Step 3: Formation of a Substituted productFriedel-Crafts acylation, like Friedel-Crafts alkylation, is a classic example of electrophilic
substitution.
Source of electrophile
Reacting with Lewis acids, anhydrides and chloranhydrides of acids become strongly polar-
ized and often form acylium cations.
RCOCl + AlCl 3 →RC+O + AlCl 4 -
Mechanism of acylation
The mechanism of acylation is very similar to that of alkylation.
C 6 H 6 + RC+O→C 6 H 6 —CO—R + H+
The ketone that is formed then forms a complex with aluminum chloride, reducing its
catalytic activity.
C 6 H 6 —CO—R + AlCl 3 →C 6 H 6 —C+(R)—O—Al−Cl 3
Therefore, a much greater amount of catalyst is required for acylation than for alkylation.
Restrictions
- Although no isomerisation of cations happens, due to the reasonance stabilization pro-
vided by the acylium ion, certain cations may lose CO and alkylation will occur instead
of acylation. For example, an attempt to add pivalyl (neopentanoyl) to an aromatic ring
will result in loss of CO from the cation, which then results in the t-butyl derivative being
formed. - Acidophobic aromatic compounds, such as many heterocycles can’t exist in the presence
of both Lewis acids and anhydrides. - Formyl chloride is unstable and cannot be used to introduce the formyl group onto a ring
through Friedel-Crafts acylation. Instead, the Gattermann-Koch reaction is often used.
Applications
Friedel-Crafts acylation is used, for example, in the synthesis of anthraquinone from benzene
and phtalic anhydride.
In laboratory synthesis Friedel-Crafts acylation is often used instead of alkylation in cases
where alkylation is difficult or impossible, such as synthesis of monosubstituted alkylben-
zenes.