Acetic anhydride

Acetic anhydride
IUPAC name	acetic anhydride
Systematic name	ethanoic anhydride
Other names	Ethanoyl ethanoate Acetic acid anhydride Acetyl acetate Acetyl oxide Acetic oxide
Identifiers
CAS number	108-24-7 Y
PubChem	7918
EC number	203-564-8
RTECS number	AK1925000
SMILES	O=C(OC(=O)C)C
InChI	1/C4H6O3/c1-3(5)7-4(2)6/h1-2H3
InChI key	WFDIJRYMOXRFFG-UHFFFAOYAH
ChemSpider ID	7630
Properties
Molecular formula	C4H6O3
Molar mass	102.09 g/mol
Appearance	clear liquid
Density	1.082 g/cm3, liquid
Melting point	−73.1 °C
Boiling point	139.8 °C
Solubility in water	2.6 g/100 ml, see text
Refractive index (nD)	1.3901
Hazards
MSDS	ICSC 0209
EU Index	607-008-00-9
EU classification	Corrosive (C)
R-phrases	R10, R20/22, R34
S-phrases	(S1/2), S26, S36/37/39, S45
NFPA 704	2 2 1 W
Flash point	49 °C
Autoignition temperature	316 ºC
Explosive limits	2.7–10.3%
Related compounds
Related acid anhydrides	Propionic anhydride
Related compounds	Acetic acid Acetyl chloride
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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

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 process, 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 at 45–55 ºC and low pressure (0.05–0.2 bar).^[2]

H₂C=C=O + CH₃COOH → (CH₃CO)₂O (ΔH = −63 kJ/mol)

Ketene is generated by dehydrating acetic acid at 700–750 ºC in the presence of triethyl phosphate as a catalyst or (in Switzerland and the CIS) by the thermolysis of acetone at 600–700 ºC in the presence of carbon disulfide as a catalyst.^[2]

CH₃COOH H₂C=C=O + H₂O (ΔH = +147 kJ/mol)

CH₃COCH₃ → H₂C=C=O + CH₄

The route from acetic acid to acetic anhydride via ketene was developed by Wacker Chemie in 1922,^[3] when the demand for acetic anhydride increased due to the production of cellulose acetate.

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

Reactions

Acetic anhydride is a versatile reagent for acetylations, the introduction of acetyl groups to organic substrates.^[4] In these conversions, acetic anhydride is viewed as a source of CH₃CO⁺. Alcohols and amines are readily acetylated.^[5] For example, the reaction of acetic anhydride with ethanol yields ethyl acetate:

(CH₃CO)₂O + CH₃CH₂OH → CH₃CO₂CH₂CH₃ + CH₃COOH

Often a base such as pyridine is added to function as catalyst. In specialized applications, Lewis acidic scandium salts have also proven effective catalysts.^[6]

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:^[7]

Cp₂Fe + (CH₃CO)₂O → CpFe(C₅H₄COCH₃)

Hydrolysis

Acetic anhydride dissolves in water to approximately 2.6% by weight.^[8] Aqueous solutions have limited stability because, like most acid anhydrides, acetic anhydride hydrolyses to give acetic acid:^[9]

(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, acetylsalicylic acid, which is prepared by the acetylation of salicylic acid.^[10] 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 is listed as a U.S. DEA List II precursor, and restricted in many other countries.^[11]

Safety

Acetic anhydride is an irritant and flammable. Because of its reactivity toward water, alcohol foam or carbon dioxide are preferred for fire suppression.^[12] The vapour of acetic anhydride is harmful.^[13]

References

External links

• ICSC 0209
• NIOSH Pocket Guide to Chemical Hazards