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Properties & Reactions of Acid Chlorides | chemca

Properties & Reactions of Acid Chlorides | chemca
Organic Chemistry

Properties and Reactions of Acid Chlorides

Reactivity, Nucleophilic Substitution, Reduction, and Friedel-Crafts Acylation.

By chemca Team • Updated Jan 2026

Acid Chlorides (Acyl Chlorides, $R-COCl$) are the most reactive derivatives of carboxylic acids. They are widely used as acylating agents because the chloride ion is an excellent leaving group.

1. Physical Properties

  • State & Smell: Lower members are colorless, volatile liquids with a pungent, irritating smell.
  • Boiling Point: Lower than parent carboxylic acids because they cannot form intermolecular Hydrogen bonds. However, they are polar molecules with dipole-dipole interactions.
  • Solubility: Insoluble in water (they react/hydrolyze instead of dissolving). Soluble in organic solvents like ether, chloroform.

2. High Reactivity of Acid Chlorides

Why are they so reactive?

Acid chlorides undergo Nucleophilic Acyl Substitution very easily due to two factors:

  1. Inductive Effect: The Chlorine atom is strongly electronegative ($-I$ effect), making the carbonyl carbon highly electron-deficient (electrophilic).
  2. Good Leaving Group: The Chloride ion ($Cl^-$) is a very weak base, making it an excellent leaving group during the elimination step.
Reactivity Order: $RCOCl > (RCO)_2O > RCOOR' > RCONH_2$

3. Nucleophilic Acyl Substitution Reactions

Conversion to Other Derivatives

A. Hydrolysis (Reaction with Water): Forms Carboxylic Acids. Reacts vigorously with cold water.
$$ R-COCl + H_2O \rightarrow R-COOH + HCl $$
B. Alcoholysis (Reaction with Alcohol): Forms Esters. Pyridine is added to neutralize the $HCl$ produced.
$$ R-COCl + R'OH \xrightarrow{\text{Pyridine}} R-COOR' + HCl $$
C. Ammonolysis (Reaction with Ammonia/Amines): Forms Amides. Requires 2 equivalents of amine (one to react, one to neutralize HCl).
$$ R-COCl + 2NH_3 \rightarrow R-CONH_2 + NH_4Cl $$

4. Reduction Reactions

A. Rosenmund Reduction

Specific reduction to Aldehydes. Hydrogenation over Palladium supported on Barium Sulphate ($Pd/BaSO_4$) poisoned with Quinoline/Sulphur to prevent further reduction to alcohol.

$$ R-COCl + H_2 \xrightarrow{Pd/BaSO_4} R-CHO + HCl $$

B. Reduction to Alcohols

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

$$ R-COCl \xrightarrow{1. LiAlH_4, 2. H_3O^+} R-CH_2OH $$

5. Friedel-Crafts Acylation

Formation of Aromatic Ketones

Acid chlorides react with benzene or activated benzene derivatives in the presence of a Lewis Acid ($Anhydrous AlCl_3$) to introduce the acyl group ($RCO-$) into the ring.

$$ C_6H_6 + CH_3COCl \xrightarrow{Anhyd. AlCl_3} \underset{\text{Acetophenone}}{C_6H_5-COCH_3} + HCl $$
Mechanism: $AlCl_3$ abstracts $Cl^-$ to generate the Acylium ion ($R-C^+=O$), which acts as the electrophile.

6. Reaction with Organometallics

A. With Grignard Reagent ($RMgX$): Yields Tertiary Alcohols (Reaction doesn't stop at ketone).
$$ R-COCl + 2R'MgX \xrightarrow{H_3O^+} R-C(OH)(R')_2 $$
B. With Dialkyl Cadmium ($R_2Cd$): Yields Ketones. Cadmium reagents are less reactive than Grignard and do not react further with the ketone formed.
$$ 2R-COCl + R'_2Cd \rightarrow 2R-CO-R' + CdCl_2 $$

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