Acetic acid, CH₃COOH, also known as ethanoic acid, is an organic acid An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids whose acidity is associated with their carboxyl group -COOH. Sulfonic acids, containing the group -SO2OH, are relatively stronger acids. The relative stability of the conjugate base of the acid determines its acidity. Other groups that gives vinegar Vinegar is an acidic liquid produced from the fermentation of ethanol in a process that yields its key ingredient, acetic acid . It also may come in a diluted form. The acetic acid concentration typically ranges from 4% to 8% by volume for table vinegar and up to 18% for pickling. Natural vinegars also contain small amounts of tartaric acid, its sour taste and pungent smell. It is a weak acid A weak acid is an acid that dissociates incompletely. It does not release all of its hydrogens in a solution, donating only a partial amount of its protons to the solution. These acids have higher pKa than strong acids, which release all of their hydrogen atoms when dissolved in water, in that it is only a partially dissociated Dissociation in chemistry and biochemistry is a general process in which ionic compounds separate or split into smaller particles, ions, or radicals, usually in a reversible manner. When a Bronsted-Lowry acid is put in water, a covalent bond between an electronegative atom and a hydrogen atom is broken by heterolytic fission, which gives a proton acid in an aqueous solution An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending to the relevant formula. The word aqueous means pertaining to, related to, similar to, or dissolved in water. As water is an excellent solvent as well as naturally abundant, it is a ubiquitous solvent in chemistry. Pure, water Water is a chemical substance with the chemical formula H2O. Its molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state, water vapor or steam-free acetic acid (glacial acetic acid) is a colourless liquid Liquid is one of the three classical states of matter. Like a gas, a liquid is able to flow and take the shape of a container, but, like a solid, it resists compression. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly constant density. A distinctive property of the liquid state is surface tension, that absorbs water from the environment (hygroscopy Hygroscopy is the ability of a substance to attract water molecules from the surrounding environment through either absorption or adsorption), and freezes at 16.5 °C Celsius is a temperature scale that is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale two years before his death. The degree Celsius (°C) can refer to a specific temperature on the Celsius scale as well as a unit to indicate a temperature interval (a difference between two temperatures (62 °F Fahrenheit is the temperature scale proposed in 1724 by, and named after, the physicist Daniel Gabriel Fahrenheit . Today, the temperature scale has been replaced by the Celsius scale in most countries. It is still in use in few nations, such as United States and Belize) to a colourless crystalline A crystal or crystalline solid is a solid material, whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is crystallography. The process of crystal formation via mechanisms of crystal growth is called solid Solid is one of the major states of matter. It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a gas does. The atoms in a solid are tightly bound to each other,. The pure acid and its concentrated solutions are dangerously corrosive.

Acetic acid is one of the simplest carboxylic acids Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. A carboxyl group is a functional group consisting of a carbonyl and a hydroxyl, which has the formula -C(=O)OH, usually written -COOH or -CO2H. Carboxylic acids are Brønsted-Lowry acids — they are proton donors. Salts and anions of carboxylic acids. It is an important chemical reagent A reagent is a "substance or compound that is added to a system in order to bring about a chemical reaction or is added to see if a reaction occurs". Such a reaction is used to confirm the presence of another substance. Examples of such analytical reagents include Fehling's reagent, Millon's reagent and Tollens' reagent and industrial chemical, used in the production of polyethylene terephthalate Polyethylene terephthalate (sometimes written poly), commonly abbreviated PET, PETE, or the obsolete PETP or PET-P), is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination with glass fiber mainly used in soft drink bottles; cellulose acetate Cellulose acetate , first prepared in 1865, is the acetate ester of cellulose. Cellulose acetate is used as a film base in photography, as a component in some adhesives, and as a frame material for eyeglasses; it is also used as a synthetic fiber and in the manufacture of cigarette filters, mainly for photographic film Photographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts (bonded by gelatin) with variable crystal sizes that determine the sensitivity, contrast and resolution of the film. When the emulsion is sufficiently exposed to light (or other forms of electromagnetic radiation such as X-rays), it forms; and polyvinyl acetate Polyvinyl acetate is a rubbery synthetic polymer with the formula (C4H6O2)n for wood glue An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials. Adhesives cure by either evaporating a solvent or by chemical reactions, as well as synthetic fibres and fabrics. In households, diluted acetic acid is often used in descaling agents An anti-scaling agent, also known as descaling agent, anti-limestone, anti-limescale, anti-lime or anti-scale are preparations to prevent the buildup or to remove limescale and fouling. Anti-scaling agent agents usually contain acids such as acetic acid, lactic acid, citric acid, phosphoric acid, hydrochloric acid, sulfamic acid to remove. In the food industry The food industry is a complex, global collective of diverse businesses that together supply much of the food energy consumed by the world population. Only subsistence farmers, those who survive on what they grow, can be considered outside of the scope of the modern food industry, acetic acid is used under the food additive code E numbers are number codes for food additives that have been assessed for use within the European Union . They are commonly found on food labels throughout the European Union. Safety assessment and approval are the responsibility of the European Food Safety Authority. The numbering scheme follows that of the International Numbering System (INS) as E260 as an acidity regulator Acidity regulators, or pH control agents, are food additives added to change or maintain pH . They can be organic or mineral acids, bases, neutralizing agents, or buffering agents and as a condiment.

