Acetic acid /əˈsiːtᵻk/, systematically named ethanoic acid /ˌɛθəˈnoʊᵻk/, is a colourless liquid organic compound with the chemical formula CH3COOH (also written as CH3CO2H or C2H4O2). When undiluted, it is sometimes called glacial acetic acid. Vinegar is roughly 3–9% acetic acid by volume, making acetic acid the main component of vinegar apart from water. Acetic acid has a distinctive sour taste and pungent smell. In addition to household vinegar, it is mainly produced as a precursor to polyvinyl acetateand cellulose acetate. Although it is classified as a weak acid, concentrated acetic acid is corrosive and can attack the skin.
Acetic acid is the second simplest carboxylic acid (after formic acid) and consists of two small functional groups, an acetyl group (sometimes symbolized as Ac) and a hydroxyl group (AcOH); it can also be viewed as a methyl group and a carboxyl group linked. It is an important chemical reagent and industrial chemical, 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. As a food additive it is approved for usage in many countries, including Canada, the European Union, the United States, Australia and New Zealand. In biochemistry, the acetyl group, derived from acetic acid, is fundamental to all forms of life. When bound tocoenzyme A, it is central to the metabolism of carbohydrates and fats.
The global demand for acetic acid is about 6.5 million metric tons per year (Mt/a), of which approximately 1.5 Mt/a is met by recycling; the remainder is manufactured from petrochemical feedstock. As a chemical reagent, biological sources of acetic acid are of interest, but generally cannot compete economically. Vinegar is dilute acetic acid, often produced by fermentation and subsequent oxidation of ethanol.
The trivial name acetic acid is the most commonly used and preferred IUPAC name. The systematic name ethanoic acid, a validIUPAC name, is constructed according to the substitutive nomenclature. The name acetic acid derives from acetum, the Latin word for vinegar, and is related to the word acid itself.
Glacial acetic acid is a name for water-free (anhydrous) acetic acid. Similar to the German name Eisessig (ice-vinegar), the name comes from the ice-like crystals that form slightly below room temperature at 16.6 °C (61.9 °F) (the presence of 0.1% water lowers its melting point by 0.2 °C).
The hydrogen centre in the carboxyl group (−COOH) in carboxylic acids such as acetic acid can separate from the molecule by ionization:
- CH3CO2H → CH3CO2− + H+
Because of this release of the proton (H+), acetic acid has acidic character. Acetic acid is a weak monoprotic acid. In aqueous solution, it has a pKa value of 4.76.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.
In solid acetic acid, the molecules form pairs (dimers), being connected by hydrogen bonds. The dimers can also be detected in the vapour at 120 °C (248 °F). Dimers also occur in the liquid phase in dilute solutions in non-hydrogen-bonding solvents, and 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 lower carboxylic acids dimerize in a similar fashion.
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 dissolves 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, and its miscibility 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.
At physiological pHs, acetic acid is usually fully ionised to acetate. 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. Unlike longer-chain carboxylic acids (the fatty acids), acetic acid does not occur in natural triglycerides. However, the artificial triglyceride triacetin (glycerine triacetate) is a common food additive and is found in cosmetics and topical medicines.
Acetic acid is produced and excreted by acetic acid bacteria, notably 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 mildantibacterial agent.
Most acetic acid is produced by methanol carbonylation. In this process, methanol and carbon monoxide react to produce acetic acid according to the equation:
The process involves iodomethane as an intermediate, and occurs in three steps. A catalyst, metal carbonyl, is needed for the carbonylation (step 2).
CH3OH + HI → CH3I + H2O
- CH3I + CO → CH3COI
- CH3COI + H2O → CH3COOH + HI
Two related processes for the carbonylation of methanol: the rhodium-catalyzed Monsanto process, and the iridium-catalyzed Cativa process. The latter process is greener and more efficient and has largely supplanted the former process, often in the same production plants. Catalytic amounts of water are used in both processes, but the Cativa process requires less, so the water-gas shift reaction is suppressed, and fewer by-products are formed.
By altering the process conditions, acetic anhydride may also be produced on the same plant using the rhodium catalysts.
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 usually not competitive with the carbonylation of methanol.
The acetaldehyde may be produced by oxidation of butane or light naphtha, or by hydration of ethylene. When butane or lightnaphtha is heated with air in the presence of various metal ions, including those of manganese, cobalt, and chromium, peroxidesform and then decompose to produce acetic acid according to the chemical equation:
- 2 C4H10 + 5 O2 → 4 CH3COOH + 2 H2O
The typical reaction is conducted at temperatures 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. 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, the oxygen in air to produce acetic acid can oxidize acetaldehyde.
