Acetic anhydride

Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula (CH₃CO)₂O. Commonly abbreviated Ac₂O, it is one the simplest anhydrides of a carboxylic acid and is widely used in the production of cellulose acetate as well 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.

Acetic anhydride

Acetic anhydride
Acetic anhydride
Names
Preferred 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
3D model (JSmol)	Interactive image Interactive image
ChEBI	CHEBI:36610 Y
ChEMBL	ChEMBL1305819
ChemSpider	7630 Y
ECHA InfoCard	100.003.241
EC Number	203-564-8
PubChem CID	7918
RTECS number	AK1925000
UNII	2E48G1QI9Q Y
UN number	1715
CompTox Dashboard (EPA)	DTXSID0024395
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	102.089 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, reacts (see text)
Vapor pressure	4 mmHg (20 °C)[1]
Magnetic susceptibility (χ)	−52.8·10−6 cm3/mol
Refractive index (nD)	1.3901
Thermochemistry[2]
Std enthalpy of formation (ΔfH⦵298)	−624.4 kJ/mol
Pharmacology
Legal status	AU: S6 (Poison) CA: Schedule VI
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H226, H302, H314, H330
Precautionary statements	P210, P233, P240, P241, P242, P243, P260, P261, P264, P270, P271, P280, P301+P312, P301+P330+P331, P303+P361+P353, P304+P312, P304+P340, P305+P351+P338, P310, P312, P321, P330, P363, P370+P378, P403+P235, P405, P501
NFPA 704 (fire diamond)	3 2 1 W
Flash point	49 °C (120 °F; 322 K)
Autoignition temperature	316 °C (601 °F; 589 K)
Explosive limits	2.7–10.3%
Lethal dose or concentration (LD, LC):
LC50 (median concentration)	1000 ppm (rat, 4 h)[3]
NIOSH (US health exposure limits):
PEL (Permissible)	TWA 5 ppm (20 mg/m3)[1]
REL (Recommended)	C 5 ppm (20 mg/m3)[1]
IDLH (Immediate danger)	200 ppm[1]
Safety data sheet (SDS)	ICSC 0209
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). Y verify (what is YN ?) Infobox references

Structure and properties

Acetic anhydride in a glass bottle

Acetic anhydride, like most organic acid anhydrides, is a flexible molecule with a nonplanar structure. The C=O and C-O distances are 1.19 and 1.39 Å.^[4] 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.^[5]

Production

Acetic anhydride was first synthesized in 1852 by the French chemist Charles Frédéric Gerhardt (1816-1856) by heating potassium acetate with benzoyl chloride.^[6]

Acetic anhydride is produced by carbonylation of methyl acetate:^[7]

CH₃CO₂CH₃ + CO → (CH₃CO)₂O

The Tennessee Eastman acetic anhydride process involves the conversion of methyl acetate to methyl iodide. Carbonylation of the methyl iodide produces acetyl iodide, which reacts with acetate source to give the desired anhydride. Rhodium chloride in the presence of lithium iodide is employed as the catalyst. 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).^[8]

H₂C=C=O + CH₃COOH → (CH₃CO)₂O

(ΔH = −63 kJ/mol)

The route from acetic acid to acetic anhydride via ketene was developed by Wacker Chemie in 1922,^[9] when the demand for acetic anhydride increased due to the production of cellulose acetate.

Due to its low cost, acetic anhydride is usually 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.^[10] In these conversions, acetic anhydride is viewed as a source of CH₃CO⁺.

Acetylation of alcohols, amines, aromatics

Alcohols and amines are readily acetylated.^[11] 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.^[12]

Aromatic rings are acetylated by acetic anhydride. Usually acid catalysts are used to accelerate the reaction. Illustrative are the conversions of benzene to acetophenone^[13] and ferrocene to acetylferrocene:^[14]

(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.^[15] It is also used for the preparation of mixed anhydrides such as that with nitric acid, acetyl nitrate.

Precursor to geminal diacetates

Aldehydes react with acetic anhydride in the presence of an acidic catalyst to give geminal diacetates.^[16] A former industrial route to vinyl acetate involved the intermediate ethylidene diacetate, the geminal diacetate obtained from acetaldehyde and acetic anhydride:^[17]

CH₃CHO + (CH₃CO)₂O → (CH₃CO₂)₂CHCH₃

Hydrolysis

Acetic anhydride dissolves in water to approximately 2.6% by weight.^[18] Aqueous solutions have limited stability because, like most acid anhydrides, acetic anhydride hydrolyses to give carboxylic acids. In this case, acetic acid is formed, this reaction product being fully water miscible:^[19]

(CH₃CO)₂O + H₂O → 2 CH₃COOH

Enolate formation

Acetic anhydride forms the enolate in the presence of acetate as base. The enolate can be trapped by condenation with benzaldehyde. In the 19th century, this chemistry, the Perkin reaction, was used for the production of cinnamic acid:^[20]

(CH₃CO)₂O + C₆H₅CHO → C₆H₅CH=CHCO₂H + CH₃CO₂H

Lewis base properties

The carbonyl groups in acetic anhydride are weakly basic. A number of adducts are known, such as the derivative of titanium tetrachloride, TiCl₄((CH₃CO)₂O).^[21]

Applications

As indicated by its organic chemistry, acetic anhydride 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.^[22] It is also used as an active modification agent via autoclave impregnation and subsequent acetylation to make a durable and long-lasting timber.^[23]

Acetic anhydride is commonly used for the production of modified starches (E1414, E1420, E1422).

Legal status

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 is restricted in many other countries.^[24][25]

Safety

Acetic anhydride is an irritant and combustible liquid; it is highly corrosive to skin and any direct contact will result in burns. Because of its reactivity toward water and alcohol, foam or carbon dioxide are preferred for fire suppression.^[26] The vapour of acetic anhydride is harmful.^[27]