AQA A Level Chemistry

Revision Notes

7.3.5 Acylation

Acyl Groups

  • Esters are organic compounds with an -COR functional group
  • They have characteristic smells and are used in perfumes, cosmetics and as solvents
  • Esters can be prepared from the condensation reaction between alcohols and carboxylic acids
    • This is also called an esterification reaction
  • A more effective way of preparing esters is from the condensation reaction between alcohols and acyl chlorides instead
  • Unlike the reactions with carboxylic acids, acyl chlorides are more reactive (so the reactions happen faster) and their reactions go to completion (so no equilibrium mixture is formed and the yield of the ester is maximum)
  • Examples of esterification reactions include:
    • Formation of ethyl ethanoate from ethanol and ethanoyl chloride
    • Formation of phenyl benzoate from phenol and benzoyl chloride

Carboxylic Acids _ Derivatives - Esterification Reactions, downloadable AS & A Level Chemistry revision notes

Formation of esters from the reaction of alcohols with acyl chlorides

Nucleophilic Addition–Elimination

  • 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

 

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 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

Carboxylic Acids & Derivatives - Esterification of Acyl Chlorides, downloadable AS & A Level Chemistry revision notes

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 non-substituted amide (when reacted with ammonia) or substituted amide (when reacted with primary and secondary 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

Carboxylic Acids & Derivatives - Condensation of Acyl Chlorides, downloadable AS & A Level Chemistry revision notes

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

Nucleophilic Addition–Elimination Mechanism

  • 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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