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 react with ammonia and primary amines in a condensation reaction to form amides

  • This is an example of a nucleophilic addition-elimination reaction

• The lone pair of electrons on the nitrogen atom in ammonia or an amine attacks the electron-deficient carbonyl carbon of the acyl chloride

• The initial products are an amide and a molecule of hydrogen chloride (HCl)

  • The HCl produced is an acid.

• Since ammonia and amines are bases, a second molecule of the amine/ammonia will react with the HCl in a neutralisation reaction.

• Therefore, the overall reaction requires two moles of ammonia or amine for every one mole of acyl chloride.

• The final products are an amide and an ammonium salt.

Reaction with ammonia

• Reacting an acyl chloride with ammonia produces a primary amide.

• For example:

CH₃CH₂COCl + 2NH₂ → CH₃CH₂CONH₂ + NH₄Cl

Reaction with Primary Amines

• Reacting an acyl chloride with a primary amine produces a secondary amide (or N-substituted amide).

• For example:

CH₃CH₂COCl + 2CH₃NH₂ → CH₃CH₂CONHCH₃ + CH₃NH₃Cl