Acetic acid/əˈsiːtɪk/, systematically namedethanoic acid/ˌɛθəˈnoʊɪk/, is an acidic, colourless liquid andorganic compound with thechemical formulaCH3COOH (also written asCH3CO2H,C2H4O2, orHC2H3O2).Vinegar is at least 4% acetic acid by volume, making acetic acid the main component of vinegar apart from water. It has been used, as a component of vinegar, throughout history from at least the third century BC.
The global demand for acetic acid as of 2023 is about 17.88 millionmetric tonnes per year (t/a). Most of the world's acetic acid is produced via thecarbonylation ofmethanol. Its production and subsequent industrial use poses health hazards to workers, including incidental skin damage and chronic respiratory injuries from inhalation.
Thetrivial name "acetic acid" is the most commonly used andpreferred IUPAC name. The systematic name "ethanoic acid", a validIUPAC name, is constructed according to the substitutive nomenclature.[8] The name "acetic acid" derives from theLatin word forvinegar, "acetum", which is related to the word "acid" itself.
"Glacial acetic acid" is a name for water-free (anhydrous) acetic acid. Similar to theGerman name "Eisessig" ("ice vinegar"), the name comes from the solid ice-like crystals that form with agitation, slightly below room temperature at 16.6 °C (61.9 °F). 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.[9]
A commonsymbol for acetic acid is AcOH (or HOAc), where Ac is thepseudoelement symbol representing theacetylgroupCH3−C(=O)−; theconjugate base,acetate (CH3COO−), is thus represented asAcO−.[10] Acetate is theion resulting from loss ofH+ from acetic acid. The name "acetate" can also refer to asalt containing this anion, or anester of acetic acid.[11] (The symbol Ac for the acetyl functional group is not to be confused with the symbol Ac for the elementactinium; context prevents confusion among organic chemists). To better reflect its structure, acetic acid is often written asCH3−C(O)OH,CH3−C(=O)−OH,CH3COOH, andCH3CO2H. In the context ofacid–base reactions, the abbreviation HAc is sometimes used,[12] where Ac in this case is a symbol for acetate (rather than acetyl).
The carboxymethyl functional group derived from removing one hydrogen from themethyl group of acetic acid has thechemical formula−CH2−C(=O)−OH.
Vinegar was known early in civilization as the natural result of exposure ofbeer andwine to air because acetic acid-producing bacteria are present globally. The use of acetic acid inalchemy extends into the third century BC, when the Greek philosopherTheophrastus described how vinegar acted on metals to producepigments useful in art, includingwhite lead (lead carbonate) andverdigris, a green mixture ofcopper salts includingcopper(II) acetate. AncientRomans boiled soured wine to produce a highly sweet syrup calledsapa.Sapa that was produced in lead pots was rich inlead acetate, a sweet substance also calledsugar of lead orsugar ofSaturn, which contributed tolead poisoning among the Roman aristocracy.[13]
In the 16th-centuryGerman alchemistAndreas Libavius described the production ofacetone from thedry 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 chemistPierre Adet proved them identical.[13][14]
By 1910, most glacial acetic acid was obtained from thepyroligneous liquor, a product of the distillation of wood. The acetic acid was isolated by treatment withmilk of lime, and the resultingcalcium acetate was then acidified withsulfuric acid to recover acetic acid. At that time, Germany was producing 10,000tons of glacial acetic acid, around 30% of which was used for the manufacture ofindigo dye.[13][16]
Because bothmethanol andcarbon monoxide are commodity raw materials, methanol carbonylation long appeared to be attractive precursors to acetic acid.Henri Dreyfus atBritish Celanese developed a methanol carbonylation pilot plant as early as 1925.[17] However, a lack of practical materials that could contain the corrosive reaction mixture at the highpressures needed (200atm or more) discouraged commercialization of these routes. The first commercial methanol carbonylation process, which used acobalt catalyst, was developed by German chemical companyBASF in 1963. In 1968, arhodium-based catalyst (cis−[Rh(CO)2I2]−) was discovered that could operate efficiently at lower pressure with almost no by-products. US chemical companyMonsanto Company built the first plant using this catalyst in 1970, and rhodium-catalyzed methanol carbonylation became the dominant method of acetic acid production (seeMonsanto process). In the late 1990s,BP Chemicals commercialised the Cativa catalyst ([Ir(CO)2I2]−), which is promoted byiridium for greater efficiency.[18] Known as theCativa process, theiridium-catalyzed production of glacial acetic acid isgreener, and has largely supplanted the Monsanto process, often in the same production plants.[19]
