Acyl Chlorides (CIE A Level Chemistry)

• 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

Using propanoic acid to form propanoyl chloride

Propanoic acid can be used to produce propanoyl chloride with different by-products depending on the reagent used

• 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

Hydrolysis of acyl chlorides

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

Esterification reactions using acyl chlorides

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

Amide formation reactions using acyl chlorides

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