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Properties & Reactions of Acid Anhydrides | chemca

Properties & Reactions of Acid Anhydrides | chemca
Organic Chemistry

Properties and Reactions of Acid Anhydrides

Preparation, Reactivity, and Acylation Reactions.

By chemca Team • Updated Jan 2026

Acid Anhydrides ($(RCO)_2O$) are derived from carboxylic acids by the removal of water. They are less reactive than acid chlorides but more reactive than esters and amides, making them valuable acylating agents in organic synthesis.

1. Preparation Methods

Dehydration and Substitution

A. From Carboxylic Acids: Heating acids with strong dehydrating agents like Phosphorus Pentoxide ($P_2O_5$).
$$ 2CH_3COOH \xrightarrow{P_2O_5, \Delta} (CH_3CO)_2O + H_2O $$
B. From Acid Chlorides: Reaction of Acid Chloride with Sodium Carboxylate.
$$ RCOCl + RCOONa \xrightarrow{\Delta} (RCO)_2O + NaCl $$

2. Physical Properties

  • State & Smell: Lower anhydrides are colorless liquids with a pungent, irritating smell.
  • Boiling Point: Higher than parent acids due to higher molecular mass, even though they lack intermolecular hydrogen bonding.
  • Solubility: Slightly soluble in water (slowly hydrolyze). Soluble in organic solvents.

3. Chemical Reactivity

Nucleophilic Acyl Substitution

Anhydrides react similarly to acid chlorides but are less vigorous. The leaving group is the carboxylate ion ($RCOO^-$), which is a weaker base than $NH_2^-$ or $RO^-$ but stronger than $Cl^-$.

A. Hydrolysis (Reaction with Water): Forms two molecules of Carboxylic Acid.
$$ (RCO)_2O + H_2O \rightarrow 2RCOOH $$
B. Alcoholysis (Reaction with Alcohol): Forms an Ester and a Carboxylic Acid. Used in acetylation (e.g., Aspirin synthesis).
$$ (RCO)_2O + R'OH \rightarrow RCOOR' + RCOOH $$
C. Ammonolysis (Reaction with Ammonia/Amines): Forms an Amide and a Carboxylic Acid (or its ammonium salt).
$$ (RCO)_2O + 2NH_3 \rightarrow RCONH_2 + RCOONH_4 $$
Reactivity Order: Acid Chloride > Acid Anhydride > Ester > Amide

4. Friedel-Crafts Acylation

Introduction of Acyl Group

Anhydrides react with benzene or activated benzene derivatives in the presence of anhydrous Aluminium Chloride ($AlCl_3$) to form aromatic ketones.

$$ C_6H_6 + (CH_3CO)_2O \xrightarrow{Anhyd. AlCl_3} \underset{\text{Acetophenone}}{C_6H_5-COCH_3} + CH_3COOH $$
Advantage over Acid Chlorides: Anhydrides are less corrosive, less volatile, and easier to handle.

5. Reduction Reactions

Conversion to Alcohols

Strong reducing agents like Lithium Aluminium Hydride ($LiAlH_4$) reduce anhydrides to primary alcohols.

$$ (RCO)_2O \xrightarrow{1. LiAlH_4, 2. H_3O^+} 2RCH_2OH $$

6. Synthesis of Aspirin

Acetylation of Salicylic Acid

Salicylic acid reacts with Acetic Anhydride in the presence of acid catalyst to form Acetylsalicylic acid (Aspirin).

$$ \underset{\text{Salicylic Acid}}{o\text{-}HO\text{-}C_6H_4\text{-}COOH} + (CH_3CO)_2O \xrightarrow{H^+} \underset{\text{Aspirin}}{o\text{-}(CH_3COO)\text{-}C_6H_4\text{-}COOH} + CH_3COOH $$

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