The hydrogen centre in thecarboxyl group (−COOH) in carboxylic acids such as acetic acid can separate from the molecule by ionization:
CH3COOH ⇌ CH3CO−2 + H+
Because of this release of theproton (H+), acetic acid has acidic character. Acetic acid is a weakmonoprotic acid. In aqueous solution, it has apKa value of 4.76.[21] Itsconjugate base isacetate (CH3COO−). A 1.0 M solution (about the concentration of domestic vinegar) has apH of 2.4, indicating that merely 0.4% of the acetic acid molecules are dissociated.[a]
Cyclic dimer of acetic acid; dashedgreen lines represent hydrogen bonds
In solid acetic acid, the molecules form chains of individual molecules interconnected byhydrogen bonds.[22] In the vapour phase at 120 °C (248 °F),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,[23] but are disrupted by hydrogen-bonding solvents. The dissociationenthalpy of the dimer is estimated at 65.0–66.0 kJ/mol, and the dissociation entropy at 154–157 J mol−1 K−1.[24] Other carboxylic acids engage in similar intermolecular hydrogen bonding interactions.[25]
Liquid acetic acid is ahydrophilic (polar)protic solvent, similar toethanol andwater. With arelative static permittivity (dielectric constant) of 6.2, it dissolves not only polar compounds such as inorganic salts andsugars, but also non-polar compounds such as oils as well as polar solutes. It is miscible with polar and non-polarsolvents such as water,chloroform, andhexane. With higher alkanes (starting withoctane), acetic acid is notmiscible at all compositions, and solubility of acetic acid in alkanes declines with longer n-alkanes.[26] The solvent andmiscibility properties of acetic acid make it a useful industrial chemical, for example, as a solvent in the production ofdimethyl terephthalate.[27]
At physiological pHs, acetic acid is usually fully ionised toacetate in aqueous solution.[28]
Theacetylgroup, formally derived from acetic acid, is fundamental to all forms of life. Typically, it is bound tocoenzyme A byacetyl-CoA synthetase enzymes,[29] where it is central to themetabolism ofcarbohydrates andfats. Unlike longer-chain carboxylic acids (thefatty acids), acetic acid does not occur in naturaltriglycerides. Most of the acetate generated in cells for use inacetyl-CoA is synthesized directly fromethanol orpyruvate.[30] However, the artificial triglyceridetriacetin (glycerine triacetate) is a common food additive and is found in cosmetics and topical medicines; this additive is metabolized toglycerol and acetic acid in the body.[31]
Purification and concentration plant for acetic acid in 1884
Acetic acid is produced industrially both synthetically and by bacterialfermentation. About 75% of acetic acid made for use in the chemical industry is made by thecarbonylation ofmethanol, explained below.[27] 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 aremethyl formate isomerization, conversion ofsyngas to acetic acid, and gas phase oxidation ofethylene andethanol.[33]
Acetic acid can be purified viafractional freezing using an ice bath. The water and otherimpurities will remain liquid while the acetic acid willprecipitate out. As of 2003–2005, total worldwide production of virgin acetic acid[b] was estimated at 5 Mt/a (million tonnes per year), 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.[34][35] Since then, the global production has increased from 10.7 Mt/a in 2010[36] to 17.88 Mt/a in 2023.[37] The two biggest producers of virgin acetic acid areCelanese andBP Chemicals. Other major producers includeMillennium Chemicals,Sterling Chemicals,Samsung,Eastman, andSvensk Etanolkemi [sv].[38]
Most acetic acid is produced by methanolcarbonylation. In this process,methanol andcarbon monoxide react to produce acetic acid according to the equation:
The process involvesiodomethane as an intermediate, and occurs in three steps. Ametal carbonylcatalyst is needed for the carbonylation (step 2).[33]
CH3OH + HI → CH3I + H2O
CH3I + CO → CH3COI
CH3COI + H2O → CH3COOH + HI
Two related processes exist for the carbonylation of methanol: the rhodium-catalyzedMonsanto process, and the iridium-catalyzedCativa process. The latter process isgreener and more efficient and has largely supplanted the former process.[19] Catalytic amounts of water are used in both processes, but the Cativa process requires less, so thewater-gas shift reaction is suppressed, and fewer by-products are formed.
