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)
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
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
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
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
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
This is the end of topic!
Drafted by Cherry (Chemistry)