The chemical compound acetic acid (from the Latin word acetum, meaning "vinegar"), systematically called ethanoic acid, is the acid that gives vinegar its sour taste. It is a carboxylic acid with chemical formula C2H4O2, often written as CH3COOH to better reflect the structure shown at right.
Acetic acid is a molecule central to biochemistry, and is produced in some amount by nearly all forms of life. The Acetobacter genus of bacteria is named for its tendency to produce acetic acid, and these bacteria are found universally in foodstuffs, water, and soil. As such, acetic acid is produced naturally as fruits and some other foods spoil, and it is one of the oldest chemicals known to humanity.
Pure acetic acid is a colorless, corrosive, flammable liquid that freezes at 16.6 °C. Because pure acetic acid freezes only slightly below room temperature and has an ice-like appearance when it does so, it is often called glacial acetic acid.
In aqueous solution, acetic acid can lose the proton of its carboxyl group, turning into the acetate ion CH3COO-. The pKa of acetic acid is about 4.8 at 25 °C, meaning that about half of the acetic acid molecules are in the acetate form at a pH of 4.8.
As a vapor, acetic acid consists of pairs of dimers held together by hydrogen bonds. As a result, the ideal gas law describes relatively poorly the behaviour of acetic acid vapour, since the "law" does not take into account the intermolecular interactions present. The dimers look like this:
H O ..... H--O H
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H O--H ..... O H
Chemically, acetic acid shares most of the properties of carboxylic acids in general, including the ability to react with alcohols and amines to produce esters and amides, respectively. In addition, it can react with alkenes to produce acetate esters. When heated above 440°C, it decomposes to produce carbon dioxide and methane, or to produce ketene and water.
Acetic acid, when complexed with coenzyme A, is central to the metabolism and biosynthetic processes of almost all forms of life. It results naturally from the action of certain bacteria in foods or liquids containing sugars or ethanol.
As an example of its importance in biology, acetic acid is produced in the human body after the consumption of alcoholic beverages. The ethanol is first converted into acetaldehyde, which is then converted into acetic acid by the enzyme acetaldehyde dehydrogenase and converted further to acetyl-CoA by acetate-CoA ligase .
Vinegar is as old as civilization itself, if not older. Acetic acid-producing bacteria are universally present, and any culture practicing the brewing of beer or wine inevitably discovered vinegar as the natural result of these alcoholic beverages being exposed to air.
The use of acetic acid in chemistry extends into antiquity. The Greek philosopher Theophrastos described in the third century BC how vinegar acted on metals to produce pigments useful in art, including white lead (lead carbonate) and verdigris (a green mixture of copper salts including cupric 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.
Renaissance-era alchemists prepared glacial acetic acid through the dry distillation of metal acetates. 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 glacial acetic acid and the acid found in vinegar were believed to be two different substances. The French chemist Pierre Adet proved them to be identical, and in 1847, the German chemist Hermann Kolbe synthesized acetic acid from inorganic materials for the first time.
Vinegar is manufactured by fermenting various starchy, sugary, or alcoholic foodstuffs with Acetobacter bacteria. Commonly used feeds include apple cider, wine, and grain or potato mashes. The vinegar is then distilled from the fermentation broth. Most nations have laws that the acetic acid found in vinegar must be produced by fermentation rather than by non-biological means. Vinegar is ususally 4-8% acetic acid by volume.
Most acetic acid made for industrial use is made by one of three chemical processes: methanol carbonylation, butane oxidation, or acetaldehyde oxidation.
In methanol carbonylation, methanol and carbon monoxide react to produce acetic acid according to the chemical equation
CH3OH + CO → CH3COOH
Because both methanol and carbon monoxide are extremely inexpensive, methanol carbonylation long appeared to be an attractive method for acetic acid production, and patents on such processes were granted as early as the 1920's. However, 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 BASF in the early 1960's. In 1968, however, a rhodium-based catalyst was discovered that could operate efficiently at lower pressure, with almost no byproducts. The first plant using this catalyst was built by Monsanto in 1970, and rhodium-catalyzed methanol carbonylation became the dominant method of acetic acid production. In the late 1990's, BP Chemicals commercialised an iridium-catalyzed process, CativaTM, which now operates on a number of plants.
When butane 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
Typically, the 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. Several 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 if this is economically useful.
Butane oxidation was the major source of acetic acid before methanol carbonylation became widely commercialized in the 1980s. Today, however, it produces less than 10% of the acetic acid supply.
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 have lower boiling points than acetic acid and are readily separated by distillation. Today, acetaldehyde oxidation is the second most widely-used method of acetic acid production.
In the form of vinegar, acetic acid is used directly as a condiment, and also in the pickling of vegetables and other foodstuffs. Acetic acid is also sprayed onto silage as a preservative to discourage bacterial and fungal growth.
The glacial acetic acid produced by the chemical industry is used in the manufacture of photographic films and stop bath and sometimes in the production of the plastic polyethylene terephthalate (PET). It is also used as an intermediate for the production of vinyl acetate , an important chemical in the paint and adhesives industry, and for cellulose acetate, a synthetic textile.
Dilute solutions (4% - 6%) of acetic acid are extremely useful in treating the sting of the box jellyfish; the acid disables the stinging cells of the jellyfish, and can prevent serious injury or death if immediately applied.
Some of the esters of acetic acid are commonly used solvents and artificial flavorings.
Concentrated acetic acid (also known as glacial acetic acid) is corrosive and has to be handled with extra care since it can cause skin burns, permanent eye damage, and irritation to the mucous membranes. It can also penetrate the skin, and cause acidic burns or blisters to appear several hours after exposure.
Dilute acetic acid (in the form of vinegar) is harmless and has been consumed for millennia. However, ingestion of stronger solutions is dangerous. It can cause severe damage to the digestive system, and a potentially lethal change in the acidity of the blood.
Acetic acid poses no known cancer risk.
See the MSDS (Material Safety Data Sheet)