By altering the process conditions,acetic anhydride may also be produced in plants using rhodium catalysis.[39]
Prior to the commercialization of the Monsanto process, most acetic acid was produced by oxidation ofacetaldehyde. This remains the second-most-important manufacturing method, although it is usually not competitive with the carbonylation of methanol. The acetaldehyde can be produced byhydration of acetylene. This was the dominant technology in the early 1900s.[40]
Lightnaphtha components are readily oxidized by oxygen or even air to giveperoxides, which decompose to produce acetic acid according to thechemical equation, illustrated withbutane:
The typical reaction is conducted attemperatures and pressures designed to be as hot as possible while still keeping the butane a liquid. Typical reaction conditions are 150 °C (302 °F) and 55 atm.[41] Side-products may also form, includingbutanone,ethyl acetate,formic acid, andpropionic 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.[42]
Acetaldehyde may be prepared fromethylene via theWacker process, and then oxidised as above.
In more recent times, chemical companyShowa Denko, which opened an ethylene oxidation plant inŌita, Japan, in 1997, commercialised a cheaper single-stage conversion of ethylene to acetic acid.[42] The process is catalyzed by apalladium metal catalyst supported on aheteropoly acid such assilicotungstic acid. A similar process uses the same metal catalyst on silicotungstic acid and silica:[43]
C2H4 + O2 → CH3CO2H
It is thought to be competitive with methanol carbonylation for smaller plants (100–250 kt/a), depending on the local price of ethylene.
For most of human history, acetic acid bacteria of the genusAcetobacter have made acetic acid, in the form of vinegar. Given sufficient oxygen, these bacteria can produce vinegar from a variety of alcoholic foodstuffs. Commonly used feeds includeapple cider,wine, and fermentedgrain,malt,rice, orpotato mashes. The overall chemical reaction facilitated by these bacteria is:
C2H5OH + O2 → CH3COOH + H2O
A dilute alcohol solution inoculated withAcetobacter 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 ofoxygen to the bacteria.[44]
The first batches of vinegar produced by fermentation probably followed errors in thewinemaking process. Ifmust is fermented at too high a temperature, acetobacter will overwhelm theyeast naturally occurring on thegrapes. 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.[45]
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 orcharcoal. The alcohol-containing feed is trickled into the top of the tower, and freshair supplied from the bottom by either natural or forcedconvection. The improved air supply in this process cut the time to prepare vinegar from months to weeks.[46]
Nowadays, most vinegar is made in submerged tankculture, first described in 1949 by Otto Hromatka and Heinrich Ebner.[47] 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.[45]
Species ofanaerobic bacteria, including members of the genusClostridium orAcetobacterium, can convert sugars to acetic acid directly without creating ethanol as an intermediate. The overall chemical reaction conducted by these bacteria may be represented as:
This ability ofClostridium to metabolize sugars directly, or to produce acetic acid from less costly inputs, suggests that these bacteria could produce acetic acid more efficiently than ethanol-oxidizers likeAcetobacter. However,Clostridium bacteria are less acid-tolerant thanAcetobacter. Even the most acid-tolerantClostridium strains can produce vinegar in concentrations of only a few per cent, compared toAcetobacter strains that can produce vinegar in concentrations up to 20%. At present, it remains more cost-effective to produce vinegar usingAcetobacter, rather than usingClostridium and concentrating it. As a result, although acetogenic bacteria have been known since 1940, their industrial use is confined to a few niche applications.[48]
Acetic acid is a chemicalreagent for the production of chemical compounds. The largest single use of acetic acid is in the production of vinyl acetatemonomer, closely followed by acetic anhydride and ester production. The volume of acetic acid used in vinegar is comparatively small.[27][34]
The primary use of acetic acid is the production ofvinyl acetate monomer (VAM). In 2008, this application was estimated to consume a third of the world's production of acetic acid.[27] The reaction consists ofethylene and acetic acid withoxygen over apalladiumcatalyst, conducted in the gas phase.[49]
