Addition-Elimination Reactions of Acyl Chlorides (Cambridge (CIE) A Level Chemistry): Revision Note

Mechanism of Addition - Elimination in Acyl Chloride Reactions

• Acyl chlorides undergo addition-elimination reactions such as hydrolysis, esterification reactions to form esters, and condensation reactions to form amides

• The general mechanism of these addition-elimination reactions involves two steps:

  • Step 1 - Addition of a nucleophile across the C=O bond

  • Step 2 - Elimination of a small molecule such as HCl or H₂O

Mechanism of hydrolysis of acyl chlorides

• In the hydrolysis of acyl chlorides, the water molecule acts as a nucleophile

  • The lone pair of the oxygen atom from water carries out an initial attack on the carbonyl carbon

  • This is followed by the elimination of a hydrochloric acid (HCl) molecule

Reaction mechanism of the hydrolysis of acyl chlorides 

The two-step addition-elimination reaction mechanism of propanoyl chloride to form propanoic acid

Formation of esters: reaction mechanism

• In the esterification reaction of acyl chlorides, the alcohols or phenols act as a nucleophile

  • The lone pair of the alcohol / phenol oxygen atom carries out an initial attack on the carbonyl carbon

  • This is again followed by the elimination of an HCl molecule

• Phenols react with acyl chlorides at room temperature, more slowly than alcohols; adding a base converts phenol to the more nucleophilic phenoxide ion and speeds the reaction up

• The base deprotonates the phenol to form a phenoxide ion which is a better nucleophile than the phenol molecule

  • The phenoxide ion carries out an initial attack on the carbonyl carbon

  • A small molecule of NaCl is eliminated

Reaction mechanism of the esterification of acyl chlorides with alcohols

The two-step addition-elimination reaction mechanism of propanoyl chloride and ethanol to form ethyl propanoate and hydrogen chloride

Reaction mechanism of the esterification of acyl chlorides with phenols

The three-step addition-elimination reaction mechanism of propanoyl chloride with phenol to form phenyl propanoate

Formation of amides: reaction mechanism

• The nitrogen atom in ammonia and primary/secondary amines act as a nucleophile

  • The lone pair of the nitrogen atom carries out an initial attack on the carbonyl carbon

  • This is followed by the elimination of an HCl molecule

• Both reactions of acyl chlorides with ammonia and amines are vigorous however there are also differences

  • With ammonia - The product is a non-substituted amide and white fumes of HCl are formed

  • With amines - The product is a substituted amide and the HCl formed reacts with the unreacted amine to form a white organic ammonium salt

Reaction mechanism of the formation of amides from acyl chlorides with ammonia

The two-step addition-elimination reaction mechanism of propanoyl chloride and ammonia to form propanamide

Reaction mechanism of the formation of amides from acyl chlorides with primary amines

The addition-elimination reaction mechanism of propanoyl chloride and methylamine to form N-methylpropanamide

Reaction mechanism of the formation of amides from acyl chlorides with secondary amines

The addition-elimination reaction mechanism of propanoyl chloride and dimethylamine to form N,N-dimethylpropanamide