Acyl Chlorides & Esters (Edexcel IAL Chemistry)

Spec point — 15.13

Acyl Chlorides & Esters

Acyl groups can be built into many molecules using acyl chlorides or acid anhydrides (known as acylating agents). Acyl chlorides are derivatives of carboxylic acids by substitution of the -OH group by a chlorine atom. They are named by identifying the parent hydrocarbon chain and adding the suffix -oyl chloride. They can also be named by removing the -oic acid from the carboxylic acid and adding -oyl chloride

• Acid anhydrides are also derivatives of carboxylic acids formed by substitution of the -OH group by an alkanoate

  • Acid anhydrides are named by identifying the parent hydrocarbon chain and adding the suffix -oic anhydride
  • They can also be named by removing the -oic acid from the carboxylic acid and adding -oic anyhydride

Ethanoic acid derivatives

Worked Example Draw the displayed formula for the following:

A. Butanoyl chloride

B. Butanoic anhydride

Answer:

• Acyl chlorides are reactive organic compounds that undergo many reactions such as nucleophilic addition-elimination reactions

• In nucleophilic addition-elimination reactions, the nucleophilic addition of a small molecule across the C=O bond takes place followed by elimination of a small molecule

• Examples of these nucleophilic addition-elimination reactions include:

  • Hydrolysis
  • Reaction with alcohols to form esters
  • Reaction with ammonia and primary amines to form amides

Hydrolysis

• The hydrolysis of acyl chlorides results in the formation of a carboxylic acid and HCl **molecule

• This is a** nucleophilic 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 to form esters

• The esterification of acyl chlorides is also a nucleophilic addition-elimination reaction

  • The alcohol adds across the C=O bond
  • A HCl molecule is eliminated

Acyl chlorides undergo esterification with alcohols to form esters

Formation of amides

• Acyl chlorides react with ammonia or primary amines to form amides in a condensation reaction.

• A lone pair on the nitrogen atom attacks the carbonyl carbon in the acyl chloride.

• The reaction proceeds via a nucleophilic addition–elimination mechanism:

  • The nucleophile adds to the C=O bond
  • A chloride ion (Cl⁻) is eliminated
  • Hydrogen chloride (HCl) is formed

What happens to the HCl?

• The HCl formed does not remain unreacted.
• It is immediately neutralised by a second molecule of ammonia or amine present in excess.
• This forms an ammonium salt (e.g. NH₄Cl, CH₃NH₃Cl).

Why 2 molecules are needed

• The 1st molecule of ammonia/amine forms the amide
• The 2nd molecule of ammonia/amine neutralises the HCl and forms ammonium salt

Examples

• Reaction with ammonia
• Product: Primary amide (propanamide) and ammonium chloride

CH₃CH₂COCl + 2NH₃ → CH₃CH₂CONH₂ + NH₄Cl

• Reaction with methylamine
• Product: Secondary (substituted) amide and methylammonium chloride

CH₃COCl + 2CH₃NH₂ → CH₃CONHCH₃ + CH₃NH₃Cl

• Reaction with ethylamine
• Product: Secondary (substituted) amide and ethylammonium chloride

CH₃COCl + 2CH₃CH₂NH₂ → CH₃CONHCH₂CH₃ + CH₃CH₂NH₃Cl

Summary for formation of amides

• All reactions form HCl, which is not observed as a separate product
• The HCl is neutralised by excess ammonia or amine
• The final products are an amide and an ammonium salt

Exam Hint Acyl chlorides also react with secondary amines to form secondary amides. They do not react with tertiary amines.

• acyl chloride + primary amine → secondary amide + HCl
• acyl chloride + secondary amine → tertiary amide + HCl
• acyl chloride + tertiary amine → no reaction