ACETIC ACID: HISTORY, STRUCTURE, PROPERTIES, USES - CHEMISTRY - 2024

The acetic acid or ethanoic acid is an organic colorless liquid having the chemical formula CH ₃ COOH. When dissolved in water, a well-known mixture called vinegar is obtained, which has been used as an additive in food for a long time. Vinegar is an aqueous solution of acetic acid with a concentration of approximately 5%.

As its name indicates, it is an acid compound, and therefore vinegar has pH values ​​lower than 7. In the presence of its acetate salt, it constitutes a buffer system that is effective in regulating the pH between 2.76 and 6.76; that is, it maintains the pH within that range with moderate additions of a base or an acid.

Source: Pixabay

Its formula is enough to realize that it is formed by the union of a methyl group (CH ₃) and a carboxyl group (COOH). After formic acid, HCOOH, is one of the simplest organic acids; which also represents the end point of many fermentation processes.

Thus, acetic acid can be produced by aerobic and anaerobic bacterial fermentation, and by chemical synthesis, with the methanol carbonylation process being the main mechanism of its production.

In addition to its daily use as a salad dressing, in industry it represents the raw material for the production of cellulose acetate, a polymer that is used to make photographic films. Additionally, acetic acid is used in the synthesis of polyvinyl acetate, used in the manufacture of a glue for wood.

When the vinegar has become highly concentrated, it is no longer called as such and is called glacial acetic acid. At these concentrations, even though it is a weak acid, it is highly corrosive and can cause skin and respiratory irritation just by shallow breathing. Glacial acetic acid finds use as a solvent in organic syntheses.

History

The man belonging to many cultures, has used the fermentation of many fruits, legumes, cereals, etc., to obtain alcoholic beverages, product of the transformation of sugars, such as glucose, in ethanol, CH ₃ CH ₂ OH.

Probably because the initial method for the production of alcohol and vinegar is fermentation, perhaps trying to produce alcohol at an indeterminate time, many centuries ago, vinegar was obtained by mistake. Note the similarity between the chemical formulas of acetic acid and ethanol.

As early as the 3rd century BC, the Greek philosopher Theophastus described the action of vinegar on metals for the production of pigments, such as lead white.

1800

In 1823, a tower-shaped equipment was designed in Germany for the aerobic fermentation of different products, in order to obtain acetic acid in the form of vinegar.

In 1846, Herman Foelbe first achieved the synthesis of acetic acid through the use of inorganic compounds. The synthesis began with the chlorination of carbon disulfide and concluded, after two reactions, with an electrolytic reduction to acetic acid.

At the end of the 19th century and the beginning of the 20th century, due to the research of J. Weizmann, the bacterium Clostridium acetobutylicum began to be used for the production of acetic acid, through anaerobic fermentation.

1900

At the beginning of the 20th century, the dominant technology was the production of acetic acid through the oxidation of acetaldehyde.

In 1925, Henry Dreyfus of the British company Celanese, designed a pilot plant for the carbonylation of methanol. Later, in 1963, the German company BASF introduced the use of cobalt as a catalyst.

Otto Hromatka and Heinrich Ebner (1949), designed a tank with a stirring system and air supply for aerobic fermentation, intended for the production of vinegar. This implement, with some adaptations, is still in use.

In 1970 the North American company Montsanto, used a catalyst system based on rhodium for the carbonylation of methanol.

Later, the BP company in 1990, introduced the Cativa process with the use of the iridium catalyst for the same purpose. This method turned out to be more efficient and less environmentally aggressive than the Montsanto method.

Structure of acetic acid

Source: Pixabay

The upper image shows the structure of acetic acid represented with a spheres and bars model. The red spheres correspond to the oxygen atoms, which in turn belong to the carboxyl group, –COOH. Therefore, it is a carboxylic acid. On the right side of the structure is the methyl group, –CH ₃.

As can be seen, it is a very small and simple molecule. It presents a permanent dipole moment due to the –COOH group, which also allows acetic acid to form two hydrogen bonds consecutively.

