acetate

acetate anion

acetate ester

An acetate (pronounced /ˈæsɪteɪt/) is a derivative of acetic acid. This term includes salts and esters, as well as the anion found in solution. Most of the approximately 5 billion kilograms of acetic acid produced annually in industry are used in the production of acetates, which usually take the form of polymers. In nature, acetate is the most common building block for biosynthesis. For example, the fatty acids are produced by connecting C2 units derived from acetate.^[1]

Nomenclature and common formula

When part of a salt, the formula of the acetate anion is written as CH₃CO₂⁻, C₂H₃O₂⁻, or CH₃COO⁻. Chemists abbreviate acetate as OAc⁻ or AcO⁻. Thus, HOAc is the abbreviation for acetic acid, NaOAc for sodium acetate, and EtOAc for ethyl acetate.^[2] The abbreviation "Ac" (or "AC") is also sometimes encountered in chemical formulas to indicate the acetate ion. This abbreviation is not to be confused with the symbol of actinium, the first element of the actinides series. For example, the formula for sodium acetate might be abbreviated as "NaAc", rather than "CH₃COONa". Care should also be taken to avoid confusion with peracetic acid when using the OAc abbreviation; for clarity and to avoid errors when translated, HOAc should be avoided in literature mentioning both compounds.

The IUPAC systematic name for acetate is ethanoate. Acetate is an accepted common name.

Salts

The acetate anion, [CH₃COO]⁻, is one of the carboxylate family. It is the conjugate base of acetic acid. Above a pH of 5.5, acetic acid converts to acetate:^[2]

CH₃COOH ⇌ CH₃COO⁻ + H⁺

Many acetate salts are ionic, indicated by their tendency to dissolve well in water. A commonly encountered acetate in the home is sodium acetate, a white solid that can be prepared by combining vinegar and sodium bicarbonate ("bicarb"):

CH₃COOH + NaHCO₃ → CH₃COO⁻Na⁺ + H₂O + CO₂

Transition metals can be complexed by acetate. Acetate is considered a relatively weak ligand in coordination chemistry because it can only form monodentate complexes. Examples of acetate complexes include chromium(II) acetate and basic zinc acetate.

Commercially important acetate salts are aluminium acetate, used in dyeing, ammonium acetate, a precursor to acetamide, and potassium acetate, used as a diuretic. All three salts are colourless and highly soluble in water.^[3]

Esters

Acetate esters have the general formula CH₃CO₂R, where R is an organyl group. The esters are the dominant forms of acetate in the marketplace. Unlike the acetate salts, acetate esters are often liquids, lipophilic, and sometimes volatile. They are popular because they have inoffensive, often sweet odors, they are inexpensive, and they are usually of low toxicity.

Almost half of acetic acid production is consumed in the production of vinyl acetate, precursor to polyvinyl alcohol, which is a component of many paints. The second largest use of acetic acid is consumed in the production of cellulose acetate. In fact, "acetate" is jargon for cellulose acetate, which is used in the production of fibres or diverse products, e.g. the acetate discs used in audio record production. Cellulose acetate can be found in many household products. Many industrial solvents are acetates, including methyl acetate, ethyl acetate, isopropyl acetate, ethylhexyl acetate. Butyl acetate is a fragrance used in food products.^[3]

Acetate in biology

Acetate is a common anion in biology. It is mainly utilized by organisms in the form of acetyl coenzyme A.^[4]

Intraperitoneal injection of sodium acetate (20 or 60 mg per kg body mass) was found to induce headache in sensitized rats, and it has been proposed that acetate resulting from oxidation of ethanol is a major factor in causing hangovers. Increased serum acetate levels lead to accumulation of adenosine in many tissues including the brain, and administration of the adenosine receptor antagonist caffeine to rats after ethanol was found to decrease nociceptive behavior.^[5][6]

Fermentation of acetate

Acetic acid can also undergo a dismutation reaction to produce methane and carbon dioxide:^[7][8]

CH₃COO^– + H⁺ → CH₄ + CO₂       ΔG° = -36 kJ/reaction

This disproportionation reaction is catalysed by methanogen archaea in their fermentative metabolism. One electron is transferred from the carbonyl function (e^– donor) of the carboxylic group to the methyl group (e^– acceptor) of acetic acid to respectively produce CO₂ and methane gas.

Structures

• 

  space-filling model of the acetate anion
• 

  ball-and-stick model of the acetate anion
• 

  resonance hybrid of the acetate anion
• 

  canonical forms of the acetate anion

See also

Wikimedia Commons has media related to: acetates

• Acetic acid
• Acetylation
• Cellulose acetate
• Copper(II) acetate
• Fermentation (biochemistry)
• Sodium acetate

References

1. ^ March, J. “Advanced Organic Chemistry” 4th Ed. J. Wiley and Sons, 1992: New York. ISBN 0-471-60180-2.
2. ^ ^{a b} Zumdahl, S. S. “Chemistry” Heath, 1986: Lexington, MA. ISBN 0-669-04529-2.
3. ^ ^{a b} Hosea Cheung, Robin S. Tanke, G. Paul Torrence "Acetic acid" in Ullmann's Encyclopedia of Industrial Chemistry Weinheim, Germany: Wiley-VCH, 2005. doi:10.1002/14356007.a01 045
4. ^ Nelson, D. L.; Cox, M. M. "Lehninger, Principles of Biochemistry" 3rd Ed. Worth Publishing: New York, 2000. ISBN 1-57259-153-6.
5. ^ 'Acetate Causes Alcohol Hangover Headache in Rats' by Christina Maxwell et al., PLoS ONE 5(12): e15963.
6. ^ 'Is coffee the real cure for a hangover?' by Bob Holmes, New Scientist, Jan. 15 2011, p. 17.
7. ^ Ferry, J.G. (1992). "Methane from acetate". Journal of Bacteriology 174 (17): 5489–5495. http://jb.asm.org/content/174/17/5489.abstract. Retrieved 2011-11-05. 
8. ^ Vogels, G.D.; Keltjens J.T., Van Der Drift C. (1988). "Biochemistry of methane production". In Zehnder A.J.B.. Biology of anaerobic microorganisms. New York: Wiley. pp. 707–770.