The global demand of acetic acid is around 6.5 million tonnes The tonne or metric ton (U.S.), often redundantly referred to as a metric tonne, is a unit of mass equal to 1,000 kg (2,205 lb) or approximately the mass of one cubic metre of water at four degrees Celsius. It is sometimes abbreviated as mt in the United States, but this conflicts with other SI symbols. The tonne is not a unit in the International per year (Mt/a), of which approximately 1.5 Mt/a is met by recycling; the remainder is manufactured from petrochemical Petrochemicals are chemical products derived from petroleum. Some chemical compounds made from petroleum are also obtained from other fossil fuels such as coal or natural gas, or renewable sources such as corn or sugar cane feedstocks or from biological sources. Dilute acetic acid produced by natural fermentation is called vinegar Vinegar is an acidic liquid produced from the fermentation of ethanol in a process that yields its key ingredient, acetic acid . It also may come in a diluted form. The acetic acid concentration typically ranges from 4% to 8% by volume for table vinegar and up to 18% for pickling. Natural vinegars also contain small amounts of tartaric acid,.

Nomenclature

The trivial name In chemistry and biology, a trivial name is a non-systematic name or non-scientific name. That is, the name is not recognised according to the rules of any formal (e.g. IUPAC) system of nomenclature. Many trivial names continue to be used because their sanctioned equivalents are considered too cumbersome for everyday use. For example, " acetic acid is the most commonly used and preferred IUPAC name. The systematic name ethanoic acid may be used as a valid IUPAC name. ^[1] The name acetic acid derives from acetum, the Latin Latin or sometimes Roman is an Italic language originally spoken in Latium and Ancient Rome. Although often considered a dead language, in view of the fact that it has no native, fluent speakers, Latin continues to be taught in schools and has been, and currently is, used in the process of new word production in modern languages from many word for vinegar Vinegar is an acidic liquid produced from the fermentation of ethanol in a process that yields its key ingredient, acetic acid . It also may come in a diluted form. The acetic acid concentration typically ranges from 4% to 8% by volume for table vinegar and up to 18% for pickling. Natural vinegars also contain small amounts of tartaric acid,, and is related to the word acid itself. The synonym ethanoic acid is constructed according to the substitutive nomenclature of the IUPAC.

Glacial acetic acid is a trivial name for water-free acetic acid. Similar to the German German (Deutsch, [ˈdɔʏtʃ] ) is a West Germanic language, thus related to and classified alongside English and Dutch. It is one of the world's major languages and the most widely spoken first language in the European Union. Globally, German is spoken by approximately 120 million native speakers and also by about 80 million non-native speakers name Eisessig (ice-vinegar), the name comes from the ice-like crystals that form slightly below room temperature at 16.7 °C (62 °F).

The most common abbreviation for acetic acid is HOAc where Ac stands for the acetyl In organic chemistry, acetyl is a functional group, the acyl with chemical formula C group In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of. However, its relative reactivity can be CH₃−C(=O)−. In the context of acid-base reactions An acid-base reaction is a chemical reaction that occurs between an acid and a base. Several concepts that provide alternative definitions for the reaction mechanisms involved and their application in solving related problems exist. Despite several differences in definitions, their importance becomes apparent as different methods of analysis when, the abbreviation HAc is often used where Ac instead stands for the acetate An acetate is either a salt or an ester of acetic acid. The formula of the acetate anion (part of a salt) is written both as CH3CO2− and C2H3O2−. Chemists abbreviate acetate as OAc− and AcO−. Thus HOAc is the abbreviation for acetic acid, NaOAc for sodium acetate, and EtOAc for ethyl acetate. Acetate is a common anion in biology anion An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. An anion , from the Greek word ἀνω (anο), meaning "up", is an ion with more electrons than protons, giving it a net negative charge (since electrons are negatively (CH₃COO⁻, abbreviated AcO⁻), although this use is often regarded as misleading. In either case, the Ac is not to be confused with the abbreviation for the chemical element A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. The term is also used to refer to a pure chemical substance composed of atoms with the same number of protons. Common examples of elements are iron, copper, silver, gold, hydrogen, carbon, actinium Actinium is a radioactive chemical element with the symbol Ac and atomic number 89, which was discovered in 1899. It was the first non-primordial radioactive element to be isolated. Polonium, radium and radon were observed before actinium, but they were not isolated until 1902. Actinium gave the name to the actinoid series, a group of 15 similar. Acetic acid has the molecular formula C₂H₄O₂ (empirical formula A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound CH₂O).