It is these bridges that spatially orient the CH ₃ COOH molecules to form dimers in the liquid (and gaseous) state.

Source: Pixabay

Above in the image you can see how the two molecules are arranged to form the two hydrogen bonds: OHO and OHO. To evaporate acetic acid, enough energy must be supplied to break these interactions; which is why it is a liquid with a boiling point higher than that of water (approximately 118 ° C).

Physical and chemical properties

Chemical names

Acid:

-Acetic

-Etanoic

-Ethyl

Molecular formula

C ₂ H ₄ O ₂ or CH ₃ COOH.

Physical appearance

Colorless liquid.

Odor

Characteristic acre.

Taste

Burning

Boiling point

244 ° F to 760 mmHg (117.9 ° C).

Melting point

61.9 ° F (16.6 ° C).

ignition point

112ºF (open cup) 104ºF (closed cup).

Water solubility

10 ⁶ mg / mL at 25 ºC (it is miscible in all proportions).

Solubility in organic solvents

It is soluble in ethanol, ethyl ether, acetone, and benzene. It is also soluble in carbon tetrachloride.

Density

1.051 g / cm ³ at 68 ° F (1.044 g / cm ³ at 25 ° C).

Vapor density

2.07 (relative to air = 1).

Vapor pressure

15.7 mmHg at 25 ° C.

Decomposition

When heated above 440 ºC, it decomposes to produce carbon dioxide and methane.

Viscosity

1,056 mPascal at 25 ° C.

Corrosiveness

Glacial acetic acid is highly corrosive and its ingestion can cause severe lesions of the esophagus and pylorus in man.

Heat of combustion

874.2 kJ / mol.

Heat of vaporization

23.70 kJ / mol at 117.9 ° C.

23.36 kJ / mol at 25.0 ° C.

pH

-A 1 M concentration solution has a pH of 2.4

- For a 0.1M solution, its pH is 2.9

- And 3.4 if the solution is 0.01M

Surface tension

27.10 mN / m at 25 ° C.

pKa

4.76 at 25th C.

Chemical reactions

Acetic acid is corrosive to many metals, releasing H ₂ gas and forming metal salts called acetates. With the exception of chromium (II) acetate, acetates are soluble in water. Its reaction with magnesium is represented by the following chemical equation:

Mg (s) + 2 CH ₃ COOH (ag) => (CH ₃ COO) ₂ Mg (ag) + H ₂ (g)

By reduction acetic acid forms ethanol. It can also form acetic anhydride from the loss of water from two water molecules.

Production

As previously stated, fermentation produces acetic acid. This fermentation can be aerobic (in the presence of oxygen) or anaerobic (without oxygen).

Oxidative or aerobic fermentation

Bacteria of the genus Acetobacter can act on ethanol or ethyl alcohol, causing its oxidation to acetic acid in the form of vinegar. By this method, vinegar with a concentration of 20% acetic acid can be produced.

These bacteria are capable of producing vinegar, acting on a wide variety of inputs that include different fruits, fermented legumes, malt, cereals such as rice or other vegetables that contain or can produce ethyl alcohol.

The chemical reaction facilitated by bacteria of the genus Acetobacter is as follows:

CH ₃ CH ₂ OH + O ₂ => CH ₃ COOH + H ₂ O

Oxidative fermentation is carried out in tanks with mechanical stirring and oxygen supply.

Anaerobic fermentation

It is based on the ability of some bacteria to produce acetic acid by acting directly on sugars, without requiring intermediates for the production of acetic acid.

C ₆ H ₁₂ O ₆ => 3CH ₃ COOH

The bacterium that intervenes in this process is Clostridium acetobutylicum, which is capable of intervening in the synthesis of other compounds, in addition to acetic acid.

Acetogenic bacteria can produce acetic acid, acting on molecules made up of only one carbon atom; such is the case with methanol and carbon monoxide.

Anaerobic fermentation is less expensive than oxidative fermentation, but it has the limitation that bacteria of the genus Clostridium are not very resistant to acidity. This limits its ability to produce vinegar with a high concentration of acetic acid, such as is achieved in oxidative fermentation.

