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AS/A-level - Acylation (Acyl Chlorides) 

Organic Chemistry, Acylation, Acyl Chlorides

· AS Chemistry,A-level Chemistry,Organic Chemistry,Acylation,Acyl Chlorides

Acylation reactions 

  • Involve the insertion of an acyl group into an organic compound.
  • The acyl group is RCO (where R is an alkyl or aromatic group)
  • The reagents use in acylation reactions are acyl chlorides or acid anhydrides. 

Acyl Chlorides 

  • Named after the parent carboxylic acid from which it is derived
  • Remove the ‘-oic acid’ suffix and replace with ‘-oyl chloride’ for the COCl bond system
  • e.g. propanoic acid ➔ propanoyl chloride
  • Very reactive and so very useful in organic synthesis
  • They are easily converted into carboxylic acid derivatives e.g. esters and amides with good yields
  • React with nucleophiles by losing the chloride ion whilst retaining the C=O double bond

Preparation of Acyl Chlorides

  • Must be prepared in a fume cupboard as the products are harmful- produce SO2 and HCl as gases
  • Prepared from their parent carboxylic acid + thionyl chloride (SOCl2)

e.g. propanoic acid(aq) + thionyl chloride(aq) ➔ propanoyl chloride(aq) + SO2(g) + HCl(g)

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Make sure to remember all the products of the reactions, these questions always come out in AS/A-level Chemistry!📚

Reaction of Acyl Chlorides with Alcohols

Acyl chloride + alcohol ➔ ester + HCl (hydrogen chloride gas)

  • Do you remember the content of A-level Chemistry regarding esterification?
  • Esterification reaction
  • Alcoholic conditions

e.g. ethanoyl chloride + propan-1-ol ➔ propyl ethanoate + HCl

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Acyl chloride + phenol ➔ phenyl ester + HCl

  • Carboxylic acids are not reactive enough to form esters with phenols
  • Acyl chlorides and acid anhydrides are both much more reactive than carboxylic acids

e.g. ethanoyl chloride + phenol ➔ phenylethanoate + HCl

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Reaction of Acyl Chlorides with Water

Acyl chloride + water ➔ carboxylic acid + hydrogen chloride

  • Hydrolysis
  • React instantly in cold water
  • Violent reaction takes place- evolution of dense, steamy hydrogen chloride fumes (white)
  • Very exothermic

e.g. ethanoyl chloride + water ➔ ethanoic acid + HCl

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Reaction of Acyl Chlorides with Ammonia 

Acyl chloride + ammonia ➔ primary amide + ammonium chloride

  • Ammonia can act as a nucleophile by donating the lone pair of electrons on the Nitrogen atom to an electron deficient species to form primary amides
  • A primary amide is when the Nitrogen atom is attached to one Carbon only and 2 Hydrogen atoms

e.g. ethanoyl chloride + ammonia ➔ ethanamide + ammonium chloride

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Reactions of Acyl Chlorides with Primary Amines

Acyl chloride + primary amine ➔ secondary amide + ammonium chloride

  • Amines can act as nucleophiles by donating the lone pair of electrons on the Nitrogen atom to an electron deficient species to form secondary amides

e.g. ethanoyl chloride + methylamine ➔ N-methylethanamide + methyl ammonium chloride

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References:

  • Acylation - Wikipedia
  • Acyl Chloride: Reactions & Synthesis | Study.com

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