To emphasize the role of the active hydrogen in forming the salt sodium acetate, some people write the molecular formula as HC₂H₃O₂.^[2] To better reflect its structure, acetic acid is often written as CH₃-CO₂-H, CH₃COOH, or CH₃CO₂H. The ion An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. An anion , from the Greek word ἀνω (anο), meaning "up", is an ion with more electrons than protons, giving it a net negative charge (since electrons are negatively resulting from loss of H⁺ The proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom, along with neutrons, but is also stable by itself and has a second identity as the hydrogen ion, H+. It is composed of three fundamental particles: two up quarks and one down quark from acetic acid is the acetate anion. The name acetate can also refer to a salt Salt is a mineral that is composed primarily of sodium chloride. It is essential for animal life in small quantities, but is harmful to animals and plants in excess. Salt flavor is one of the basic tastes, making salt one of the oldest, most ubiquitous food seasonings. Salting is an important method of food preservation containing this anion, or an ester Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH (hydroxyl) group is replaced by an -O-alkyl (alkoxy) group, and most commonly from carboxylic acids and alcohols. Basically, esters are of acetic acid.

History

Vinegar Vinegar is an acidic liquid produced from the fermentation of ethanol in a process that yields its key ingredient, acetic acid . It also may come in a diluted form. The acetic acid concentration typically ranges from 4% to 8% by volume for table vinegar and up to 18% for pickling. Natural vinegars also contain small amounts of tartaric acid, was known early in civilization as the natural result of air exposure of beer Beer is the world's most widely consumed and probably oldest alcoholic beverage, and the third most popular drink overall after water and tea. It is produced by the brewing and fermentation of starches, mainly derived from cereal grains—most commonly malted barley, although wheat, maize , and rice are widely used. Most beer is flavored with hops, and wine Wine is an alcoholic beverage, typically made of fermented grape juice. The natural chemical balance of grapes is such that they can ferment without the addition of sugars, acids, enzymes or other nutrients. Wine is produced by fermenting crushed grapes using various types of yeast. Yeast consumes the sugars found in the grapes and converts them, as acetic acid-producing bacteria are present globally. The use of acetic acid in alchemy Alchemy, derived from the Arabic word al-kimia , is both a philosophy and an ancient practice focused on the attempt to change base metals into gold, investigating the preparation of the "elixir of longevity", and achieving ultimate wisdom, involving the improvement of the alchemist as well as the making of several substances described extends into the third century BC, when the Greek Greece (English: /ˈɡriːs/ ; Greek: Ελλάδα, Elláda, IPA: [eˈlaða] ( listen); Ancient Greek: Ἑλλάς, Hellás, IPA: [helːás]), also known as Hellas and officially the Hellenic Republic (Ελληνική Δημοκρατία, Ellīnikī́ Dīmokratía, IPA: [eliniˈci ðimokraˈtia]), is a country in southeastern Europe, situated on philosopher Theophrastus Theophrastus , a Greek native of Eresos in Lesbos, was the successor of Aristotle in the Peripatetic school. He came to Athens at a young age, and initially studied in Plato's school. After Plato's death he attached himself to Aristotle. Aristotle bequeathed to Theophrastus his writings, and designated him as his successor at the Lyceum described how vinegar acted on metals to produce pigments A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light useful in art, including white lead (lead carbonate Lead carbonate, is the chemical compound PbCO3. It is prepared industrially from lead acetate and carbon dioxide. It occurs naturally as the mineral cerussite ) and verdigris, a green mixture of copper salts including copper(II) acetate. Ancient Romans boiled soured wine in lead pots to produce a highly sweet syrup called sapa. Sapa 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.^[3]

In the 8th century, Jabir Ibn Hayyan (Geber) was the first to concentrate acetic acid from vinegar through distillation. In the Renaissance, glacial acetic acid was prepared through the dry distillation of certain metal acetates (the most noticeable one being copper(II) acetate). The 16th-century German alchemist Andreas Libavius described such a procedure, and he compared the glacial acetic acid produced by this means to vinegar. 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 to be identical.^[3]

In 1847 German chemist Hermann Kolbe synthesized acetic acid from inorganic materials 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.^[4]

By 1910, most glacial acetic acid was obtained from the "pyroligneous liquor" from distillation of wood. The acetic acid was isolated from this by treatment with milk of lime, and the resultant 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.^[3][5]

Chemical properties

The hydrogen (H) atom in the carboxyl group (−COOH) in carboxylic acids such as acetic acid can be given off as an H⁺ ion (proton), giving them their acidic character. Acetic acid is a weak, effectively monoprotic acid in aqueous solution, with a pK_a value of 4.75. Its conjugate base is acetate (CH₃COO⁻). A 1.0 M solution (about the concentration of domestic vinegar) has a pH of 2.4, indicating that merely 0.4% of the acetic acid molecules are dissociated.

