Reactions of Acyl Halides
Carboxylic Acid Derivatives. Nucleophilic Acyl Substitution
of Acyl Chlorides
Nucleophilic Acyl Substitution
- Acyl chlorides are the
most reactive of the carboxylic acid derivatives and therefore can be readily
converted into other carboxylic acid derivatives (see above).
- They are sufficiently
reactive that they react quite readily with cold water and hydrolyze to the
- The HCl by-product is
usually removed by adding a base such as pyridine or triethyl amine.
Acylation of Benzene
(review of Chapter 12)
Reaction type: Electrophilic
Aromatic Substitution (of the aromatic), or
Nucleophilic Acyl Substitution (of the acyl halide)
- Overall transformation
: Ar-H to Ar-COR
- Named after Friedel
and Crafts who discovered the reaction.
- Reagent : normally the
acyl halide (e.g. usually RCOCl) with aluminum trichloride, AlCl3,
a Lewis acid catalyst.
- The AlCl3
enhances the electrophilicity of the acyl halide by complexing with the halide.
- Electrophilic species
: the acyl cation or acylium ion (i.e. RCO + ) formed
by the "removal" of the halide by the Lewis acid catalyst.
- Friedel-Crafts reactions
are limited to arenes as or more reactive than mono-halobenzenes.
- Other sources of acylium
can also be used such as acid anhydrides with AlCl3
- Note how the reaction
can still be reviewed as a Nucleophilic Acyl Substitution of
the acyl halide since overall we have a nucleophile (here the p bond of an
aromatic ring) replaces the leaving group (chloride) at the electrophilic