Acetic acid

Acetic acid, systematically named ethanoic acid, is an acidic, colourless liquid and organic compound with the chemical formula . Acetic acid is the active component of vinegar. Historically, vinegar was produced from the third millennium BC, making acetic acid likely the first acid to be produced in large quantities.
Acetic acid is the second simplest carboxylic acid. It is an important chemical reagent and industrial chemical across various fields, used primarily in the production of cellulose acetate for photographic film, polyvinyl acetate for wood glue, and synthetic fibres and fabrics. In households, diluted acetic acid is often used in descaling agents. In the food industry, acetic acid is controlled by the food additive code E260 as an acidity regulator and as a condiment. In biochemistry, the acetyl group, derived from acetic acid, is fundamental to all forms of life. When bound to coenzyme A, it is central to the metabolism of carbohydrates and fats.
The global demand for acetic acid as of 2023 is about 17.88 million metric tonnes per year. Most of the world's acetic acid is produced via the carbonylation of methanol. Its production and subsequent industrial use poses health hazards to workers, including incidental skin damage and chronic respiratory injuries from inhalation.

Nomenclature

The trivial name "acetic acid" is the most commonly used and preferred IUPAC name. The systematic name "ethanoic acid", a valid IUPAC name, is constructed according to the substitutive nomenclature. The name "acetic acid" derives from the Latin word for vinegar, "", which means "to sour" in relation to the bitter taste of fermented fruits.
"Glacial acetic acid" is a name for water-free acetic acid. Similar to the German name "Eisessig", the name comes from the solid ice-like crystals that form with agitation, slightly below room temperature at. Acetic acid can never be truly water-free in an atmosphere that contains water, so the presence of 0.1% water in glacial acetic acid lowers its melting point by 0.2 °C.
A common symbol for acetic acid is AcOH, where Ac is the pseudoelement symbol representing the acetyl group ; the conjugate base, acetate, is thus represented as. Acetate is the ion resulting from loss of from acetic acid. The name "acetate" can also refer to a salt containing this anion, or an ester of acetic acid.. To better reflect its structure, acetic acid is often written as,,, and. In the context of acid–base reactions, the abbreviation HAc is sometimes used, where Ac in this case is a symbol for acetate.
The carboxymethyl functional group derived from removing one hydrogen from the methyl group of acetic acid has the chemical formula.

History

Acetic acid, in the dilute solution known as vinegar, is likely the first acid to be produced in large quantities in history. Vinegar has been used for at least 10,000 years as it is the natural result of exposure of beer and wine to air because acetic acid-producing bacteria are present globally. The use of acetic acid in alchemy extends into the third century BC, when the Greek philosopher Theophrastus described how vinegar acted on metals to produce pigments useful in art, including white lead and verdigris, a green mixture of copper salts including copper acetate. Hippocrates used vinegar as an antiseptic and as a remedy for numerous conditions including fever, constipation, ulcers, and pleurisy. Ancient Romans boiled soured wine to produce a highly sweet syrup called sapa. Sapa that was produced in lead pots was rich in lead acetate, a sweet substance also called sugar of lead or sugar of Saturn, which contributed to lead poisoning among the Roman aristocracy.
In the 16th-century German alchemist Andreas Libavius described the production of acetone from the dry distillation of lead acetate, ketonic decarboxylation. The presence of water in vinegar has such a profound effect on acetic acid's properties that for centuries chemists believed that glacial acetic acid and the acid found in vinegar were two different substances. French chemist Pierre Adet proved them identical.
In 1845 German chemist Hermann Kolbe synthesized acetic acid from inorganic compounds for the first time. This reaction sequence consisted of chlorination of carbon disulfide to carbon tetrachloride, followed by pyrolysis to tetrachloroethylene and aqueous chlorination to trichloroacetic acid, and concluded with electrolytic reduction to acetic acid.
By 1910, most glacial acetic acid was obtained from the pyroligneous liquor, a product of the distillation of wood. The acetic acid was isolated by treatment with milk of lime, and the resulting calcium acetate was then acidified with sulfuric acid to recover acetic acid. At that time, Germany was producing 10,000 tons of glacial acetic acid, around 30% of which was used for the manufacture of indigo dye.
Because both methanol and carbon monoxide are commodity raw materials, methanol carbonylation long appeared to be attractive precursors to acetic acid. Henri Dreyfus at British Celanese developed a methanol carbonylation pilot plant as early as 1925. However, a lack of practical materials that could contain the corrosive reaction mixture at the high pressures needed discouraged commercialization of these routes. The first commercial methanol carbonylation process, which used a cobalt catalyst, was developed by German chemical company BASF in 1963. In 1968, a rhodium-based catalyst was discovered that could operate efficiently at lower pressure with almost no by-products. US chemical company Monsanto Company built the first plant using this catalyst in 1970, and rhodium-catalyzed methanol carbonylation became the dominant method of acetic acid production. In the late 1990s, BP Chemicals commercialized the Cativa catalyst, which is promoted by iridium for greater efficiency. Known as the Cativa process, the iridium-catalyzed production of glacial acetic acid is greener, and has largely supplanted the Monsanto process, often in the same production plants.