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.
Acetaldehyde may be prepared from ethylene via the Wacker process, and then oxidised as above. In more recent times, chemical company Showa Denko, which opened an ethylene oxidation plant in Ōita, Japan, in 1997, commercialised a cheaper single-stage conversion of ethylene to acetic acid. The process is catalyzed 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. The approach will be based on utilizing a novel selective photocatalytic oxidation technology for the selective oxidation of ethylene and ethane to acetic acid. Unlike traditional oxidation catalysts, the selective oxidation process will use UV light to produce acetic acid at ambient temperatures and pressure.
For most of human history, acetic acid bacteria of the genus Acetobacter 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 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.
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.
Nowadays, most vinegar is made in submerged tank culture, first described in 1949 by Otto Hromatka and Heinrich Ebner. 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.
Species of anaerobic bacteria, including members of the genus Clostridium or Acetobacterium 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:
- C6H12O6 → 3 CH3COOH
These acetogenic bacteria produce acetic acid from one-carbon compounds, including methanol, carbon monoxide, or a mixture ofcarbon dioxide and hydrogen:
- 2 CO2 + 4 H2 → CH3COOH + 2 H2O
This ability of Clostridium 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 like Acetobacter. However, Clostridium bacteria are less acid-tolerant than Acetobacter. Even the most acid-tolerant Clostridium strains can produce vinegar in concentrations of only a few per cent, compared to Acetobacter strains that can produce vinegar in concentrations up to 20%. At present, it remains more cost-effective to produce vinegar using Acetobacter, rather than using Clostridium and concentrating it. As a result, although acetogenic bacteria have been known since 1940, their industrial use is confined to a few niche 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.
Vinyl acetate monomer
The primary use of acetic acid is the production of vinyl acetate monomer (VAM). In 2008, this application was estimated to consume a third of the world’s production of acetic acid.The reaction consists of ethylene and acetic acid with oxygen over a palladium catalyst, conducted in the gas phase.
- 2 H3C−COOH + 2 C2H4 + O2 → 2 H3C−CO−O−CH=CH2 + 2 H2O
Vinyl acetate can be polymerised to polyvinyl acetate or other polymers, which are components in paints and adhesives.
The major esters of acetic acid are commonly used as solvents for inks, paints and coatings. The esters include ethyl acetate, n-butyl acetate, isobutyl acetate, and propyl acetate. They are typically produced by catalyzed 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, 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.
The product of the condensation 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. The main process involves dehydration of acetic acid to give ketene at 700–750 °C. Ketene is thereafter reacted with acetic acid to obtain the anhydride:
CH3CO2H → CH2=C=O + H2O
- CH3CO2H + CH2=C=O → (CH3CO)2O
Acetic anhydride is an 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 heroin and other compounds.
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. Acetic acid is used as asolvent in the production of terephthalic acid (TPA), the raw material for polyethylene terephthalate (PET). In 2006, about 20% of acetic acid was used for TPA production.
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 syntheticcamphor 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 rearrangedcarbocation.
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.
Diluted acetic acid is used in physical therapy using iontophoresis.
Acetic acid undergoes the typical chemical reactions of a carboxylic acid. Upon treatment with a standard base, it converts to metal acetate and water. With strong bases (e.g., organolithium reagents), it can be doubly deprotonated to give LiCH2CO2Li. Reduction of acetic acid gives ethanol. The OH group is the main site of reaction, as illustrated by the conversion of acetic acid to acetyl chloride. 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 be formed. When heated above 440 °C (824 °F), acetic acid decomposes to producecarbon dioxide and methane, or to produce ketene and water:
CH3COOH → CH4 + CO2
- CH3COOH → CH2CO + H2O
Reactions with inorganic compounds
Acetic acid is mildly corrosive to metals including iron, magnesium, and zinc, forming hydrogen gas and salts called acetates:
- Mg + 2 CH3COOH → (CH3COO)2Mg + H2
Because aluminium forms a passivating acid-resistant film of aluminium oxide, aluminium tanks are used 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:
- NaHCO3 + CH3COOH → CH3COONa + CO2 + H2O
A colour reaction for salts of acetic acid is iron(III) chloride solution, which results in a deeply red colour that disappears after acidification.A more sensitive test uses lanthanum nitrate with iodine and ammonia to give a blue solution. Acetates when heated with arsenic trioxide form cacodyl oxide, which can be detected by its malodorousvapours.