AQA A Level Chemistry

Revision Notes

7.3.5 Acylation

Acyl Groups

Acyl groups

  • 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
    • Acyl chlorides 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) butanoyl anhydride

Answer:

7.3.5 Worked example answer, downloadable AS & A Level Chemistry revision notes

 

Nucleophilic Addition–Elimination

  • 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

 

Carboxylic 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 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 can form amides with primary amines and ammonia
  • The nitrogen atom in ammonia and primary amine has a lone pair of electrons which can be used to attack the carbonyl carbon atom in the acyl chlorides
  • The product is an amide (when reacted with ammonia) or N-substituted amide (when reacted with primary amines)
  • This is also an example of a nucleophilic addition-elimination reaction as
    • The amine or ammonia molecule adds across the C=O bond
    • A HCl molecule is eliminated

 

 

 

 

 

 

 

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

Nucleophilic Addition–Elimination Mechanism

  • Acyl chlorides undergo nucleophilic addition-elimination reactions such as hydrolysis, esterification reactions to form esters, and condensation reactions to form amides
  • The general mechanism of these nucleophilic addition-elimination reactions involve two steps:
    • Step 1 – Addition of a nucleophile across the C=O bond
    • Step 2 – Elimination of a small molecule such as HCl or H2O

Mechanism of hydrolysis of acyl chlorides

  • In the hydrolysis of acyl chlorides, the water molecule acts as a nucleophile
    • Step 1 – Nucleophilic addition; the lone pair on the oxygen atoms carry out an initial attack on the carbonyl carbon
    • Step 2 – Elimination; this is followed by the elimination of a hydrochloric acid (HCl) molecule

 

Carboxylic Acids _ Derivatives - Mechanism Hydrolysis, downloadable AS & A Level Chemistry revision notes

Reaction mechanism of the hydrolysis of acyl chlorides

Formation of esters: reaction mechanism

  • In the esterification reaction of acyl chlorides, the alcohols act as a nucleophile
    • Step 1 – Nucleophilic addition; the lone pair on the oxygen atoms carry out an initial attack on the carbonyl carbon
    • Step 2 – Elimination; this is again followed by the elimination of an HCl molecule

Carboxylic Acids _ Derivatives - Mechanism Esterification with Alcohol, downloadable AS & A Level Chemistry revision notes

Reaction mechanism of the esterification of acyl chlorides with alcohols

Formation of amides: reaction mechanism

  • The nitrogen atom in ammonia and primary amines act as a nucleophile
    • Step 1 – Nucleophilic addition; the lone pair on the nitrogen atoms carry out an initial attack on the carbonyl carbon
    • Step 2 – Elimination; this is followed by the elimination of an HCl molecule
    • Step 3 – Acid-Base reaction; the HCl formed would immediately react with excess ammonia to give ammonium chloride in an acid-base reaction

 

 

 

 

 

 

 

Reaction mechanism of the formation of amides from acyl chlorides with ammonia

Carboxylic Acids _ Derivatives - Mechanism Condensation with Primary Amines (1), downloadable AS & A Level Chemistry revision notes Carboxylic Acids _ Derivatives - Mechanism Condensation with Primary Amines (2), downloadable AS & A Level Chemistry revision notes

Reaction mechanism of the formation of amides from acyl chlorides with primary amines

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