The crystal structure of acetic acid shows that the molecules pair up into dimers connected by hydrogen bonds.^[6] The dimers can also be detected in the vapour at 120 °C. They also occur in the liquid phase in dilute solutions in non-hydrogen-bonding solvents, and a certain extent in pure acetic acid,^[7] 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.^[8] This dimerization behaviour is shared by other lower carboxylic acids.

Liquid acetic acid is a hydrophilic (polar) protic solvent, similar to ethanol and water. With a moderate relative static permittivity (dielectric constant) of 6.2, it can dissolve not only polar compounds such as inorganic salts and sugars, but also non-polar compounds such as oils and elements such as sulfur and iodine. It readily mixes with other polar and non-polar solvents such as water, chloroform, and hexane. With higher alkanes (starting with octane) acetic acid is not completely miscible anymore, and its miscibility continues to decline with longer n-alkanes.^[9] This dissolving property and miscibility of acetic acid makes it a widely used industrial chemical.

Chemical reactions

Acetic acid is corrosive to metals including iron, magnesium, and zinc, forming hydrogen gas and metal salts called acetates. Aluminium, when exposed to oxygen, forms a thin layer of aluminium oxide on its surface, which is relatively resistant to the acid, allowing aluminium tanks to transport acetic acid. Metal acetates can also be prepared from acetic acid and an appropriate base, as in the popular "baking soda + vinegar" reaction. With the notable exception of chromium(II) acetate, almost all acetates are soluble in water.

Mg(s) + 2 CH₃COOH(aq) → (CH₃COO)₂Mg(aq) + H₂(g)

NaHCO₃(s) + CH₃COOH(aq) → CH₃COONa(aq) + CO₂(g) + H₂O(l)

Acetic acid undergoes the typical chemical reactions of a carboxylic acid, such as producing water and a metal ethanoate when reacting with alkalis, producing a metal ethanoate when reacted with a metal, and producing a metal ethanoate, water, and carbon dioxide when reacting with carbonates and hydrogencarbonates. Most notable of all its reactions is the formation of ethanol by reduction, and formation of derivatives such as acetyl chloride via nucleophilic acyl substitution. Other substitution derivatives include acetic anhydride; this anhydride is produced by loss of water from two molecules of acetic acid. Esters of acetic acid can likewise be formed via Fischer esterification, and amides can also be formed. When heated above 440 °C, acetic acid decomposes to produce carbon dioxide and methane, or to produce ethenone and water.

Acetic acid can be detected by its characteristic smell. A colour reaction for salts of acetic acid is iron(III) chloride solution, which results in a deeply red colour that disappears after acidification. Acetates when heated with arsenic trioxide form cacodyl oxide, which can be detected by its malodorous vapours.

Biochemistry

The acetyl group, derived from acetic acid, is fundamental to the biochemistry of all forms of life. When bound to coenzyme A, it is central to the metabolism of carbohydrates and fats. However, the concentration of free acetic acid in cells is kept at a low level to avoid disrupting the control of the pH of the cell contents. Unlike longer-chain carboxylic acids (the fatty acids), acetic acid does not occur in natural triglycerides. However, the artificial triglyceride triacetin (glycerin triacetate) is a common food additive, and is found in cosmetics and topical medicines.

Acetic acid is produced and excreted by acetic acid bacteria, notable ones being the Acetobacter genus 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.^[10]

Production

Acetic acid is produced both synthetically and by bacterial fermentation. Today, the biological route accounts for only about 10% of world production, but it remains important for vinegar production, as many nations' food purity laws stipulate that vinegar used in foods must be of biological origin. About 75% of acetic acid made for use in the chemical industry is made by methanol carbonylation, explained below. Alternative methods account for the rest.^[11] Total worldwide production of virgin acetic acid is estimated at 5 Mt/a (million tonnes per year), approximately half of which is produced in the United States. European production stands at approximately 1 Mt/a and is declining, and 0.7 Mt/a is produced in Japan. Another 1.5 Mt are recycled each year, bringing the total world market to 6.5 Mt/a.^[12][13] The two biggest producers of virgin acetic acid are Celanese and BP Chemicals. Other major producers include Millennium Chemicals, Sterling Chemicals, Samsung, Eastman, and Svensk Etanolkemi.

