Acetic anhydride

Acetic anhydride

Names
IUPAC name acetic anhydride
Systematic IUPAC name ethanoic anhydride
Other names Ethanoyl ethanoate Acetic acid anhydride Acetyl acetate Acetyl oxide Acetic oxide
Identifiers
CAS Number	108-24-7 Y
ChEBI	CHEBI:36610 Y
ChemSpider	7630 Y
EC Number	203-564-8
Jmol 3D model	Interactive image Interactive image
PubChem	7918
RTECS number	AK1925000
UNII	2E48G1QI9Q Y
InChI InChI=1S/C4H6O3/c1-3(5)7-4(2)6/h1-2H3 Y Key: WFDIJRYMOXRFFG-UHFFFAOYSA-N Y InChI=1/C4H6O3/c1-3(5)7-4(2)6/h1-2H3 Key: WFDIJRYMOXRFFG-UHFFFAOYAH
SMILES O=C(OC(=O)C)C CC(=O)OC(=O)C
Properties
Chemical formula	C4H6O3
Molar mass	102.09 g·mol−1
Appearance	colorless liquid
Density	1.082 g cm−3, liquid
Melting point	−73.1 °C (−99.6 °F; 200.1 K)
Boiling point	139.8 °C (283.6 °F; 412.9 K)
Solubility in water	2.6 g/100 mL, see text
Vapor pressure	4 mmHg (20 °C)
Refractive index (nD)	1.3901
Pharmacology
Legal status	AU: S6 (Poison)
Vapor pressure	{{{value}}}
Related compounds
Related acid anhydrides	Propionic anhydride
Related compounds	Acetic acid Acetyl chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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

Structure and properties

Acetic anhydride, like most acid anhydrides, is a flexible molecule with a nonplanar structure. The pi system linkage through the central oxygen offers very weak resonance stabilization compared to the dipole-dipole repulsion between the two carbonyl oxygens. The energy barriers to bond rotation between each of the optimal aplanar conformations are quite low.

Like most acid anhydrides, the carbonyl carbon of acetic anhydride has electrophilic character, as the leaving group is carboxylate. The internal asymmetry may contribute to acetic anhydride's potent electrophilicity as the asymmetric geometry makes one side of a carbonyl carbon more reactive than the other, and in doing so tends to consolidate the electropositivity of a carbonyl carbon to one side (see electron density diagram).

Production

Acetic anhydride was first synthesized in 1852 by the French chemist Charles Frédéric Gerhardt (1816-1856) by heating potassium acetate with benzoyl chloride.

Acetic anhydride is produced by carbonylation of methyl acetate:

CH₃CO₂CH₃ + CO → (CH₃CO)₂O

The Tennessee Eastman acetic anhydride 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 chloride in the presence of lithium iodide is employed as catalysts. Because acetic anhydride is not stable in water, the conversion is conducted under anhydrous conditions.

To a decreasing extent, acetic anhydride is also prepared by the reaction of ketene (ethenone) with acetic acid at 45–55 °C and low pressure (0.05–0.2 bar).

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

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

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, 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. In these conversions, acetic anhydride is viewed as a source of CH₃CO⁺.

Acetylation of alcohols and amines

Alcohols and amines are readily acetylated. 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.

Acetylation of aromatic rings

Aromatic rings are acetylated by acetic anhydride. Usually acid catalysts are used to accelerate the reaction. Illustrative are the conversions of benzene to acetophenone and ferrocene to acetylferrocene:

(C₅H₅)₂Fe + (CH₃CO)₂O → (C₅H₅)Fe(C₅H₄COCH₃) + CH₃CO₂H

Preparation of other acid anhydrides

Dicarboxylic acids are converted to the anhydrides upon treatment with acetic anhydride.

Precursor to geminal diacetates

Aldehydes react with acetic anhydride in the presence of an acidic catalyst to give geminal diacetates. A former industrial route to vinyl acetate involved the intermediate ethylidene diacetate, the geminal diacetate obtained from acetaldehyde and acetic anhydride:

CH₃CHO + (CH₃CO)₂O → (CH₃CO₂)₂CHCH₃

Hydrolysis

Acetic anhydride dissolves in water to approximately 2.6% by weight. Aqueous solutions have limited stability because, like most acid anhydrides, acetic anhydride hydrolyses to give carboxylic acids. In this case, acetic acid is formed:

(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, and is used in the manufacture of cigarette filters. Similarly it is used in the production of aspirin (acetylsalicylic 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.

In starch industry, acetic anhydride is a common acetylation compound, used for the production of modified starches (E1414, E1420, E1422)

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.

Safety

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

When mixed with hydrogen peroxide, an excess of acetic anhydride reacts with one of the reaction products peracetic acid and forms highly shock sensitive and explosive diacetyl peroxide.

External links

• ICSC 0209
• NIOSH Pocket Guide to Chemical Hazards