Acetic acid
, also known as ethanoic acid, is an organic chemical compound best recognized for giving vinegar its sour taste and pungent smell. Its structural
formula is represented as CH3COOH. Pure, water-free acetic acid (glacial acetic acid) is a colourless liquid that attracts water from the environment (hygroscopy), and freezes below 16.7°C (62°F) to a colourless crystalline solid.
Acetic acid is corrosive, and its vapour causes irritation to the eyes, a dry and burning nose, sore throat and congestion to the lungs. It is considered a weak acid because at standard temperature and pressure the dissociated acid
exists in equilibrium with the undissociated form in aqueous solutions, in contrast to strong acids, which are fully dissociated.
Acetic acid is one of the simplest carboxylic acids (the second-simplest, next to formic
acid). It is an important chemical reagent and industrial chemical that is used in the production of polyethylene terephthalate mainly used in soft drink bottles; cellulose acetate, mainly for photographic film; and polyvinyl
acetate for wood glue, as well as many synthetic fibres and fabrics. In households diluted acetic acid is often used in descaling agents. In the food industry acetic acid is used under the food additive code E260 as an acidity
regulator.
The global demand of acetic acid is around 6.5 million tonnes per year (Mt/a), of which approximately 1.5 Mt/a is met by recycling; the remainder is manufactured from petrochemical feedstocks or from biological
sources.
Nomenclature
The trivial name acetic acid is the most commonly used and officially preferred name by the IUPAC. This name derives from acetum, the Latin word for vinegar. The synonym ethanoic acid
is a systematic name that is sometimes used in introductions to chemical nomenclature.
Glacial acetic acid is a trivial name for water-free acetic acid. Similar to the German name Eisessig (literally, ice-vinegar), the name
comes from the ice-like crystals that form slightly below room temperature at 16.7 °C (about 62 °F).
The most common and official abbreviation for acetic acid is AcOH or HOAc where Ac stands for the acetyl group CH3¥C(=O)¥;.
In the context of acid-base reactions the abbreviation HAc is often used where Ac instead stands for the acetate anion (CH3COO¥), although this use is regarded by many as misleading. In either case, the Ac is not to be confused
with the abbreviation for the chemical element actinium.
Acetic acid has the empirical formula CH2O and the molecular formula C2H4O2 or HC2H3O2 (to emphasize the role of the 'active' hydrogen in forming the salt sodium
acetate). To better reflect its structure, acetic acid is often written as CH3-CO2-H, CH3COOH, CH3CO2H, or HOCOCH3. The ion resulting from loss of H+ from acetic acid is the acetate anion. The name acetate can also refer to a salt
containing this anion, or an ester of acetic acid.
History
Crystallized acetic acid
Vinegar is at least as old as civilization. Acetic acid-producing bacteria are present throughout the world, and any culture practicing the
brewing of beer or wine inevitably discovered vinegar as the natural result of their exposure to air.
The use of acetic acid in alchemy extends into antiquity. In the 3rd century BC, the Greek philosopher Theophrastos
described 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 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.
In the 8th
century, the Arab alchemist 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 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 many chemists believed that glacial acetic acid and the acid found in vinegar were two different substances. The French chemist Pierre Adet proved them to be identical.
In 1847 the German
chemist Hermann Kolbe synthesised 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.
Detail of acetic acid crystals
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 this time Germany was producing 10,000
tons of glacial acetic acid, around 30% of which was used for the manufacture of indigo dye.
Solvent
Liquid acetic acid is a hydrophilic (polar) protic solvent, similar to ethanol and water. With a moderate 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 many other polar and non-polar solvents such as
water, chloroform, and hexane. This dissolving property and miscibility of acetic acid makes it a widely used industrial chemical.
Chemical reactions
Acetic acid is corrosive to many 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, so that aluminium tanks can be 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. 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)
Two typical organic reactions of acetic acid
Acetic acid undergoes the typical chemical reactions of a carboxylic acid, such 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 ketene and water.
Detection
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 virtually 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 some 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
certain 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 some 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.
Applications
2.5-litre bottle of acetic acid in a laboratory.
Acetic acid is a
chemical reagent for the production of many 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 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 polymerised 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, where R = a general alkyl group
Most acetate esters, however, are produced from acetaldehyde using the Tishchenko reaction. Additionally, some 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.
Some of these ether acetates, for example EEA, have been shown to be harmful to human reproduction.
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 plants can be adapted
for anhydride production.
Condensation of acetic acid to acetic anhydride
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 5% 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 foodstuffs. Table vinegar tends to be more diluted (5% to 8% acetic acid), while commercial food
pickling generally employs more concentrated solutions. The amount of acetic acid used as vinegar on a worldwide scale is not large, but historically, this is by far the oldest and most well-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 recrystallisation to purify organic compounds. Pure molten acetic acid is used as a solvent in the production of terephthalic
acid (TPA), the raw material for polyethylene terephthalate (PET). Although currently accounting for 5%–10% of acetic acid use worldwide, this specific application is expected to grow significantly in the next decade, as PET
production increases.
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 an 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. 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.
Equivalently, 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.
Several 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.
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 several hours after exposure. Latex gloves offer no protection, so
specially resistant gloves, such as those made of nitrile rubber, should be worn when handling the compound. Concentrated acetic acid can be ignited with some 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%).
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.
* Vinegar
* Pickling, a food preservation method
* Acetic acid bacteria, produce vinegar from alcoholic solutions
* Acetic acid (data page)
* Acetobacter, an important genus of acetic acid bacteria
* Descaling agent, often contains acetic acid |
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