Acyl Chlorides

Specification Reference Organic Chemistry, Carboxylic Acids and Derivatives 33.3

Quick Notes

Acyl chlorides can be made from carboxylic acids using PCl₅, PCl₃ + heat, or SOCl₂.
Reactions at room temperature:

With water: forms a carboxylic acid + HCl
With alcohols: forms an ester + HCl
With phenol: forms an ester + HCl
With ammonia: forms an amide + HCl
With amines: forms a substituted amide + HCl

All reactions follow a nucleophilic addition–elimination mechanism.
Hydrolysis: Acyl chlorides hydrolyse more easily than alkyl or aryl chlorides due to stronger polarity and reactivity of the carbonyl group.

Full Notes

Acyl chlorides (R–COCl) are highly reactive derivatives of carboxylic acids.

Formation of Acyl Chlorides from Carboxylic Acids

They are commonly made by replacing the –OH group of a carboxylic acid with a –Cl atom using one of the following reagents:

PCl₅ (phosphorus pentachloride)

CIE A-Level Chemistry reaction of carboxylic acid with PCl5 to form acyl chloride, POCl3, and HCl.

RCOOH + PCl₅ → RCOCl + POCl₃ + HCl

PCl₃ (phosphorus trichloride) and heat

CIE A-Level Chemistry reaction of carboxylic acid with PCl3 on heating to form acyl chloride and H3PO3.

3RCOOH + PCl₃ → 3RCOCl + H₃PO₃

SOCl₂ (thionyl chloride)

CIE A-Level Chemistry reaction of carboxylic acid with thionyl chloride to form acyl chloride, SO2, and HCl.

RCOOH + SOCl₂ → RCOCl + SO₂ + HCl
(Cleanest method – all by-products are gases)

Reactions of Acyl Chlorides at Room Temperature

Due to the polarised C=O and C–Cl bonds, acyl chlorides are very reactive to nucleophiles, especially in nucleophilic addition–elimination reactions.

Hydrolysis with Water

CIE A-Level Chemistry hydrolysis of acyl chloride with water forming carboxylic acid and HCl gas.

RCOCl + H₂O → RCOOH + HCl
This is a vigorous reaction; steamy fumes of HCl are released.

Reaction with Alcohols

CIE A-Level Chemistry reaction of acyl chloride with alcohol forming ester and HCl.

RCOCl + R'OH → RCOOR' + HCl
Forms an ester without needing a catalyst (faster and more direct than using carboxylic acids).

Reaction with Phenol

CIE A-Level Chemistry reaction of acyl chloride with phenol forming ester and HCl.

RCOCl + PhOH → RCOOPh + HCl
Phenol is less reactive than alcohols but still forms an ester readily due to acyl chloride’s high reactivity.

Reaction with Ammonia (NH₃)

CIE A-Level Chemistry reaction of acyl chloride with ammonia forming amide and HCl.

RCOCl + NH₃ → RCONH₂ + HCl
Forms a primary amide. HCl produced reacts with excess NH₃ to form NH₄Cl.

Reaction with Primary Amines

CIE A-Level Chemistry reaction of acyl chloride with primary amine forming substituted amide and HCl.

RCOCl + R'NH₂ → RCONHR' + HCl
(Can also react with Secondary amines to form tertiary amide RCOCl + R'₂NH → RCONR'₂ + HCl)
Forms substituted amides. HCl is neutralised by excess amine.

Nucleophilic Addition–Elimination Mechanism

Because of the partially positively charged in the C=O group, acyl chlorides react with nucleophiles in addition–elimination.

step 1: Nucleophile attracted to δ+C in the C=O bond in the acyl chloride and uses lone pair of electrons to form bond to C
step 2: C=O double bond breaks, with bonding pair of electrons going to the oxygen
step 3: Carbon–oxygen bond reforms as carbon–chlorine bond breaks
step 4: Nu–H bond breaks

This is common to all reactions in 33.3.2 (a)–(e).

Ease of Hydrolysis: Acyl Chlorides vs. Alkyl Chlorides vs. Aryl Chlorides

In the hydrolysis of acyl chlorides, alkyl chlorides and aryl chlorides, water is added and the corresponding carboxylic acid gets formed. However, how easily the reaction occurs is based on the type of chloride reacting.

Acyl chlorides: Very reactive and easily hydrolysed due to electron-deficient carbonyl carbon and good leaving group (Cl⁻).
Alkyl chlorides (e.g. CH₃CH₂Cl): Much less reactive and hydrolysis requires stronger conditions.
Aryl chlorides (e.g. chlorobenzene): Much lower reactivity and resistant to hydrolysis because the C–Cl bond is strengthened by delocalisation into the benzene ring, resisting nucleophilic attack.

Summary

Acyl chlorides are prepared from carboxylic acids using PCl₅, PCl₃, or SOCl₂.
They react vigorously at room temperature

With water: forms a carboxylic acid + HCl
With alcohols: forms an ester + HCl
With phenol: forms an ester + HCl
With ammonia: forms an amide + HCl
With amines: forms a substituted amide + HCl

All reactions follow the nucleophilic addition–elimination mechanism.
Acyl chlorides hydrolyse much more easily than alkyl or aryl chlorides.