Acyl Chlorides (OCR A Level Chemistry)

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Formation of 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

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Uses of Acyl Chlorides

  • 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 HClCarboxylic Acids & Derivatives - Overall Hydrolysis Acyl Chlorides, downloadable AS & A Level Chemistry revision notes

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 eliminatedCarboxylic Acids & Derivatives - Esterification of Acyl Chlorides, downloadable AS & A Level Chemistry revision notes

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

Formation of amide reaction
Acyl chlorides undergo condensation reactions with ammonia and amines to form amides

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Sonny

Author: Sonny

Expertise: Chemistry

Sonny graduated from Imperial College London with a first-class degree in Biomedical Engineering. Turning from engineering to education, he has now been a science tutor working in the UK for several years. Sonny enjoys sharing his passion for science and producing engaging educational materials that help students reach their goals.