Methanol carbonylation

Most virgin acetic acid is produced by methanol carbonylation. In this process, methanol and carbon monoxide react to produce acetic acid according to the chemical equation:

CH₃OH + CO → CH₃COOH

The process involves iodomethane as an intermediate, and occurs in three steps. A catalyst, usually a metal complex, is needed for the carbonylation (step 2).

1. CH₃OH + HI → CH₃I + H₂O
2. CH₃I + CO → CH₃COI
3. CH₃COI + H₂O → CH₃COOH + HI

By altering the process conditions, acetic anhydride may also be produced on the same plant. Because both methanol and carbon monoxide are commodity raw materials, methanol carbonylation long appeared to be an attractive method for acetic acid production. Henry Drefyus at British Celanese developed a methanol carbonylation pilot plant as early as 1925.^[14] However, a lack of practical materials that could contain the corrosive reaction mixture at the high pressures needed (200 atm or more) 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 (cis−[Rh(CO)₂I₂]⁻) was discovered that could operate efficiently at lower pressure with almost no by-products. The first plant using this catalyst was built by US chemical company Monsanto Company in 1970, and rhodium-catalysed methanol carbonylation became the dominant method of acetic acid production (see Monsanto process). In the late 1990s, the chemicals company BP Chemicals commercialized the Cativa catalyst ([Ir(CO)₂I₂]⁻), which is promoted by ruthenium. This iridium-catalysed Cativa process is greener and more efficient^[15] and has largely supplanted the Monsanto process, often in the same production plants.

Acetaldehyde oxidation

Prior to the commercialization of the Monsanto process, most acetic acid was produced by oxidation of acetaldehyde. This remains the second-most-important manufacturing method, although it is uncompetitive with methanol carbonylation.

The acetaldehyde may be produced via oxidation of butane or light naphtha, or by hydration of ethylene. When butane or light naphtha is heated with air in the presence of various metal ions, including those of manganese, cobalt, and chromium, peroxides form and then decompose to produce acetic acid according to the chemical equation

2 C₄H₁₀ + 5 O₂ → 4 CH₃COOH + 2 H₂O

The typical reaction is run at a combination of temperature and pressure designed to be as hot as possible while still keeping the butane a liquid. Typical reaction conditions are 150 °C and 55 atm. Side-products may also form, including butanone, ethyl acetate, formic acid, and propionic acid. These side-products are also commercially valuable, and the reaction conditions may be altered to produce more of them where needed. However, the separation of acetic acid from these by-products adds to the cost of the process.

Under similar conditions and using similar catalysts as are used for butane oxidation, acetaldehyde can be oxidized by the oxygen in air to produce acetic acid

2 CH₃CHO + O₂ → 2 CH₃COOH

Using modern catalysts, this reaction can have an acetic acid yield greater than 95%. The major side-products are ethyl acetate, formic acid, and formaldehyde, all of which have lower boiling points than acetic acid and are readily separated by distillation.^[16]

Ethylene oxidation

Acetaldehyde may be prepared from ethylene via the Wacker process, and then oxidized as above. In more recent times, a cheaper, single-stage conversion of ethylene to acetic acid was commercialized by chemical company Showa Denko, which opened an ethylene oxidation plant in Ōita, Japan, in 1997.^[17] The process is catalysed by a palladium metal catalyst supported on a heteropoly acid such as tungstosilicic acid. It is thought to be competitive with methanol carbonylation for smaller plants (100–250 kt/a), depending on the local price of ethylene.

Oxidative fermentation

For most of human history, acetic acid, in the form of vinegar, has been made by acetic acid bacteria of the genus Acetobacter. Given sufficient oxygen, these bacteria can produce vinegar from a variety of alcoholic foodstuffs. Commonly used feeds include apple cider, wine, and fermented grain, malt, rice, or potato mashes. The overall chemical reaction facilitated by these bacteria is:

C₂H₅OH + O₂ → CH₃COOH + H₂O

A dilute alcohol solution inoculated with Acetobacter and kept in a warm, airy place will become vinegar over the course of a few months. Industrial vinegar-making methods accelerate this process by improving the supply of oxygen to the bacteria.

