Acyl Chloride

What is an acyl chloride, and how is one made from a carboxylic acid?

An acyl chloride contains the –COCl functional group. It is made by reacting a carboxylic acid with thionyl chloride (SOCl₂), which replaces the –OH of the carboxyl group with –Cl:
RCOOH + SOCl₂ → RCOCl + SO₂ + HCl
Example: CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl (ethanoic acid → ethanoyl chloride)
The reaction produces SO₂ and HCl gases as by-products. Acyl chlorides are much more reactive than carboxylic acids, making them useful synthetic intermediates.

Why are acyl chlorides more reactive than carboxylic acids?

The C–Cl bond in an acyl chloride is more polarised than the C–OH bond in a carboxylic acid, making the carbonyl carbon even more δ+. Additionally, Cl⁻ is a much better leaving group than OH⁻, so nucleophilic substitution occurs more readily and rapidly. Acyl chlorides react vigorously with water, alcohols, and amines even at room temperature and without a catalyst

What is the difference between a primary and secondary amide?

A primary amide has the –CONH₂ group — the nitrogen is bonded to one carbon (the carbonyl carbon) and two hydrogens. It is formed from an acyl chloride and ammonia. A secondary amide has the –CONHR group — the nitrogen is bonded to two carbons (the carbonyl carbon and one from R). It is formed from an acyl chloride and a primary amine. The terms “primary” and “secondary” for amides refer to the degree of substitution on the nitrogen.

What is a primary amide and how does it react with acyl chloride

A primary amide has the –CONH₂ group — the nitrogen is bonded to one carbon (the carbonyl carbon) and two hydrogens. It is formed from an acyl chloride and ammonia.

CH₃CH₂COCl + 2NH₃ → CH₃CH₂CONH₂ + NH₄Cl ( ethanamide + ammonium chloride)

What is a secondary amide and how is it formed from acyl chloride

A secondary amide has the –CONHR group — the nitrogen is bonded to two carbons (the carbonyl carbon and one from R). It is formed from an acyl chloride and a primary amine.

CH₃CH₂COCl + 2CH₃NH₂ → CH₃CH₂CONHCH₃ + CH₃NH₃Cl

Why does phenol react poorly with carboxylic acids during esterification?

Phenol is a weaker nucleophile than aliphatic alcohols because the lone pairs on its oxygen atom are partially delocalised into the benzene ring, reducing electron density. Consequently, carboxylic acids are not electrophilic enough to react with phenol under normal conditions.

Why can phenol react with an acyl chloride to form an ester at room temperature?

Acyl chlorides are significantly more reactive than carboxylic acids because the carbonyl carbon is more electron-deficient (δ+) and Cl− acts as a much better leaving group. This enables the weakly nucleophilic phenol to readily attack and form an ester:

CH₃COCl + C₆H₅OH → CH₃COOC₆H₅ + HCl (ethanoyl chloride + phenol → phenyl ethanoate)