Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula2O.
Commonly abbreviated Ac2O, it is the simplest isolatable 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.
Structure and properties Acetic anhydride, like most acid anhydrides,
are flexible molecules with nonplanar structures. 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 is a potent electrophile as the leaving group for each carbonyl carbon
is a good electron-withdrawing leaving group. 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. Production
Acetic anhydride was first synthesized in 1852 by the French chemist Charles Frédéric
Gerhardt by heating potassium acetate with benzoyl chloride.
Acetic anhydride is produced by carbonylation of methyl acetate:
CH3CO2CH3 + CO →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. 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 °C and low pressure. H2C=C=O + CH3COOH →2O
Ketene is generated by dehydrating acetic acid at 700–750 °C in the presence of
triethyl phosphate as a catalyst or by the thermolysis of acetone at 600–700 °C in
the presence of carbon disulfide as a catalyst. CH3COOH H2C=C=O + H2O
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+. 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. 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 +2O →Fe(C5H4COCH3) + CH3CO2H A former industrial route to vinyl acetate
involved the intermediate ethylidene diacetate. This geminal diacetate was obtained by reaction
of acetaldehyde and acetic anhydride in the presence of a ferric chloride catalyst:
CH3CHO +2O →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. Similarly it is used in the production of
aspirin, 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 anydride is a common acetylation compound, used for the production
of modified starches 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.
References
External links ICSC 0209
NIOSH Pocket Guide to Chemical Hazards