Acetic acid
| Acetic acid | |
|---|---|
| |
| General | |
| Systematic name | Ethanoic acid |
| Other names | Methanecarboxylic acid Ethylic acid |
| Molecular formula | C2H4O2 |
| SMILES | CC(=O)O |
| Molar mass | 60.05 g/mol |
| Appearance | Colourless crystals or viscous liquid |
| CAS number | [64-19-7] |
| Properties | |
| Density and phase | 1.049 g/cm3, liquid 1.266 g/cm3, solid |
| Solubility in water | Fully miscible |
| In ethanol,acetone In toluene, hexane In carbon disulfide |
Fully miscible Fully miscible Pract. insoluble |
| Melting point | 16.5°C (289.6 K) |
| Boiling point | 118.1°C (391.2 K) |
| Acidity (pKa) | 4.76 |
| Viscosity | 1.22 cP at 25°C |
| Dipole moment | 1.74 D (gas) |
| Hazards | |
| MSDS | External MSDS |
| EU classification | Corrosive (C) |
| NFPA 704 | Template:Nfpa |
| Flash point | 43 °C |
| R-phrases | R10, R35 |
| S-phrases | S1/2, S23, S26, S45 |
| Supplementary data page | |
| Structure & properties |
n, εr, etc. |
| Thermodynamic data |
Phase behaviour Solid, liquid, gas |
| Spectral data | UV, IR, NMR, MS |
| Related compounds | |
| Related carboxylic acidss |
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 | |
The chemical compound acetic acid, systematically called ethanoic acid, is the acid that gives vinegar its sour taste and pungent smell. It derives its name from the Latin word for vinegar: acetum. Acetic acid is a carboxylic acid with chemical formula C2H4O2, also written as HC2H3O2 or CH3COOH to reflect its chemical structure. In pure form it has an ice crystal form, which is called glacial acetic acid.
Acetic acid is central in many biochemical reactions, 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. Also, at one time acetic acid was produced as a product of fermentation with Clostridium acetobutylicum. 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.
Nowadays, acetic acid is a key organic intermediate used in the preparation of metal acetates used in printing processes, vinyl acetate, acetic anhydride, and volatile organic esters such as ethyl acetate and butyl acetate. The worldwide market is estimated at 6.5 million metric tonnes per year.
History
Vinegar is as old as civilisation 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. In the third 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 seventh century, it was Jabir Ibn Hayyan (Geber) who separated three important acids for biochemistry: acetic acid from vinegar, citric acid from lemon juice, and tartaric acid from wine residue.
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 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.
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 tetrachloromethane, followed by pyrolysis to tetrachloroethylene and aqueous chlorination to trichloroacetic acid, and was concluded with reduction by electrolysis to acetic acid [1].
Chemical and physical properties
Pure acetic acid is a colourless, corrosive, flammable liquid that freezes at 16.6 °C. It is called glacial acetic acid because it freezes with long ice-like crystals.
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. A 1.0 M solution has a pH of 2.4.
In its gaseous state, acetic acid consists of pairs of dimers held together by hydrogen bonds. As a result, the ideal gas law does not accurately describe the behaviour of acetic acid vapour, since it does not take intermolecular interactions into account. The dimers look like this:
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.
Biochemistry
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.
Production
Of old, all acetic acid was made by a fermentation process, still amounting to approximately 10% of world production. About 75% of all acetic acid made for industrial use is made by methanol carbonylation, explained below. Alternative methods account for the rest [2].
Total worldwide production of virgin acetic acid is estimated at 5 Mt/a, approximately half of which is US, approx 1 Mt/a is European and declining, and .7 Mt/a in Japan. Additionally, another 1.5 Mt/a is recovered acetic acid, totalling the world market to 6.5 Mt/a and growing.[3][4] The two biggest producers of virgin acetic acid are Celanese, and BP Chemicals. Other major producers include Millennium, Sterling Chemicals, Samsung, Eastman, Pardies Acetiques and Svensk Etanolkemi.
