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Acetic acid

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                             Acetic acid
                      Acetic acid Acetic acid
                               General
     Systematic name                                      Acetic acid
                                                        Ethanoic acid
         Other names               Methanecarboxylic acid
                                   Acetyl hydroxide (AcOH)
                                               Hydrogen acetate (HAc)
   Molecular formula                        C[2]H[4]O[2] or CH[3]COOH
              SMILES                                          CC(=O)O
          Molar mass                                   60.05 g mol^−1
          Appearance                                Colourless liquid
                                                          or crystals
          CAS number                                        [64-19-7]
                             Properties
   Density and phase                          1.049 g cm^−3, liquid
                                                 1.266 g cm^−3, solid
 Solubility in water                                   Fully miscible
 In ethanol, acetone
  In toluene, hexane
 In carbon disulfide               Fully miscible
                                         Fully miscible
                                                Practically insoluble
       Melting point                        16.7°C (289.9 K) (62.1°F)
       Boiling point                     118.1°C (391.2 4K) (244.6°F)
     Acidity (pK[a])                                     4.76 at 25°C
           Viscosity                               1.22 mPa·s at 25°C
       Dipole moment                                     1.74 D (gas)
                               Hazards
                MSDS                                    External MSDS
   EU classification                                    Corrosive (C)
            NFPA 704

                     2
                     2
                     0

         Flash point                                             43°C
           R-phrases                                         R10, R35
           S-phrases                              S1/2, S23, S26, S45
    U.S. Permissible
exposure limit (PEL)                                           10 ppm
                       Supplementary data page
           Structure
        & properties                                    n, ε[r], etc.
       Thermodynamic
                data                                  Phase behaviour
                                                   Solid, liquid, gas
       Spectral data                                  UV, IR, NMR, MS
                          Related compounds
  Related carboxylic
               acids                                Formic acid
                                                    Propionic acid
                                                         Butyric acid
   Related compounds                      Acetamide
                                          Ethyl acetate
                                          Acetyl chloride
                                          Acetic anhydride
                                          Acetonitrile
                                          Acetaldehyde
                                                              Ethanol
          Except where noted otherwise, data are given for
                materials in their standard state (at 25°C, 100 kPa)
                                    Infobox disclaimer and references

   Acetic acid, also known as ethanoic acid, is an organic chemical
   compound best recognized for giving vinegar its sour taste and pungent
   smell. Pure water-free acetic acid (glacial acetic acid) is a colorless
   hygroscopic liquid and freezes below 16.7 °C (62 °F) to a colourless
   crystalline solid. Acetic acid is corrosive, and its vapour is
   irritating to eyes and nose, although it is a weak acid based on its
   ability to dissociate in aqueous solutions.

   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 CH[3]−C(=O)−;. In the context
   of acid-base reactions the abbreviation HAc is often used where Ac
   instead stands for the acetate anion (CH[3]COO^−), 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 CH[2]O and the molecular formula
   C[2]H[4]O[2]. The latter is often written as CH[3]-COOH, CH[3]COOH, or
   CH[3]CO[2]H to better reflect its structure. 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

   Frozen acetic acid
   Enlarge
   Frozen acetic acid

   Vinegar is as old as civilization itself, perhaps older. 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 these alcoholic beverages being
   exposed to air.

   The use of acetic acid in chemistry 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. The 8th century Persian alchemist Jabir Ibn Hayyan (Geber)
   concentrated 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
   Enlarge
   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.

Chemical properties

   Acidity

   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 4.8. 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.

   Deprotonation equilibrium of acetic acid in water

   Cyclic dimer

   Cyclic dimer of acetic acid; dashed lines represent hydrogen bonds.
   Enlarge
   Cyclic dimer of acetic acid; dashed lines represent hydrogen bonds.

   The crystal structure of acetic acid shows that the molecules pair up
   into dimers connected by hydrogen bonds. The dimers can also be
   detected in the vapour at 120 °C. They probably also occur in the
   liquid phase of pure acetic acid, but are rapidly disrupted if any
   water is present. This dimerisation behaviour is shared by other lower
   carboxylic acids.

   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 CH[3]COOH( aq) → (CH[3]COO)[2]Mg(aq) + H[2](g)

          NaHCO[3](s) + CH[3]COOH(aq) → CH[3]COONa(aq) + CO[2](g) + H[2]O(
          l)

   Two typical organic reactions of acetic acid

   Acetic acid undergoes the typical chemical reactions of a carboxylic
   acid, notably 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.

Production

   Purification and concentration plant for acetic acid in 1884
   Enlarge
   Purification and concentration plant for acetic acid in 1884

   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 of the world 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.

   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. 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:

          CH[3]OH + CO → CH[3]COOH

   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) CH[3]OH + HI → CH[3]I + H[2]O

          (2) CH[3]I + CO → CH[3]COI

          (3) CH[3]COI + H[2]O → CH[3]COOH + 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. However, a lack of practical materials that could
   contain the corrosive reaction mixture at the high pressures needed
   (200 atm or more) discouraged commercialisation of these routes for
   some time. 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)[2]I[2]]^−) 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 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 commercialised the Cativa catalyst ([Ir(CO)[2]I[2]]^−), which
   is promoted by ruthenium. This iridium-catalysed process is greener and
   more efficient and has largely supplanted the Monsanto process, often
   in the same production plants.

Acetaldehyde oxidation

   Prior to the commercialisation 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 C[4]H[10] + 5 O[2] → 4 CH[3]COOH + 2 H[2]O

   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. 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 oxidised by the oxygen in air to
   produce acetic acid

          2 CH[3]CHO + O[2] → 2 CH[3]COOH

   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.

Ethylene oxidation

Fermentation

   Oxidative fermentation

   For most of human history, acetic acid, in the form of vinegar, has
   been made by 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

          C[2]H[5]OH + O[2] → CH[3]COOH + H[2]O

   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.

   Most vinegar today 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 this method,
   vinegar of 15% acetic acid can be prepared in only 2–3 days.

   Anaerobic fermentation

   Some species of anaerobic bacteria, including several 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:

          C[6]H[12]O[6] → 3 CH[3]COOH

   More interestingly from the point of view of an industrial chemist,
   many of these acetogenic bacteria can produce acetic acid from
   one-carbon compounds, including methanol, carbon monoxide, or a mixture
   of carbon dioxide and hydrogen:

          2 CO[2] + 4 H[2] → CH[3]COOH + 2 H[2]O

   This ability of Clostridium to utilise sugars directly, or to produce
   acetic acid from less costly inputs, means that these bacteria could
   potentially produce acetic acid more efficiently than ethanol-oxidisers
   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 some
   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.

Applications

   2.5-litre bottle of acetic acid in a laboratory.
   Enlarge
   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 H[3]C-COOH + 2 C[2]H[4] + O[2] → 2 H[3]C-CO-O-CH=CH[2] + 2
          H[2]O

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

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 dilute (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.

   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.

   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.

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%).

   The hazards of solutions of acetic acid depend on the concentration.
   The following table lists the EU classification of acetic acid
   solutions:
   Safety symbol
   Enlarge
   Safety symbol
   Concentration
   by weight        Molarity     Classification R-Phrases
   10%–25%      1.67–4.16 mol/L  Irritant (Xi)  R36/38
   25%–90%      4.16–14.99 mol/L Corrosive (C)  R34
   >90%         >14.99 mol/L     Corrosive (C)  R10, R35

   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.

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