Acetic Anhydride or Ethanoic Anhydride

Preparation of Acetic Anhydride

Acetic acid: By heating acetic acid with phosphorus pentoxide, one of its two molecules removes water and forms acetic anhydride.

2CH₃COOH + P₂O₅ → (CH₃CO)₂O + H₂O

Laboratory Method: In the laboratory, acetic anhydride is obtained by distillation of a mixture of acetyl chloride and anhydrous sodium acetate.

CH₃COCl + CH₃COONa → (CH₃CO)₂O + NaCl

Method: Take about 50 grams of anhydrous sodium acetate powder in a distillation flask and arrange it with a point funnel, condenser and receiver. Take about 40 grams of acetyl chloride in the funnel and pour it slowly into the flask.

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At this time, keep the flask in cold water. After adding complete acetyl chloride, shake the flask thoroughly and place it on the sand boiler.

The flask is heated to 135-140°C to distill the mixture kept in it. The acetic anhydride is distilled and collected in the receptor. The anhydrous calcium chloride placed in the receptacle’s lateral tube absorbs air moisture and does not allow it to come in contact with acetic anhydride.

A small amount of anhydrous calcium acetate added to the acetic anhydride thus obtained gives a pure acetic anhydride upon re-distillation.

A small amount of anhydrous calcium acetate added to the
acetic anhydride thus obtained gives a pure acetic anhydride upon
re-distillation.

Industrial Manufacture: A commercial quantity of acetic anhydride is obtained by the following methods.

On heating an excess of anhydrous sodium acetate with thionyl chloride, acetyl chloride is first formed which reacts with the remaining sodium acetate to form acetic anhydride.

CH₃COONa + SOCl₂ → CH₃COCl + SO₂ + NaCl

CH₃COONa + CH₃COCl → (CH₃CO)₂O + NaCl

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Ethylidene acetate is obtained when acetylene gas is precipitated in the glacial acetic acid in the presence of a marcuric sulfate catalyst whose distillation in the presence of zinc chloride yields acetic anhydride.

CH≡CH + 2CH₃COOH → CH₃CH(CH₃COO)₂

CH₃.CH(CH₃COO)₂ → (CH₃CO)₂O + CH₃CHO

Physical Properties

It is a colorless, pungent odor, less soluble in cold water and soluble in ether and benzene. Its boiling point is 139.5°C and relative density is 1.087.

Chemical Properties

The chemical reactions of acetic anhydride are similar to acetyl chloride but are less reactive than acetyl chloride. Following are its major chemical reactions.

Water Decomposition: It does not decompose with cold water. At normal or high temperatures, this water decomposes to form acetic acid.

(CH₃CO)₂O + H₂O → 2CH₃COOH

Acetylation: Like acetyl chloride, it reacts with compounds containing -OH and -NH₂ group and displaces the hydrogen atom by acetyl group. Since it is less reactive than acetyl chloride, H₂SO₄ or sodium acetate are used as catalysts in its acetylation reaction. Following is its major acetylation reactions.

Reactions with alcohols: It reacts with alcohols to form ester.

(CH₃CO)₂O + C₂H₅OH → CH₃COOC₂H₅ + CH₃COOH

Reaction with ammonia: By acting with ammonia, it forms acetamide.

(CH₃CO)₂O + NH₃ → CH₃CONH₂ + CH₃COOH

Reaction with primary and secondary amines: By reacting with primary and secondary amines, it forms N substituted acetamide.

(CH₃CO)₂O + C₂H₅NH₂ → CH₃CONH₂ + CH₃COOH

(CH₃CO)₂O + (C₂H₅)₂NH → CH₃CON(C₂H₅)₂ + CH₃COOH

Reaction with hydrogen chloride: It reacts with dry hydrogen chloride gas to form acetyl chloride and acetic acid.

(CH₃CO)₂O + HCl → CH₃COCl + CH₃COOH

Phosphorus pentachloride: It reacts with Phosphorus pentachloride to form acetyl chloride.

(CH₃CO)₂ + PCl₅ → 2CH₃COCl + POCl₃

Reduction: It is reduced by lithium aluminum hydride to form ethanol.

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(CH₃CO)₂O + LiAlH₄ → 2C₂H₅OH + H₂O

The above reduction can also be done by H₂/Ni, Na/alcohol or NaBH₄.

Acetic Anhydride Uses

To find the number of -OH and -NH₂ groups in carbonic compounds,

As acetylation reagent in the manufacture of carbonic compounds,

In the manufacture of pigments and medical such as Aspirin

As reagent in the laboratory.