Acetic anhydride

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Acetic anhydride
Acetic anhydride
Acetic anhydride
IUPAC name Ethanoyl ethanoate
Other names Acetic anhydride
Acetic acid anhydride
Acetylacetate
Acetyl oxide
Acetic oxide
Ethanoic anhydride
Identifiers
CAS number [108-24-7]
RTECS number AK1925000
SMILES CC(=O)OC(=O)C
InChI 1/C4H6O3/c1-3(5)
7-4(2)6/h1-2H3
Properties
Molecular formula C4H6O3
Molar mass 102.1 g/mol
Appearance clear liquid
Density 1.08 g/cm³, liquid
Melting point

−73.1 °C

Boiling point

139.8 °C

Solubility in water 2.6% by weight; rapidly reacts to
form acetic acid
Hazards
EU classification Corrosive (C)
R-phrases R10, R20/22, R34
S-phrases (S1/2), S26, S36/37/39, S45
Flash point 54 °C
Related compounds
Related acid anhydrides Propionic anhydride
Related compounds Acetic acid
Acetyl chloride
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

Acetic anhydride is the chemical compound with the formula (CH3CO)2O. Commonly abbreviated Ac2O, it is one of the simplest acid anhydrides and is a widely used reagent in organic synthesis. It is a colorless liquid that smells strongly of acetic acid, which is formed by its reaction with the moisture in the air.

Contents

[edit] Production

Acetic anhydride is produced by carbonylation of methyl acetate:[1]

CH3CO2CH3 + CO → (CH3CO)2O

This process involves the conversion of methyl acetate to methyl iodide and an acetate salt. Carbonylation of the methyl iodide in turn affords acetyl iodide, which reacts with acetate salts or acetic acid to give the product. Rhodium and lithium iodides are employed as catalysts. Because acetic anhydride is not stable in water, the conversion is conducted under anhydrous conditions. In contrast, the Monsanto acetic acid synthesis, which also involves a rhodium catalyzed carbonylation of methyl iodide, is at least partially aqueous.

To a decreasing extent, acetic anhydride is also prepared by the reaction of ketene with acetic acid. Ketene is generated by dehydrating acetic acid at elevated temperatures. This production method was developed by Wacker Chemie in 1922, when the demand for acetic anhydride increased due to the production of acetic fibre.

Due to its low cost, acetic anhydride is purchased, not prepared, for use in research laboratories.

[edit] Organic chemistry

Acetic anhydride is a versatile reagent for acetylations, the introduction of acetyl groups to organic substrates. In these conversions, acetic anhydride is viewed as a source of CH3CO+. Alcohols and amines are readily acetylated.[2] For example, the reaction of acetic anhydride with ethanol is:

(CH3CO)2O + CH3CH2OH → CH3CO2CH2CH3 + CH3CO2H

Often a base such as pyridine is added to function as catalyst. In specialized applications, Lewis acidic scandium salts have also proven effective catalysts.[3]

Aromatic rings are acetylated, usually in the presence of an acid catalyst. Illustrative is the conversion of benzene to acetophenone:

(CH3CO)2O + C6H6 → CH3COC6H5 + CH3CO2H

[edit] Hydrolysis

Acetic anhydride dissolves in water to approximately 2.6% by weight.[4] Aqueous solutions have limited stability because, like most acid anhydrides, acetic anhydride hydrolyses to give acetic acid:[5]

(CH3CO)2O + H2O → 2 CH3CO2H

[edit] Applications

As indicated by its organic chemistry, Ac2O is mainly used for acetylations leading to commercially significant materials. Its largest application is for the conversion of cellulose to cellulose acetate, which is a component of photographic film and other coated materials. Similarly it is used in the production of aspirin, acetyl salicylic acid, which is prepared by the acetylation of salicylic acid. It ia also used as a wood preservative via autoclave impregnation to to make a longer lasting timber. Because of its use for the synthesis of heroin by the diacetylation of morphine, acetic anhydride (known as 'AA' in clandestine chemistry circles) is listed as a U.S. DEA List II Precursor,[6] and restricted in many other countries.

[edit] Safety

Acetic anhydride is an irritant and flammable. Because of its reactivity toward water, alcohol foam or carbon dioxide are preferred for fire suppression.[7] The vapour of acetic anhydride is harmful.[8]

[edit] Notes and references

  1. ^ Zoeller, J. R.; Agreda, V. H.; Cook, S. L.; Lafferty, N. L.; Polichnowski, S. W.; Pond, D. M. "Eastman Chemical Company Acetic Anhydride Process" Catalysis Today (1992), volume 13, pp.73-91. doi:10.1016/0920-5861(92)80188-S
  2. ^ Science is Fun…. Chemical of the Week. Retrieved on 2006-03-25.
  3. ^ Macor, J.; Sampognaro, A. J.; Verhoest, P. R.; Mack, R. A. "(R)-(+)-2-Hydroxy-1,2,2-Triphenylethyl Acetate" Organic Syntheses, Collected Volume 10, p.464 (2004). http://www.orgsyn.org/orgsyn/pdfs/V77P0045.pdf
  4. ^ British Petroleum. Acetic Anhydride: Frequently Asked Questions (PDF). Retrieved on 2006-05-03.
  5. ^ Celanese. Acetic Anhydride: Material Safety Data Sheet (PDF). Retrieved on 2006-05-03.
  6. ^ ChemGuide. Acid Anhydrides. Retrieved on 2006-03-25.
  7. ^ Data Sheets. International Occupational Safety and Health Information Centre. Retrieved on 2006-04-13.
  8. ^ NIOSH. Pocket Guide to Chemical Hazards. Retrieved on 2006-04-13.

[edit] See also