The first batches of vinegar produced by fermentation probably followed errors in the winemaking process. If must is fermented at too high a temperature, acetobacter will overwhelm the yeast naturally occurring on the grapes. As the demand for vinegar for culinary, medical, and sanitary purposes increased, vintners quickly learned to use other organic materials to produce vinegar in the hot summer months before the grapes were ripe and ready for processing into wine. This method was slow, however, and not always successful, as the vintners did not understand the process.^[18]

One of the first modern commercial processes was the "fast method" or "German method", first practised in Germany in 1823. In this process, fermentation takes place in a tower packed with wood shavings or charcoal. The alcohol-containing feed is trickled into the top of the tower, and fresh air supplied from the bottom by either natural or forced convection. The improved air supply in this process cut the time to prepare vinegar from months to weeks.^[19]

Nowadays, most vinegar is made in submerged tank culture, first described in 1949 by Otto Hromatka and Heinrich Ebner.^[20] In this method, alcohol is fermented to vinegar in a continuously stirred tank, and oxygen is supplied by bubbling air through the solution. Using modern applications of this method, vinegar of 15% acetic acid can be prepared in only 24 hours in batch process, even 20% in 60-hour fed-batch process.^[18]

Anaerobic fermentation

Species of anaerobic bacteria, including members of the genus Clostridium, can convert sugars to acetic acid directly, without using ethanol as an intermediate. The overall chemical reaction conducted by these bacteria may be represented as:

C₆H₁₂O₆ → 3 CH₃COOH

It is interesting to note that, from the point of view of an industrial chemist, these acetogenic bacteria can produce acetic acid from one-carbon compounds, including methanol, carbon monoxide, or a mixture of carbon dioxide and hydrogen:

2 CO₂ + 4 H₂ → CH₃COOH + 2 H₂O

This ability of Clostridium to utilize sugars directly, or to produce acetic acid from less costly inputs, means that these bacteria could potentially produce acetic acid more efficiently than ethanol-oxidizers like Acetobacter. However, Clostridium bacteria are less acid-tolerant than Acetobacter. Even the most acid-tolerant Clostridium strains can produce vinegar of only a few per cent acetic acid, compared to Acetobacter strains that can produce vinegar of up to 20% acetic acid. At present, it remains more cost-effective to produce vinegar using Acetobacter than to produce it using Clostridium and then concentrating it. As a result, although acetogenic bacteria have been known since 1940, their industrial use remains confined to a few niche applications.^[21]

Applications

Acetic acid is a chemical reagent for the production of chemical compounds. The largest single use of acetic acid is in the production of vinyl acetate monomer, closely followed by acetic anhydride and ester production. The volume of acetic acid used in vinegar is comparatively small.^[13]

Vinyl acetate monomer

The major use of acetic acid is for the production of vinyl acetate monomer (VAM). This application consumes approximately 40% to 45% of the world's production of acetic acid. The reaction is of ethylene and acetic acid with oxygen over a palladium catalyst.

2 H₃C-COOH + 2 C₂H₄ + O₂ → 2 H₃C-CO-O-CH=CH₂ + 2 H₂O

Vinyl acetate can be polymerized to polyvinyl acetate or to other polymers, which are applied in paints and adhesives.

Ester production

The major esters of acetic acid are commonly used solvents for inks, paints and coatings. The esters include ethyl acetate, n-butyl acetate, isobutyl acetate, and propyl acetate. They are typically produced by catalysed reaction from acetic acid and the corresponding alcohol:

H₃C-COOH + HO-R → H₃C-CO-O-R + H₂O, (R = a general alkyl group)

Most acetate esters, however, are produced from acetaldehyde using the Tishchenko reaction. In addition, ether acetates are used as solvents for nitrocellulose, acrylic lacquers, varnish removers, and wood stains. First, glycol monoethers are produced from ethylene oxide or propylene oxide with alcohol, which are then esterified with acetic acid. The three major products are ethylene glycol monoethyl ether acetate (EEA), ethylene glycol monobutyl ether acetate (EBA), and propylene glycol monomethyl ether acetate (PMA). This application consumes about 15% to 20% of worldwide acetic acid. Ether acetates, for example EEA, have been shown to be harmful to human reproduction.^[13]

Acetic anhydride

The condensation product of two molecules of acetic acid is acetic anhydride. The worldwide production of acetic anhydride is a major application, and uses approximately 25% to 30% of the global production of acetic acid. Acetic anhydride may be produced directly by methanol carbonylation bypassing the acid, and Cativa production plants can be adapted for anhydride production.

Acetic anhydride is a strong acetylation agent. As such, its major application is for cellulose acetate, a synthetic textile also used for photographic film. Acetic anhydride is also a reagent for the production of aspirin, heroin, and other compounds.

Vinegar

In the form of vinegar, acetic acid solutions (typically 4% to 18% acetic acid, with the percentage usually calculated by mass) are used directly as a condiment, and also in the pickling of vegetables and other foods. Table vinegar tends to be more diluted (4% to 8% acetic acid), while commercial food pickling, in general, employs more concentrated solutions. The amount of acetic acid used as vinegar on a worldwide scale is not large, but is by far the oldest and best-known application.

