consume a lot of water, some formic acid manufacturers use an indirect process with ammonia, which in turn requires two process steps. However, this indirect process has problems because the by-product ammonium sulfate is partially released:

Formic acid methyl ester reacts with ammonia to form amide and methanol.
Formamide reacts with sulfuric acid to form formic acid and ammonium sulfate.

Due to this problem, the manufacturer developed a new process of direct hydrolysis, in which the formic acid can be separated from the large amounts of water in an energy-efficient manner. One example of this is a process from BASF in which the formic acid can be separated out by liquid extraction with the aid of an organic base.

Furthermore, formic acid occurs as a by-product in the production of acetic acid from light gasoline or butane. Formic acid can also be produced with the help of hydrogen cyanide. There is a second process for production from methanol. Here, methanol is converted to formaldehyde and formic acid. However, these three processes are of little technical importance.

Occurrence

On earth, formic acid is only found in animals and plants, and in small amounts in the human body - but never in pure form. There are even traces of formic acid in space.

Formic acid is widespread in nature. It is used by many plant and animal species, especially by voices, as a component of poison cocktails for defense or attack purposes.

The caterpillars of the Great Forktail (Cerura vinula) - a species of butterfly - as well as some species of ants (members of the subfamily Formicinae) spray a liquid containing formic acid as a defense. While the fork-tailed caterpillars can only spray the formic acid a few centimeters, z. B. Wood ants in defense of their anthill to a range of about one meter. Some species of ground beetles and bees use formic acid cocktails for both defense and attack purposes. In some species of jellyfish, formic acid is a component of the poison in nettles that poison the prey.

There is a nettle poison in the stinging hairs of the nettles, which contains, among other things, formic acid and sodium formate.

In the human organism, formic acid is produced in addition to formaldehyde when methanol is metabolized. Formic acid is easily biodegradable.

In interstellar matter (gas and dust clouds) about 3/4 of the substances are carbon-containing compounds. Traces of formic acid can also be observed here. Formic acid is often found on comets. In the envelope (coma) of the comet Hale-Bopp, 0.06 mol of formic acid were found normalized to 100 mol of water. Traces of formic acid can also be found on many other comets. It was also found in small traces on a few other planets.

security

Hazards and protective measures

Contact with formic acid or concentrated vapors is irritating to the respiratory tract and eyes. It leads to severe burns and blisters on the skin from a concentration of more than ten percent. When they decay, the breath poison carbon monoxide can be formed. Therefore, caution should be exercised where ever larger amounts of formic acid are found. Formic acid can be broken down by the body. It is slightly hazardous to water. Chronic exposure can lead to skin allergies. Animal experiments have shown that formic acid is mutagenic. It also leads to liver and kidney damage.

Formic acid should be kept in a well-ventilated, cool place. Containers in which formic acid is stored must also be closed with a pressure equalization screw connection, as otherwise overpressure can arise when the gases develop. Safety glasses and gloves must be used when performing experiments with formic acid. Formic acid containers should be kept locked up and their vapors should not be inhaled. The maximum workplace concentration is 5 ml / m³.

First aid and fire fighting

If formic acid or its vapors come into contact with the eyes, they must be rinsed out with water for about 15 minutes and a doctor consulted, as it can cause permanent eye damage. Even in the event of skin contact, the affected area should be rinsed off with water. Serious damage is not to be expected after a few seconds of skin contact. Fresh air should be supplied immediately if inhaled and plenty of water should be drunk if swallowed in order to dilute the acid. However, no vomiting may be induced and no attempts at neutralization may be undertaken. In the event of any kind of accident or discomfort due to formic acid, a doctor should be consulted immediately. The measures and dangers described in this and the above section correspond to the R and S phrases prescribed in Appendix 1 of RL 67/548 / EEC: R: 35; S: (1/2) 23-26-45. More detailed information can be found there.

From a volume fraction of 12–38 percent in air, formic acid vapors form explosive mixtures. There is also a risk of explosion in the presence of nickel catalysts and nitromethane. The flash point of formic acid is 69 ° C, the ignition temperature is 480 ° C. The vapors are heavier than air and flammable. The extinguishing agent should be selected according to the situation. Water mist, foam or carbon dioxide are possible.

  • Safety data sheet
  • Medicine and dangers
  • Properties, presentation and use

literature

  • Formic acid, formates, diglycol - bis - chloroformate. VCH, Weinh, ISBN 3527285296
  • Selected C-H-O radicals. Formic acid. Acetic acid. Oxalic acid (Gmelin Handbook of Inorganic and Organometallic Chemistry - 8th edition ELEM C TL C LFG 4). Springer Verlag, ISBN 3540932836
  • Gundula Jänsch-Kaiser and Dieter Behrens: Formic acid and alkaline earth hydroxides. DECHEMA Society for Chemical Technology and Biotechnology e.V., ISBN 3926959002
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