Acetic anhydride, or ethanoic anhydride, is the
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
with the
formula
In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwee ...
(CH
3CO)
2O. Commonly abbreviated
Ac2O, it is the simplest isolable
anhydride
An organic acid anhydride is an acid anhydride that is an organic compound. An acid anhydride is a compound that has two acyl groups bonded to the same oxygen atom. A common type of organic acid anhydride is a carboxylic anhydride, where the pa ...
of a
carboxylic acid
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...
and is widely used as a
reagent in
organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
. It is a colorless liquid that smells strongly of
acetic acid
Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component ...
, 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
An intermolecular force (IMF) (or secondary force) is the force that mediates interaction between molecules, including the electromagnetic forces of attraction
or repulsion which act between atoms and other types of neighbouring particles, e.g. a ...
repulsion between the two
carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containing a ...
oxygens. The energy barriers to bond rotation between each of the optimal aplanar conformations are quite low.
Like most acid anhydrides, the carbonyl carbon atom of acetic anhydride has
electrophilic character, as the leaving group is
carboxylate
In organic chemistry, a carboxylate is the conjugate base of a carboxylic acid, (or ). It is an ion with negative charge.
Carboxylate salts are salts that have the general formula , where M is a metal and ''n'' is 1, 2,...; ''carboxylat ...
. The internal asymmetry may contribute to acetic anhydride's potent electrophilicity as the asymmetric geometry makes one side of a carbonyl carbon atom more reactive than the other, and in doing so tends to consolidate the electropositivity of a carbonyl carbon atom 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
Potassium acetate (CH3COOK) is the potassium salt of acetic acid. It is a hygroscopic solid at room temperature.
Preparation
It can be prepared by treating a potassium-containing base such as potassium hydroxide or potassium carbonate with acetic ...
with
benzoyl chloride
Benzoyl chloride, also known as benzenecarbonyl chloride, is an organochlorine compound with the formula . It is a colourless, fuming liquid with an irritating odour, and consists of a benzene ring () with an acyl chloride () substituent. It is ...
.
Acetic anhydride is produced by
carbonylation
Carbonylation refers to reactions that introduce carbon monoxide into organic and inorganic substrates. Carbon monoxide is abundantly available and conveniently reactive, so it is widely used as a reactant in industrial chemistry. The term carbony ...
of
methyl acetate
Methyl acetate, also known as MeOAc, acetic acid methyl ester or methyl ethanoate, is a carboxylate ester with the formula CH3COOCH3. It is a flammable liquid with a characteristically pleasant smell reminiscent of some glues and nail polish remo ...
:
: CH
3CO
2CH
3 + CO → (CH
3CO)
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
Acetyl iodide is an organoiodine compound with the formula CH3COI. It is a colourless liquid. It is formally derived from acetic acid. Although far rarer in the laboratory than the related acetyl bromide and acetyl chloride, acetyl iodide is p ...
, which reacts with acetate salts or acetic acid to give the product.
Rhodium chloride
Rhodium(III) chloride refers to inorganic compounds with the formula RhCl3(H2O)''n'', where ''n'' varies from 0 to 3. These are diamagnetic solids featuring octahedral Rh(III) centres. Depending on the value of ''n'', the material is either a den ...
in the presence of
lithium iodide
Lithium iodide, or LiI, is a compound of lithium and iodine. When exposed to air, it becomes yellow in color, due to the oxidation of iodide to iodine. It crystallizes in the NaCl motif. It can participate in various hydrates.Wietelmann, Ulrich ...
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
In organic chemistry, ethenone is the formal name for ketene, an organic compound with formula or . It is the simplest member of the ketene class. It is an important reagent for acetylations.
Properties
Ethenone is a highly reactive gas (a ...
) with acetic acid at 45–55 °C and low pressure (0.05–0.2 bar).
[.]
:H
2C=C=O + CH
3COOH → (CH
3CO)
2O (Δ''H'' = −63 kJ/mol)
The route from acetic acid to acetic anhydride via ketene was developed by
Wacker Chemie
Wacker Chemie AG is a German multinational chemical company which was founded in 1914 by Alexander Wacker. The company is controlled by the Wacker family holding more than 50 percent of the shares. The corporation is operating more than 25 prod ...
in 1922, when the demand for acetic anhydride increased due to the production of
cellulose acetate
In biochemistry, cellulose acetate refers to any acetate ester of cellulose, usually cellulose diacetate. It was first prepared in 1865. A bioplastic, cellulose acetate is used as a film base in photography, as a component in some coatings, and ...
.
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. In these conversions, acetic anhydride is viewed as a source of CH
3CO
+.
Acetylation of alcohols and amines
Alcohols and
amine
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituen ...
s are readily acetylated. For example, the reaction of acetic anhydride with
ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an Alcohol (chemistry), alcohol with the chemical formula . Its formula can be also written as or (an ethyl ...
yields
ethyl acetate
Ethyl acetate ( systematically ethyl ethanoate, commonly abbreviated EtOAc, ETAC or EA) is the organic compound with the formula , simplified to . This colorless liquid has a characteristic sweet smell (similar to pear drops) and is used in glues ...
