Acetic Anhydride which is also known as ethanoic anhydride with a molecular formula of (CH3CO)2O or C4H6O3 is an organic compound and an insoluble anhydride of a carboxylic acid. It is also a flexible molecule with a non-planar structure.
It is a colorless liquid that is corrosive to tissue and metal and has a strong and pungent vinegar-like odor that is attributed to its reaction with the moisture in your nose and the water vapor in the atmosphere to form ethanoic acid.
It was first produced by French chemist Charles Frederic Gerhardt in 1852 and can be used for many purposes which include being a re-agent in chemical synthesis amongst others.
Preparation of Acetic Anhydride
Acetic Anhydride can also be prepared when two molecules of acetic acid react and are then dehydrated or when two molecules of acetic acid are heated with phosphorus pentoxide then one of its molecules is dehydrated.
CH3COOH + CH3COOH = -H2O = CH3COOCOCH3 + H2O
2CH3COOH + P2O5 = (CH3CO)2O + H2O
In the laboratory, acetic anhydride is produced when acetyl chloride and anhydrous sodium acetate reacts and are then fractionally distilled.
CH3COCl + CH3COONa = (CH3CO)2O + NaCl
Industrially, it can be prepared using different methods:
• By precipitating acetylene gas in glacial acetic in the presence of a sulfate catalyst to form ethylidene acetate which is distilled in the presence of zinc chloride to produce acetic anhydride.
CH = CH + 2CH3COOH = CH3CH(CH3COO)2
CH3.CH(CH3COO)2 = (CH3CO)2O + CH3CHO
• By heating excess anhydrous sodium acetate with thionyl chloride to form acetyl chloride which then reacts with sodium acetate to form ethanoic anhydride.
CH3COONa + SOCl2 = CH3COCl + SO2 + NaCl
CH3COONa + CH3COCl = (CH3CO)2O + NaCl
• By the acylation of ketene and acetic acid or by their reaction at low pressure.
Other methods of preparing acetic anhydride include:
• By the carbonylation of methyl acetate.
CH3CO2CH3 + CO = (CH3CO)2O
• By the oxidation of acetaldehyde.
• By heating two carboxylic acids in the presence of strong dehydrating agents like concentrated sulphuric acid by the elimination of a molecule of water.
• By the reaction of carbon oxide, methyl ester, and acetic acid with the aid of a nickel catalyst.
Chemical Properties of Acetic Anhydride
• Acetic Anhydride reacts with alcoholic compounds like ethanol to form an ester and acetic acid.
(CH3CO)2O + C2H5OH = CH3COOC2H5 + CH3COOH
• Acetic anhydride reacts with phosphorus pentachloride to form acetyl chloride.
(CH3CO)2O + PCl5 = 2CH3COCl + POCl3
• It reacts with primary and secondary amines to form acetamide.
(CH3CO)2O + C2H5NH2 = CH3CONH2 + CH3COOH
(CH3CO)2O + (C2H5)2NH = CH3CON(C2H5)2 + CH3COOH
• It reacts with water which is at a normal or high temperature to form acetic acid.
(CH3CO)2O + H2O = 2CH3COOH
• It reacts with hydrogen chloride gas to form acetic acid and acetic chloride.
(CH3CO)2O + HCl = CH3COCl + CH3COOH
• It reacts with other compounds to carry out its acetylation reaction with sodium acetate and sulfuric acid as catalysts.
Physical Properties of Acetic Anhydride
Acetic Anhydride is a colourless liquid with a vinegar-like smell. It has a molar mass of 102.089 g.mol-1, a density of 1.082 g cm3 or 9 lb/gal, and a flashing point of 129°F.
It melts at -73.1°C and -99.6 in °F and boils at 139.8 °C and 283.6 °F.
This compound doesn’t necessarily dissolve in water, benzene, and alcohol but rather reacts with it.
Uses of Acetic Anhydride
• It is used to preserve wood and ensure the durability of timber.
• It is used to produce cigarette filters.
• It is used in the production of dyes.
• It is used to convert cellulose to cellulose acetate.
• It is used in the production of pharmaceuticals like aspirin.
• It is used in the manufacturing of textile and photographic films.
• It is used as a reagent of acetylation in the production of carbonic compounds.
• It is used in the production of fibers.
• It is used in the production of vinyl acetate.
• Due to its corrosive, it is used in the production of explosives.
• It is used in the production of metal acetates.
• It is used for chemical synthesis.
• It is used as a reagent in laboratory practices.
• It is used for the deacetylation of morphine
Acetic Anhydride is a very harmful and combustible liquid so it should be stored and handled with care. It is corrosive to tissue and metal and its vapour is very toxic to the human body.
Inhalation can cause damage to the nose and throat. It can also cause pulmonary edema and chemical burns to the respiratory tract. And if the inhaled acetic anhydride is concentrated, it can cause severe coughing, shortness of breathing, and damage to your lung. If ingested it can also cause burns on your gastrointestinal tract and penetration of your digestive tract.
Contact with your eye can cause eye damage and an increased flow of tears. It is also very lethal to your skin and could cause burns and irritations if it comes in contact with your skin. So you must ensure you prevent eye contact by using protective and safe eyeglasses. You can prevent skin contact with this toxic liquid by wearing gloves and protective clothing. And you can prevent inhalation by putting on a breathable and protective nose cover/ mask.
It is also very flammable and can explode if exposed to extremely high temperatures so you must keep them in well-ventilated areas, away from hot conditions, direct sunlight, fire sparks, or flames.