Addition-Elimination Reactions of Acyl Chlorides (CIE A Level Chemistry)

• 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
• With phenols, the reaction requires heat to proceed and needs to be carried out in the presence of a base
• 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 water

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 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 dimethylpropanamide