Use as solvent

Glacial acetic acid is an excellent polar protic solvent, as noted above. It is frequently used as a solvent for recrystallization to purify organic compounds. Pure acetic acid is used as a solvent in the production of terephthalic acid (TPA), the raw material for polyethylene terephthalate (PET). In 2006, about 20% of acetic acid is used for TPA production.^[13]

Acetic acid is often used as a solvent for reactions involving carbocations, such as Friedel-Crafts alkylation. For example, one stage in the commercial manufacture of synthetic camphor involves a Wagner-Meerwein rearrangement of camphene to isobornyl acetate; here acetic acid acts both as a solvent and as a nucleophile to trap the rearranged carbocation. Acetic acid is the solvent of choice when reducing an aryl nitro-group to aniline using palladium-on-carbon.

Glacial acetic acid is used in analytical chemistry for the estimation of weakly alkaline substances such as organic amides. Glacial acetic acid is a much weaker base than water, so the amide behaves as a strong base in this medium. It then can be titrated using a solution in glacial acetic acid of a very strong acid, such as perchloric acid.

Other applications

Dilute solutions of acetic acids are also used for their mild acidity. Examples in the household environment include the use in a stop bath during the development of photographic films, and in descaling agents to remove limescale from taps and kettles.

Dilute solutions of glacial acetic acid can be used in the clinical laboratory to lyse red blood cells in order to do manual white blood cell counts. Another clinical use is for lysing red blood cells, which can obscure other important constituents in urine during a microscopic examination.

The acidity is also used for treating the sting of the box jellyfish by disabling the stinging cells of the jellyfish, preventing serious injury or death if applied immediately, and for treating outer ear infections in people in preparations such as Vosol. In this manner, acetic acid is used as a spray-on preservative for livestock silage, to discourage bacterial and fungal growth. Glacial acetic acid is also used as a wart and verruca remover.

Organic or inorganic salts are produced from acetic acid, including:

• Sodium acetate, used in the textile industry and as a food preservative (E262).
• Copper(II) acetate, used as a pigment and a fungicide.
• Aluminium acetate and iron(II) acetate—used as mordants for dyes.
• Palladium(II) acetate, used as a catalyst for organic coupling reactions such as the Heck reaction.
• Silver acetate, used as a pesticide.

Substituted acetic acids produced include:

• Monochloroacetic acid (MCA), dichloroacetic acid (considered a by-product), and trichloroacetic acid. MCA is used in the manufacture of indigo dye.
• Bromoacetic acid, which is esterified to produce the reagent ethyl bromoacetate.
• Trifluoroacetic acid, which is a common reagent in organic synthesis.

Amounts of acetic acid used in these other applications together (apart from TPA) account for another 5–10% of acetic acid use worldwide. These applications are, however, not expected to grow as much as TPA production.^[13] Diluted acetic acid is also used in physical therapy to break up nodules of scar tissue via iontophoresis.

Safety

Concentrated acetic acid is corrosive and must, therefore, be handled with appropriate care, since it can cause skin burns, permanent eye damage, and irritation to the mucous membranes. These burns or blisters may not appear until hours after exposure. Latex gloves offer no protection, so specially resistant gloves, such as those made of nitrile rubber, are worn when handling the compound. Concentrated acetic acid can be ignited with difficulty in the laboratory. It becomes a flammable risk if the ambient temperature exceeds 39 °C (102 °F), and can form explosive mixtures with air above this temperature (explosive limits: 5.4–16%).

The hazards of solutions of acetic acid depend on the concentration. The following table lists the EU classification of acetic acid solutions:

Solutions at more than 25% acetic acid are handled in a fume hood because of the pungent, corrosive vapour. Dilute acetic acid, in the form of vinegar, is harmless. However, ingestion of stronger solutions is dangerous to human and animal life. It can cause severe damage to the digestive system, and a potentially lethal change in the acidity of the blood.

Due to incompatibilities, it is recommended to keep acetic acid away from chromic acid, ethylene glycol, nitric acid, perchloric acid, permanganates, peroxides and hydroxyls.

See also

• Acetyl group, the CH₃-CO– group, abbreviated Ac
• Acids in wine
• Common chemicals, where to purchase common chemicals used in experiments
• Sodium citrate

