An acid anhydride is a chemical compound which is non- metal oxide which is obtained by removal of a water molecule from an acid. The word ‘’anhydride’’ means without water. It reacts with water and form an acidic solution .
In organic chemistry it is a functional group which consists of two acyl groups joined together by an oxygen atom R(CO)O(CO)R’. It is formed when 1 equivalent of water is removed from two equivalents of an organic acid in a dehydration reaction.
In inorganic chemistry it termed as an acidic oxide which reacts with water and form oxyacid or react with a base to form a salt.
The name of organic acid anhydride is derived from carboxylic acid, undergo dehydration to form a compound. In symmetrical acid anhydride carboxylic acid the only constituent used to form the compound so the prefix is carboxylic and suffix anhydride is used. In asymmetrical acid anhydride, two different carboxylic acids are used like the dehydration of benzoic acid and propanoic acid, so the prefix is benzoic propanoic and anhydride is a suffix. Ethanoic acid forms ethanoic anhydride, propanoic forms propanoic anhydride, etc. .
It was first prepared by French chemist Charles Frederic Gerhardt in 1852 through heating potassium acetate with benzoyl chloride.
Acetic anhydride is prepared by carbonylation of methyl acetate .
CH3CO2CH3 + CO → (CH3CO)2O
It is also prepared by another process that was developed by Wacker Chemie in 1922. In this method acetic anhydride is prepared by reaction of ketene (ethenone) with acetic acid at 45-550C and pressure is about 0.05-0.2 bar.
H2C=C=O + CH3COOH → (CH3CO)2O
Acid chloride is effective precursor for the preparation of organic acid anhydride .
CH3C(O)Cl + HCO2Na → HCO2COCH3 + NaCl
Acetic anhydride or ethanoic anhydride is simplest insoluble anhydride of carboxylic acid. It is flexible molecule with a nonplanar structure.
Acetic anhydride is a colorless liquid but have vinegar like smell. Its smell is due to its reaction with water vapor in the air which produce ethanoic again.
It may not dissolve in water but it reacts to prepare ethanoic acid again.
- Electrophilic character
Its internal asymmetry contribute a lot because it makes one side of compound more reactive than the other one, that’s why it has electrophilic character as carboxylate is the leaving group.
- Boiling point
Ethanoic anhydride boiling point is 1400C because it is big polar molecule. It has both types of forces like van der Waals dispersion force and dipole- dipole force of attraction. But it does not has hydrogen bonds that’s why it boiling point is less than other carboxylic acid like pentanoic acid (1860C).
Acetic anhydride has strong irritating odor and combustible in nature.
Acetylation of alcohol and amines
In this reaction acetic anhydride is consider as a source of CH3CO. Alcohol and amines are acetylated quickly. The reaction of acetic anhydride with ethanol produced ethyl acetate.
(CH3CO)2O + CH3CH2OH → CH3CO2CH2CH3 + CH3COOH
Pyridine is added as a catalyst, for specialized conditions Lewis acidic scandium salts are used as catalyst.
These are acetylated by acetic anhydride, in which acid catalyst are used to speed up the reaction, conversion of benzene to acetophenone and ferrocene to acetylferrocene.
(CH3CO)2O + C6H6 → CH3COC6H5 + CH3CO2H
(C5H5)2Fe + (CH3CO)2O → (C2H5)Fe(C5H4COCH3) + CH3CO2H
Preparation of other acid anhydride
Upon treatment with acetic anhydride dicarboxylic acids are converted into anhydrides. Acid anhydride also used in the preparation of anhydrides like reaction with nitric acid make acetyl nitrate.
Precursor to germinal diacetates
Acetic anhydride react with aldehyde in the presence of acidic catalyst to produce germinal diacetate. On industrial scale vinyl acetate are used for the preparation of ethylidene diacetate.
CH3CHO + (CH3CO)2O → (CH3CO2)2CHCH3
It dissolves in water about 2.6% by weight. Mostly acid anhydrides hydrolyses to give carboxylic acids. In acetic anhydride it formed acetic acid.
(CH3CO)2O + H2O → 2 CH3CO2H
Chemical reaction with water, ethanol and phenol
The similarity present between them is having –OH group, in water it is attached with hydrogen atom, in ethanol it is present with alkyl group and in phenol it is present with a benzene ring.
Reaction with water
Acid anhydride react with water and produced two molecules of ethanoic acid. This reaction is carried out very slowly at room temperature.
(CH3CO)2O + H2O → 2CH3COOH
Reaction with alcohol
Alcohol react with ethanoic anhydride it produced an ester and ethanoic acid.
(CH3CO)2O + ROH → CH3COOR + CH3COOH
An ester ethanoic acid
Reaction with ethanol produced ester ethyl ethanote. This reaction required heating at a moderate temperature.
(CH3CO)2O + CH3CH2OH → CH3COOCH2CH3 + CH3COOH
Reaction with phenol
In the reaction of phenol and acid anhydride produced an ester called phenyl ethanote .
(CH3CO)2O +C6H5OH → CH3COOC6H5 + CH3COOH
Comparing acid anhydride with acyl chloride
During the reactions of ethanoic anhydride, the red part behave just like chlorine atom of acyl chloride, it is always intact and replace just like chlorine atom.
The reaction is carried out slowly, and the difference is that ethanoic acid is produced instead of HCl.
Reaction with Ammonia
Amides contain –CONH2 group, during the reaction of ethanoic anhydride and ammonia, the amide is produced which is called as ethanamide.
(CH3CO)2O +NH3 → CH3CONH2 + CH3COOH
With excess it produced to give ammonium ethanote.
CH3COOH + NH3 → CH3COO– +NH4
Overall reaction is
(CH3CO)2O + 2NH3 → CH3CONH2 + CH3COO– +NH4
The reaction is acyl chloride is
CH3COCl + 2NH3 → CH3CONH2 + NH4+Cl–
- Acetic anhydride is important solvent and acetylation agent.
- It is used in the preparation of acetylcellulose, vitamin B6, nitrofuran etc.
- It is also used as acetulizer and solvent in examining fatty and volatile oils, wool fat detection of rosin.
- It is also used in organic synthesis like dehydrating agents in nitration, sulfonations.
- It is also used in manufacturing of acetyl compounds.
- Acetic anhydride is also used as an esterifier for food starch.
- It is also used in the synthesis of heroin through diacetylation of morphine.
- It is used in the production of aspirin.
- It is also used in cigarette filters.
- It is used for the preparation of modified starch like E1414, E1420 and E1422.
- In the form of dianhydride, two acid anhydrides are present they are used for the preparation of polyimides, polyester and polyamide.
- ATP is also derived from phosphoric acid in its protonated form in an anhydride .
- They are widely used in metabolic process in the form of mixed anhydride.
- Zoeller, J. R.; Agreda, V. H.; Cook, S. L.; Lafferty, N. L.; Polichnowski, S. W.; Pond, D. M. (1992), “Eastman Chemical Company Acetic Anhydride Process”, Catal. Today, 13(1): 73–91, doi:10.1016/0920-5861(92)80188-S
- Lewis I. Krimen (1988). “Acetic Formic Anhydride”. Organic Syntheses.; Collective Volume, 6, p. 8