Acetic anhydride

Chembox new
Name = Acetic anhydride
ImageFile = Acetic-anhydride-2D-skeletal.png ImageName = Acetic anhydride
ImageFile1 = Acetic-anhydride-3D-vdW.png ImageName1 = Acetic anhydride
IUPACName = Ethanoyl ethanoate
OtherNames = Acetic anhydride
Acetic acid anhydride
Acetylacetate
Acetyl oxide
Acetic oxide
Ethanoic anhydride
Section1 = Chembox Identifiers
SMILES = CC(=O)OC(=O)C
CASNo = 108-24-7
RTECS = AK1925000
InChI=1/C4H6O3/c1-3(5)
7-4(2)6/h1-2H3
Section2 = Chembox Properties
Formula = C₄H₆O₃
MolarMass = 102.1 g/mol
Appearance = clear liquid
Density = 1.08 g/cm³, liquid
Solubility = 2.6% by weight; rapidly reacts to
form acetic acid
MeltingPt = −73.1 °C
BoilingPt = 139.8 °C
Section3 = Chembox Structure
Dipole =
Section7 = Chembox Hazards
ExternalMSDS =
EUClass = Corrosive (C)
RPhrases = R10, R20/22, R34
SPhrases = S1/2, S26, S36/37/39, S45
FlashPt = 54 °C
Section8 = Chembox Related
Function = acid anhydrides
OtherFunctn = Propionic anhydride
OtherCpds = Acetic acid
Acetyl chloride

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.

Production

Acetic anhydride is produced by carbonylation of methyl acetate: [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] : CH₃CO₂CH₃ + CO → (CH₃CO)₂OThis 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.

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. [cite web | title=Science is Fun… | work=Chemical of the Week | url=http://scifun.chem.wisc.edu/CHEMWEEK/AceticAcid/AceticAcid.html | accessdate=2006-03-25] For example, the reaction of acetic anhydride with ethanol is:: (CH₃CO)₂O + CH₃CH₂OH → CH₃CO₂CH₂CH₃ + CH₃CO₂HOften a base such as pyridine is added to function as catalyst. In specialized applications, Lewis acidic scandium salts have also proven effective catalysts. [OrgSynth | author = Macor, J.; Sampognaro, A. J.; Verhoest, P. R.; Mack, R. A. | title = (R)-(+)-2-Hydroxy-1,2,2-Triphenylethyl Acetate | collvol = 10 | collvolpages = 464 | year = 2004 | prep = V77P0045]

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

Ferrocene may be acetylated too: [http://valhalla.chem.udel.edu/ferroc.html]

: Cp₂Fe → CpFe(C₅H₄COCH₃)

Hydrolysis

Acetic anhydride dissolves in water to approximately 2.6% by weight. [cite web | title=British Petroleum | work=Acetic Anhydride: Frequently Asked Questions (PDF) | url=http://www.bp.com/liveassets/bp_internet/globalbp/STAGING/global_assets/downloads/pdfs/acetyls_aromatics_pta/AceticAnhydrideFAQ.pdf | accessdate=2006-05-03] Aqueous solutions have limited stability because, like most acid anhydrides, acetic anhydride hydrolyses to give acetic acid: [cite web | title=Celanese | work=Acetic Anhydride: Material Safety Data Sheet (PDF) | url=http://www.celanese.com/msds/pdf/570-22027587.pdf | accessdate=2006-05-03] : (CH₃CO)₂O + H₂O → 2 CH₃CO₂H

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 is also used as a wood preservative via autoclave impregnation 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, [cite web | title=ChemGuide | work=Acid Anhydrides | url=http://www.chemguide.co.uk/organicprops/anhydridemenu.html | accessdate=2006-03-25] and restricted in many other countries. The largest markets for diverted acetic anhydride continue to be heroin laboratories in Afghanistan. [cite web | title=http://www.bloomberg.com/apps/news?pid=20670001&refer=home&sid=aY1eg_RtBNNU | work=Diverted Acetic Anhydride | url=http://www.bloomberg.com/apps/news?pid=20670001&refer=home&sid=aY1eg_RtBNNU | accessdate=2008-10-07]

afety

Acetic anhydride is an irritant and flammable. Because of its reactivity toward water, alcohol foam or carbon dioxide are preferred for fire suppression. [cite web | title=Data Sheets | work=International Occupational Safety and Health Information Centre | url=http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc02/icsc0209.htm | accessdate=2006-04-13] The vapour of acetic anhydride is harmful. [cite web | title=NIOSH | work=Pocket Guide to Chemical Hazards | url=http://www.cdc.gov/niosh/npg/npgd0003.html | accessdate=2006-04-13]

Notes and references

ee also

*Acid anhydride