Acyl Chlorides

· Acyl chlorides are carboxylic acid derivatives with the functional group RCOCl.
· The carbonyl carbon is strongly δ+ because both O and Cl withdraw electron density.
· The C–Cl bond can break to release Cl⁻, making acyl chlorides very reactive towards nucleophiles.
· General reaction pattern: RCOCl + H–Nu → RCONu + HCl.
· Key mechanism type: addition–elimination.

This diagram shows the RCOCl functional group found in all acyl chlorides. The C=O and C–Cl bonds are the key reactive parts of the molecule. Source

Making Acyl Chlorides

· Acyl chlorides are made from carboxylic acids.
· Reagents and conditions to recall: PCl₃ and heat, PCl₅, or SOCl₂.
· General transformation: RCOOH → RCOCl.
· Example equations:
· RCOOH + PCl₅ → RCOCl + POCl₃ + HCl.
· 3RCOOH + PCl₃ → 3RCOCl + H₃PO₃.
· RCOOH + SOCl₂ → RCOCl + SO₂ + HCl.
· Exam tip: acyl chlorides must not be mixed with water unless hydrolysis is intended.

Hydrolysis with Water

· Reagent/condition: water, room temperature.
· Product: carboxylic acid + HCl.
· General equation: RCOCl + H₂O → RCOOH + HCl.
· This reaction is rapid because water acts as a nucleophile and chloride is a good leaving group.
· Observation often associated with low-Mr acyl chlorides: steamy acidic fumes of HCl.

This shows how water reacts with an acyl chloride to form a carboxylic acid and HCl. It is useful for seeing the addition step, the elimination of chloride, and the final proton transfer. Source

Reaction with Alcohols

· Reagent/condition: alcohol, room temperature.
· Product: ester + HCl.
· General equation: RCOCl + R′OH → RCOOR′ + HCl.
· Example: ethanoyl chloride + ethanol → ethyl ethanoate + HCl.
· Key exam phrase: alcohols react with acyl chlorides by addition–elimination.
· Compared with esterification using carboxylic acids, acyl chlorides react more readily and do not require concentrated H₂SO₄ catalyst.

This page shows the general idea of nucleophilic acyl substitution. For acyl chlorides, the nucleophile replaces Cl, giving a new carboxylic acid derivative such as an ester. Source

Reaction with Phenol

· Reagent/condition: phenol, room temperature.
· Product: ester + HCl.
· General equation: RCOCl + C₆H₅OH → RCOOC₆H₅ + HCl.
· Example: benzoyl chloride + phenol → phenyl benzoate + HCl.
· Key exam point: phenol is less reactive than alcohols, but still reacts with reactive acyl chlorides to form esters.
· Product naming: an ester made from phenol contains the phenyl group, e.g. phenyl benzoate.

Reaction with Ammonia

· Reagent/condition: ammonia, room temperature.
· Product: amide + HCl.
· General equation: RCOCl + NH₃ → RCONH₂ + HCl.
· In practice, excess NH₃ may neutralise HCl to form NH₄Cl.
· Example: ethanoyl chloride + ammonia → ethanamide + HCl.
· This is a condensation reaction because a small molecule, HCl, is eliminated.

This example supports the idea that ammonia reacts with acyl chlorides to form amides. It reinforces that the reaction occurs by replacing Cl with a nitrogen-containing group. Source

Reaction with Primary and Secondary Amines

· Reagent/condition: primary or secondary amine, room temperature.
· Product: amide + HCl.
· With a primary amine: RCOCl + R′NH₂ → RCONHR′ + HCl.
· With a secondary amine: RCOCl + R′₂NH → RCONR′₂ + HCl.
· Primary amines form secondary amides.
· Secondary amines form tertiary amides.
· Key phrase: reaction of ammonia or amines with acyl chlorides is a condensation reaction forming an amide.

Addition–Elimination Mechanism

· Step 1: nucleophile attacks the δ+ carbonyl carbon.
· Step 2: the C=O π bond breaks, forming a tetrahedral intermediate.
· Step 3: the C=O reforms.
· Step 4: Cl⁻ is eliminated as the leaving group.
· Step 5: H⁺ is lost, giving the final product and HCl.
· The mechanism is called addition–elimination because the nucleophile first adds, then chloride is eliminated.
· Curly arrow exam rule: arrows must start from a lone pair or bond, and point to where the electron pair moves.

These diagrams show the two key stages of acyl chloride reactions: nucleophilic addition followed by elimination of chloride. This is the core mechanism required for reactions with water, alcohols, phenol, ammonia, and amines. Source

Relative Ease of Hydrolysis

· Hydrolysis becomes easier in the order: halogenoarene < halogenoalkane < acyl chloride.
· Acyl chlorides hydrolyse most easily because the carbonyl carbon is highly δ+ and Cl⁻ is a good leaving group.
· Alkyl chlorides hydrolyse less readily because they need nucleophilic substitution at a saturated carbon, with no activating carbonyl group.
· Halogenoarenes hydrolyse least readily because the C–Cl bond is strengthened by interaction with the benzene ring’s delocalised π system.
· In halogenoarenes, the carbon bonded to Cl is sp² and the ring resists normal nucleophilic substitution.
· Exam comparison phrase: acyl chlorides react rapidly because they undergo addition–elimination at the carbonyl carbon, while halogenoarenes are resistant because of delocalisation and a stronger C–Cl bond.

The diagram highlights why acid chlorides are among the most reactive carboxylic acid derivatives. This supports the CIE comparison of hydrolysis ease, because acyl chlorides have a highly electrophilic carbonyl carbon and a good leaving group. Source

Common Exam Equations to Memorise

· RCOCl + H₂O → RCOOH + HCl.
· RCOCl + R′OH → RCOOR′ + HCl.
· RCOCl + C₆H₅OH → RCOOC₆H₅ + HCl.
· RCOCl + NH₃ → RCONH₂ + HCl.
· RCOCl + R′NH₂ → RCONHR′ + HCl.
· RCOCl + R′₂NH → RCONR′₂ + HCl.
· RCOOH + PCl₅ → RCOCl + POCl₃ + HCl.
· RCOOH + SOCl₂ → RCOCl + SO₂ + HCl.