AQA A Level Chemistry

Revision Notes

7.5.4 Nucleophilic Addition–Elimination

Nucleophilic Addition–Elimination

  • Acyl chlorides undergo addition-elimination reactions such as hydrolysis, esterification reactions to form esters, and condensation reactions to form amides
  • The general mechanism of these 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
    • The lone pair on the oxygen atoms carry out an initial attack on the carbonyl carbon
    • 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 or phenols act as a nucleophile
    • The lone pair on the oxygen atoms carry out an initial attack on the carbonyl carbon
    • This is again followed by the elimination of an HCl molecule
  • With phenols, the reaction requires heat to proceed and needs to be carried out in the presence of a base
  • The base deprotonates the phenol to form a phenoxide ion which is a better nucleophile than the phenol molecule
    • The phenoxide ion carries out an initial attack on the carbonyl carbon
    • A small molecule of NaCl is eliminated

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

Reaction mechanism of the esterification of acyl chlorides with alcohols

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

Reaction mechanism of the esterification of acyl chlorides with phenols

Formation of amides: reaction mechanism

  • The nitrogen atom in ammonia and primary/secondary amines act as a nucleophile
    • The lone pair on the nitrogen atoms carry out an initial attack on the carbonyl carbon
    • This is followed by the elimination of an HCl molecule
  • Both reactions of acyl chlorides with ammonia and amines are vigorous however there are also differences
    • With ammonia – The product is a non-substituted amide and white fumes of HCl are formed
    • With amines – The product is a substituted amide and the HCl formed reacts with the unreacted amine to form a white organic ammonium salt

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

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

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

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

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