Historic method: fermentation
Vinegar is manufactured by fermenting various starchy, sugary, or alcoholic foodstuffs with Acetobacter bacteria. Commonly used feeds include apple cider, wine, and grain, malt, rice or potato mashes. This is very similar to some processes for making kombucha or kvass. The vinegar is then distilled from the fermentation broth. Most nations have laws that the acetic acid found in food vinegar must be produced by fermentation rather than by non-biological means. Vinegar is usually 4%–8% acetic acid by volume.
Industrial production: catalysed methanol carbonylation
Most virgin acetic acid is produced by the carbonylation of methanol. In this process, methanol and carbon monoxide react to produce acetic acid according to the chemical equation
Because both methanol and carbon monoxide are commodity raw materials, standard methanol carbonylation long appeared to be an attractive method for acetic acid production, and patents on such processes were granted as early as the 1920s. However, the high pressures needed (200 atm or more) discouraged at the time commercialisation of these routes. The first commercial methanol carbonylation process, which used a cobalt catalyst, was developed by BASF in the early 1960s. In 1968, a rhodium-based catalyst was discovered that could operate efficiently at lower pressure with almost no by-products. The first plant using this catalyst was built by Monsanto in 1970, and rhodium-catalysed methanol carbonylation became the dominant method of acetic acid production (see Monsanto process). In the late 1990s, BP Chemicals commercialised an iridium-catalysed process, CativaTM, which now operates on many production plants.
Alternative production methods
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
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.
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
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.
Applications
Acetic acid is an chemical reagent for the production of many chemical compounds. The largest consumption is for vinyl acetate monomer, closely followed by acetic anhydride production, and ester production. The use of acetic acid in consumptive vinegar is volume-wise minor.
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 worlds production of acetic acid. The reaction is of ethylene and acetic acid with oxygen over a palladium catalyst.
- C2H4 + H3C-COOH → H3C-CO-O-CH=CH2
Vinyl acetate can be polymerized to polyvinyl acetate or to other polymers, which are applied in paints and adhesives.
Acetic anhydride
The condensation product of acetic acid is acetic anhydride, which uses approximately 25 to 30% of worlds acetic acid.
- 2 H3C-COOH → H3C-CO-O-CO-CH3 + H2O
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 the precursor for aspirin, heroin, and other compounds.
Ester production
Some of the esters of acetic acid are commonly used solvents and artificial flavourings. The esters include methyl acetate and ethyl acetate. Ester production consumes about 15 to 20% of worldwide acetic acid.
Vinegar
In the form of vinegar, acetic acid solutions (typically 5 to 18%) are used directly as a condiment, and also in the pickling of vegetables and other foodstuffs. The amount of acetic acid consumed on a worldwide scale is not large, but historically, this is by far the oldest and most known application.
Other applications
Pure molten acetic acid is used as 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, with the PET production.
Dilute solutions of acetic acids are also used for their mild acidity:
- stop bath in the developement of photographic films
- treating the sting of the box jellyfish by disabling the stinging cells of the jellyfish, and can prevent serious injury or death if immediately applied.
- reating outer ear infections in people in preparations such as Vosol.
- Silage spray as a preservative to discourage bacterial and fungal growth.
Several organic or inorganic salts are produced from acetic acid, including
- palladium(II) acetate
- monochloroacetic acid
- bromoacetic acid, which produces reagent ethyl bromoacetate
- acetoacetic acid, which produces reagent ethyl acetoacetate
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 has to be handled with appropriate care, since it can cause skin burns, permanent eye damage, irritation to the mucous membranes and can cause acidic burns or blisters to appear 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.
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.
References
- ^ Production report, Chemical and Engineering News, July 11, 2005, 67-76.
- ^ CEH Marketing Research Report Acetic Acid, December 2003, 602.5000, Chemical Economics Handbook SRI International
- ^ NewHaven bulletin of the ACS, September 2003
- ^ Yoneda, Noriyki; Kusano, Satoru; Yasui, Makoto; Pujado, Peter; Wilcher, Steve. Applied Catalysis A: General. 2001, 221, 253-265.
- Jones, R.E., Templeton, D.H., The crystal structure of acetic acid, Acta Crystallographica, Vol 11, Part 7 (July 1958), 484-487
- Mooij, W.T.M., Eijck, B.P. van, Price, S.L., Verwer, P., Kroon, J., Crystal structure predictions for acetic acid, J. Comput. Chem., Vol 19, 459 (1998)