Interstellar medium

acetic acid was first detected in the Sagittarius B2 North molecular cloud. Acetic acid has the distinction of being the first molecule discovered in the interstellar medium using solely radio interferometers; in all previous ISM molecular discoveries made in the millimetre and centimetre wavelength regimes, single dish radio telescopes were at least partly responsible for the detections.

Properties

Acidity

The hydrogen centre in the carboxyl group in carboxylic acids such as acetic acid can separate from the molecule by ionization in water:
Because of this release of the proton, acetic acid has acidic character. Acetic acid is a weak monoprotic acid. In aqueous solution, it has a pK_a value of 4.76. Its conjugate base is acetate. A 1.0 M solution has a pH of 2.4, indicating that merely 0.4% of the acetic acid molecules are ionized.

Structure

In solid acetic acid, the molecules form chains of individual molecules interconnected by hydrogen bonds. In the vapour phase at, dimers can be detected. Dimers also occur in the liquid phase in dilute solutions with non-hydrogen-bonding solvents, and to a certain extent in pure acetic acid, but are disrupted by hydrogen-bonding solvents. The dissociation enthalpy of the dimer is estimated at 65.0–66.0 kJ/mol, and the dissociation entropy at 154–157 J mol⁻¹ K⁻¹. Other carboxylic acids engage in similar intermolecular hydrogen bonding interactions.

Solvent properties

acetic acid is a hydrophilic protic solvent, similar to ethanol and water. With a relative static permittivity of 6.2, it dissolves not only polar compounds such as inorganic salts and sugars, but also non-polar compounds such as oils as well as polar solutes. It is miscible with polar and non-polar solvents such as water, chloroform, and hexane. With higher alkanes, acetic acid is not miscible at all compositions, and solubility of acetic acid in alkanes declines with longer n-alkanes. The solvent and miscibility properties of acetic acid make it a useful industrial chemical, for example, as a solvent in the production of dimethyl terephthalate.

Biochemistry

At physiological pHs, acetic acid is usually fully ionized to acetate in aqueous solution.
The acetyl group, formally derived from acetic acid, is fundamental to all forms of life. Typically, it is bound to coenzyme A by acetyl-CoA synthetase enzymes, where it is central to the metabolism of carbohydrates and fats. Unlike longer-chain carboxylic acids, acetic acid does not occur in natural triglycerides. Most of the acetate generated in cells for use in acetyl-CoA is synthesized directly from ethanol or pyruvate. However, the artificial triglyceride triacetin is a common food additive and is found in cosmetics and topical medicines; this additive is metabolized to glycerol and acetic acid in the body.
Acetic acid is produced and excreted by acetic acid bacteria, notably the genus Acetobacter and Clostridium acetobutylicum. These bacteria are found universally in foodstuffs, water, and soil, and acetic acid is produced naturally as fruits and other foods spoil. Acetic acid is also a component of the vaginal lubrication of humans and other primates, where it appears to serve as a mild antibacterial agent.

Production

Acetic acid is produced industrially both naturally via bacterial fermentation and synthetically. About 75% of acetic acid made for use in the chemical industry is made by the carbonylation of methanol, explained below. The biological route accounts for only about 10% of world production, but it remains important for the production of vinegar because many food purity laws require vinegar used in foods to be of biological origin. Other processes are methyl formate isomerization, conversion of syngas to acetic acid, and gas phase oxidation of ethylene and ethanol.
Acetic acid can be purified via fractional freezing using an ice bath. The water and other impurities will remain liquid while the acetic acid will precipitate out. As of 2003–2005, total worldwide production of virgin acetic acid was estimated at 5 Mt/a, approximately half of which was produced in the United States. European production was approximately 1 Mt/a and declining, while Japanese production was 0.7 Mt/a. Another 1.5 Mt were recycled each year, bringing the total world market to 6.5 Mt/a. Since then, the global production has increased from 10.7 Mt/a in 2010 to 17.88 Mt/a in 2023. The two biggest producers of virgin acetic acid are Celanese and BP Chemicals. Other major producers include Millennium Chemicals, Sterling Chemicals, Samsung, Eastman, and.