6.3.3 Acyl Chlorides

• Due to the increased reactivity of acyl chlorides compared to carboxylic acids, they are often used as starting compounds in organic reactions
• Acyl chlorides are compounds that contain an -COCl functional group and can be prepared from the reaction of carboxylic acids with:
  • Liquid sulfur dichloride oxide (SOCl₂)
• Propanoyl chloride can this way be prepared from propanoic acid using the reaction above

• Acyl chlorides are reactive organic compounds that undergo many reactions such as addition-elimination reactions
• In addition-elimination reactions, the addition of a small molecule across the C=O bond takes place followed by elimination of a small molecule
• Examples of these addition-elimination reactions include:
  • Hydrolysis
  • Reaction with alcohols and phenols to form esters
  • Reaction with ammonia and amines to form amides

Hydrolysis

• The hydrolysis of acyl chlorides results in the formation of a carboxylic acid and HCl molecule
• This is an addition-elimination reaction
  • A water molecule adds across the C=O bond
  • A hydrochloric acid (HCl) molecule is eliminated

• An example is the hydrolysis of propanoyl chloride to form propanoic acid and HCl

Acyl chlorides are hydrolysed to carboxylic acids

Formation of esters

• Acyl chlorides can react with alcohols and phenols to form esters
  • The reaction with phenols requires heat and a base

• Esters can also be formed from the reaction of carboxylic acids with phenol and alcohols however, this is a slower reaction as carboxylic acids are less reactive and the reaction does not go to completion (so less product is formed)
• Acyl chlorides are therefore more useful in the synthesis of esters
• The esterification of acyl chlorides is also an addition-elimination reaction
  • The alcohol or phenol adds across the C=O bond
  • A HCl molecule is eliminated

Acyl chlorides undergo esterification with alcohols and phenols to form esters

Formation of amides

• Acyl chlorides can form amides from their condensation reaction with amines and ammonia
• The nitrogen atom in ammonia and amines has a lone pair of electrons which can be used to attack the carbonyl carbon atom in the acyl chlorides
• The product is a primary  amide (when reacted with ammonia) or secondary amide (when reacted with primary amines)
• This is also an example of an addition-elimination reaction as
  • The amine or ammonia molecule adds across the C=O bond
  • A HCl molecule is eliminated

Acyl chlorides undergo condensation reactions with ammonia and amines to form amides