• 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:
  • Solid?phosphorus(V) chloride (PCl₅)
  • Liquid?phosphorus(III) chloride (PCl₃) and heat
  • Liquid?sulfur dichloride oxide (SOCl₂)
• Propanoyl chloride can this way be prepared from propanoic acid using the reactions above

Formation of acyl chlorides from their corresponding carboxylic acids

• 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?non-substituted?amide (when reacted with ammonia) or?substituted?amide (when reacted with primary and secondary 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