References

1. ^ IUPAC Provisional Recommendations 2004 Chapter P-12.1; page 4
2. ^ Akeroyd, F. Michael (1993). "Laudan's Problem Solving Model". The British Journal for the Philosophy of Science 44 (4): 785–88. doi:10.1093/bjps/44.4.785.
3. ^ ^{a b c} Martin, Geoffrey (1917). Industrial and Manufacturing Chemistry (Part 1, Organic ed.). London: Crosby Lockwood. pp. 330–31.
4. ^ Goldwhite, Harold (September 2003). "Short summary of the career of the German organic chemist, Hermann Kolbe" (PDF). New Haven Section Bull. Am. Chem. Soc. 20 (3). http://membership.acs.org/N/NewHaven/bulletins/Bulletin_2003-09.pdf.
5. ^ Schweppe, Helmut (1979). "Identification of dyes on old textiles". J. Am. Inst. Conservation (Journal of the American Institute for Conservation, Vol. 19, No. 1) 19 (1/3): 14–23. doi:10.2307/3179569. http://aic.stanford.edu/jaic/articles/jaic19-01-003_1.html.
6. ^ Jones, R.E.; Templeton, D.H. (1958). "The crystal structure of acetic acid". Acta Crystallogr. 11 (7): 484–87. doi:10.1107/S0365110X58001341.
7. ^ Briggs, James M.; Toan B. Nguyen, William L. Jorgensen (1991). "Monte Carlo simulations of liquid acetic acid and methyl acetate with the OPLS potential functions". J. Phys. Chem. 95: 3315–22. doi:10.1021/j100161a065.
8. ^ Togeas, James B. (2005). "Acetic Acid Vapor: 2. A Statistical Mechanical Critique of Vapor Density Experiments". J. Phys. Chem. A 109 (24): 5438. doi:10.1021/jp058004j. PMID 16839071.
9. ^ Zieborak, K.; K. Olszewski (1958). Bull.Acad.Pol.Sci.Ser.Sci.Chim.Geol.Geogr. 6 (2): 3315–22.
10. ^ executive ed.: J. Buckingham (1996). Dictionary of Organic Compounds. 1 (6th ed.). London: Chapman & Hall. ISBN 0-412-54090-8.
11. ^ Yoneda, N.; Kusano, S.; Yasui, M.; Pujado, P.; Wilcher, S. (2001). "Recent advances in processes and catalysts for the production of acetic acid". Applied Catalysis A, General 221 (1-2): 253–265. doi:10.1016/S0926-860X(01)00800-6.
12. ^ "Production report". Chem. Eng. News: 67–76. 11 July 2005.
13. ^ ^{a b c d e} Suresh, Bala (2003). "Acetic Acid". Chemicals Economic Handbook. SRI International. pp. 602.5000. http://www.sriconsulting.com/CEH/Public/Reports/602.5000/.
14. ^ Wagner, Frank S. (1978). "Acetic acid". in Grayson, Martin. Kirk-Othmer Encyclopedia of Chemical Technology (3rd ed.). New York: John Wiley & Sons.
15. ^ Lancaster, Mike (2002). Green Chemistry, an Introductory Text. Cambridge: Royal Society of Chemistry. pp. 262–66. ISBN 0-85404-620-8.
16. ^ "Acetic acid". National Institute of Standards and Technology. http://webbook.nist.gov/cgi/cbook.cgi?ID=C64197&Units=SI&Mask=4#Thermo-Phase. Retrieved 2008-02-03.
17. ^ Sano, Ken-ichi; Uchida, Hiroshi; Wakabayashi, Syoichirou (1999). A new process for acetic acid production by direct oxidation of ethylene. 3. 66–60. doi:10.1023/A:1019003230537.
18. ^ ^{a b} Otto Hromatka and Heinrich Ebner (1959). "Vinegar by Submerged Oxidative Fermentation". Ind. Eng. Chem. 51 (10): 1279–1280. doi:10.1021/ie50598a033.
19. ^ Everett P. Partridge (1931). "Acetic Acid and Cellulose Acetate in the United States A General Survey of Economic and Technical Developments". Ind. Eng. Chem. 23 (5): 482 –498. doi:10.1021/ie50257a005.
20. ^ O Hromatka, H Ebner (1949). "Investigations on vinegar fermentation: Generator for vinegar fermentation and aeration procedures". Enzymologia 13: 369.
21. ^ Jia Huey Sim, Azlina Harun Kamaruddin, Wei Sing Long and Ghasem Najafpour (2007). "Clostridium aceticum—A potential organism in catalyzing carbon monoxide to acetic acid: Application of response surface methodology". Enzyme and Microbial Technology 40 (5): 1234–1243. doi:10.1016/j.enzmictec.2006.09.017.

External links

• International Chemical Safety Card 0363
• National Pollutant Inventory - Acetic acid fact sheet
• NIOSH Pocket Guide to Chemical Hazards
• Method for sampling and analysis
• 29 CFR 1910.1000, Table Z-1 (US Permissible exposure limits)
• ChemSub Online: Acetic acid
• Usage of acetic acid in Organic Syntheses
• Acetic acid pH and titration - freeware for data analysis, simulation and distribution diagram generation
• Calculation of vapor pressure, liquid density, dynamic liquid viscosity, surface tension of acetic acid

Categories: Otologicals | Acetates | Cigarette additives | Flavors | Household chemicals | Oenology | Photographic chemicals | Solvents | World Health Organization essential medicines | Acetic acids

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