Carbonylation of methanol

Methanol can react with carbon monoxide to produce acetic acid in the presence of catalysts

CH ₃ OH + CO => CH ₃ COOH

Using iodomethane as a catalyst, the carbonylation of methanol occurs in three stages:

In a first stage, hydroiodic acid (HI) reacts with methanol, producing iodomethane, which reacts in a second stage with carbon monoxide, forming the compound iodo acetaldehyde (CH ₃ COI). The CH ₃ COI is then hydrated to produce acetic acid and regenerate HI.

The Monsanto process (1966) is a method for the manufacture of acetic acid by catalytic carbonylation of methanol. It is developed at a pressure of 30 to 60 atm, at a temperature of 150-200 ° C, and using a rhodium catalyst system.

The Monsanto process was largely superseded by the Cativa process (1990) developed by BP Chemicals LTD, which uses an iridium catalyst. This process is cheaper and less polluting.

Acetaldehyde oxidation

This oxidation requires metallic catalysts such as naphthenates, manganese salts, cobalt or chromium.

2 CH ₃ CHO + O ₂ => 2 CH ₃ COOH

Acetaldehyde oxidation can have a very high yield that can reach 95% with suitable catalysts. The side products of the reaction are separated from acetic acid by distillation.

After the methanol carbonylation method, the oxidation of acetaldehyde is the second form in percentage of industrial production of acetic acid.

Applications

Industrial

-Acetic acid reacts with ethylene in the presence of oxygen to form vinyl acetate monomer, using palladium as the catalyst for the reaction. Vinyl acetate polymerizes to polyvinyl acetate, which is used as a component in paints and adhesive material.

-Reacts with different alcohols to produce esters, including ethyl acetate and propyl acetate. Acetate esters are used as solvents for inks, nitrocellulose, coatings, varnishes, and acrylic lacquers.

-By the condensation of two molecules of acetic acid, losing one molecule of a molecule, acetic anhydride is formed, CH ₃ CO-O-COCH ₃. This compound is involved in the synthesis of cellulose acetate, a polymer that constitutes a synthetic fabric and is used in the production of photographic films.

As a solvent

-It is a polar solvent with the ability to form hydrogen bonds. It is capable of dissolving polar compounds like inorganic salts and sugars, but it also dissolves non-polar compounds like oils and fats. Furthermore, acetic acid is miscible with polar and nonpolar solvents.

-The miscibility of acetic acid in alkanes depends on the extension of their chain: as the length of the alkane chain increases, its miscibility with acetic acid decreases.

Doctors

-Diluted acetic acid is used as an antiseptic, applied topically, with the ability to attack bacteria such as streptococci, staphylococci and pseudomonas. Due to this action it is used in the treatment of skin infections.

-Acetic acid is used in endoscopy of Barrett's esophagus. This is a condition in which the lining of the esophagus changes, becoming similar to the lining of the small intestine.

-A 3% acetic acid gel appears to be an effective adjuvant to treatment with the vaginal drug Misoprostol, inducing medical abortion in the middle trimester, especially in women with a vaginal pH of 5 or more.

-Used as a substitute for chemical exfoliation. However, complications have arisen with this use, since at least one case of burns suffered by a patient has been reported.

In food

Vinegar has been used as a seasoning and flavoring for foods for a long time, which is why this is the best known application of acetic acid.

References

1. Byju's. (2018). What is Ethanoic Acid? Recovered from: byjus.com
2. PubChem. (2018). Acetic acid. Recovered from: pubchem.ncbi.nlm.nih.gov
3. Wikipedia. (2018). Acetic acid. Recovered from: en.wikipedia.org
4. Chemical Book. (2017). Glacial acetic acid. Recovered from: chemicalbook.com
5. Acetic acid: what is it and what is it for? Recovered from: acidoacetico.info
6. Helmenstine, Anne Marie, Ph.D. (June 22, 2018). What Is Glacial Acetic Acid? Recovered from: thoughtco.com