Acyl Chlorides (OCR A Level Chemistry A): Revision Note

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

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 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