:
: (CH
3CO)
2O + CH
3CH
2OH → CH
3CO
2CH
2CH
3 + CH
3COOH
Often a base such as
pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid with a d ...
is added to function as catalyst. In specialized applications,
Lewis acid
A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
ic
scandium
Scandium is a chemical element with the symbol Sc and atomic number 21. It is a silvery-white metallic d-block element. Historically, it has been classified as a rare-earth element, together with yttrium and the Lanthanides. It was discovered in ...
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
Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms, ...
to
acetophenone
Acetophenone is the organic compound with the chemical formula, formula C6H5C(O)CH3. It is the simplest aromatic ketone. This colorless, viscous liquid is a precursor to useful resins and fragrances.
Production
Acetophenone is formed as a byprodu ...
and
ferrocene
Ferrocene is an organometallic compound with the formula . The molecule is a complex consisting of two cyclopentadienyl rings bound to a central iron atom. It is an orange solid with a camphor-like odor, that sublimes above room temperature, a ...
to acetylferrocene:
: (C
5H
5)
2Fe + (CH
3CO)
2O → (C
5H
5)Fe(C
5H
4COCH
3) + CH
3CO
2H
Preparation of other acid anhydrides
Dicarboxylic acids are converted to the anhydrides upon treatment with acetic anhydride. It is also used for the preparation of mixed anhydrides such as that with nitric acid,
acetyl nitrate
Acetyl nitrate is the organic compound with the formula CH3C(O)ONO2. It is classified as the mixed anhydride of nitric and acetic acids. It is a colorless explosive liquid that fumes in moist air.
Synthesis and reactions
It is prepared from ace ...
.
Precursor to geminal diacetates
Aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
s react with acetic anhydride in the presence of an acidic
catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
to give geminal diacetates. A former industrial route to
vinyl acetate
Vinyl acetate is an organic compound with the formula CH3CO2CH=CH2. This colorless liquid is the precursor to polyvinyl acetate and ethene-vinyl acetate copolymers, important industrial polymers.
Production
The worldwide production capacity of v ...
involved the intermediate
ethylidene diacetate
Ethylidene diacetate is an organic compound with the formula (CH3CO2)2CHCH3. A colorless low-melting solid, it once served as a precursor to vinyl acetate.
Preparation
A major industrial route involves the reaction of acetaldehyde and acetic anhy ...
, the geminal diacetate obtained from
acetaldehyde and acetic anhydride:
:CH
3CHO + (CH
3CO)
2O → (CH
3CO
2)
2CHCH
3
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, this reaction product being fully water miscible:
: (CH
3CO)
2O + H
2O → 2 CH
3CO
2H
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
In biochemistry, cellulose acetate refers to any acetate ester of cellulose, usually cellulose diacetate. It was first prepared in 1865. A bioplastic, cellulose acetate is used as a film base in photography, as a component in some coatings, and ...
, 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
Aspirin, also known as acetylsalicylic acid (ASA), is a nonsteroidal anti-inflammatory drug (NSAID) used to reduce pain, fever, and/or inflammation, and as an antithrombotic. Specific inflammatory conditions which aspirin is used to treat inc ...
(acetylsalicylic acid), which is prepared by the acetylation of
salicylic acid.
[.] It is also used as an active modification agent via
autoclave
An autoclave is a machine used to carry out industrial and scientific processes requiring elevated temperature and pressure in relation to ambient pressure and/or temperature. Autoclaves are used before surgical procedures to perform sterilizat ...
impregnation and subsequent acetylation to make a durable and long-lasting timber.
In starch industry, acetic anhydride is a common acetylation compound, used for the production of
modified starch
Modified starch, also called starch derivatives, are prepared by physically, enzymatically, or chemically treating native starch to change its properties. Modified starches are used in practically all starch applications, such as in food produ ...
es (E1414, E1420, E1422)
Legal status
Because of its use for the synthesis of
heroin
Heroin, also known as diacetylmorphine and diamorphine among other names, is a potent opioid mainly used as a recreational drug for its euphoric effects. Medical grade diamorphine is used as a pure hydrochloride salt. Various white and brow ...
by the diacetylation of
morphine
Morphine is a strong opiate that is found naturally in opium, a dark brown resin in poppies (''Papaver somniferum''). It is mainly used as a analgesic, pain medication, and is also commonly used recreational drug, recreationally, or to make ...
, 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; it is highly corrosive to skin and any direct contact will result in severe burns. Because of its reactivity toward water and alcohol, foam or carbon dioxide are preferred for fire suppression. The vapour of acetic anhydride is harmful.
[
]
References
External links
ICSC 0209
{{DEFAULTSORT:Acetic Anhydride
Carboxylic anhydrides
Lachrymatory agents
Solvents