Most acetateesters, however, are produced fromacetaldehyde using theTishchenko reaction. In addition, ether acetates are used as solvents fornitrocellulose,acrylic lacquers,varnish removers, and wood stains. First, glycol monoethers are produced fromethylene oxide orpropylene 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, more commonly known as PGMEA in semiconductor manufacturing processes, where it is used as a resist solvent). 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.[34]
The product of thecondensation of two molecules of acetic acid isacetic anhydride. The worldwide production of acetic anhydride is a major application, and uses approximately 25% to 30% of the global production of acetic acid. The main process involves dehydration of acetic acid to giveketene at 700–750 °C. Ketene is thereafter reacted with acetic acid to obtain the anhydride:[50]
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 weakerbase 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 asperchloric acid.[52]
Acetic acid is an effective antiseptic when used as a 1% solution, with broad spectrum of activity against streptococci, staphylococci, pseudomonas, enterococci and others.[56][57][58] It may be used to treat skin infections caused by pseudomonas strains resistant to typical antibiotics.[59]
While diluted acetic acid is used iniontophoresis, no high quality evidence supports this treatment for rotator cuff disease.[60][61]
Acetic acid has 349 kcal (1,460 kJ) per 100 g.[63] Vinegar is typically no less than 4% acetic acid by mass.[64][65][66] Legal limits on acetic acid content vary by jurisdiction. Vinegar is used directly as acondiment, and in thepickling of vegetables and other foods. Table vinegar tends to be more diluted (4% to 8% acetic acid), while commercial food pickling employs solutions that are more concentrated. The proportion of acetic acid used worldwide as vinegar is not as large as industrial uses, but it is by far the oldest and best-known application.[67]
Acetic acid undergoes the typicalchemical reactions of a carboxylic acid. Upon treatment with a standard base, it converts to metalacetate andwater. With strong bases (e.g., organolithium reagents), it can be doubly deprotonated to giveLiCH2COOLi. Reduction of acetic acid gives ethanol. The OH group is the main site of reaction, as illustrated by the conversion of acetic acid toacetyl chloride. Other substitution derivatives includeacetic anhydride; thisanhydride is produced byloss of water from two molecules of acetic acid.Esters of acetic acid can likewise be formed viaFischer esterification, andamides can be formed. When heated above 440 °C (824 °F), acetic acid decomposes to producecarbon dioxide andmethane, or to produceketene and water:[68][69][70]
Prolonged inhalation exposure (eight hours) to acetic acid vapours at 10 ppm can produce some irritation of eyes, nose, and throat; at 100 ppm marked lung irritation and possible damage to lungs, eyes, and skin may result. Vapour concentrations of 1,000 ppm cause marked irritation of eyes, nose and upper respiratory tract and cannot be tolerated. These predictions were based onanimal experiments and industrial exposure.[75]
In 12 workers exposed for two or more years to an airborne average concentration of 51 ppm acetic acid (estimated), symptoms of conjunctive irritation, upper respiratory tract irritation, and hyperkeratotic dermatitis were produced. Exposure to 50 ppm or more is intolerable to most persons and results in intensivelacrimation and irritation of the eyes, nose, and throat, with pharyngeal oedema and chronic bronchitis. Unacclimatised humans experience extreme eye and nasal irritation at concentrations in excess of 25 ppm, and conjunctivitis from concentrations below 10 ppm has been reported. In a study of five workers exposed for seven to 12 years to concentrations of 80 to 200 ppm at peaks, the principal findings were blackening and hyperkeratosis of the skin of the hands, conjunctivitis (but no corneal damage), bronchitis and pharyngitis, and erosion of the exposed teeth (incisors and canines).[76]
Concentrated acetic acid (≥ 25%) iscorrosive to skin.[77] These burns or blisters may not appear until hours after exposure.[78] The hazardous properties of acetic acid are dependent on the concentration of the (typicallyaqueous) solution, with the most significant increases in hazard levels at thresholds of 25% and 90% acetic acid concentration by weight. The following table summarizes the hazards of acetic acid solutions by concentration:[79]
Concentrated acetic acid can be ignited only with difficulty at standard temperature and pressure, but becomes a flammable risk in temperatures greater than 39 °C (102 °F), and can form explosive mixtures with air at higher temperatures withexplosive limits of 5.4–16% concentration.
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