Friedel-Crafts Acylation
The reaction of benzene with acid halides or acid anhydride in the presence of a Lewis acid
catalyst like AlCl3 results in the introduction of an acyl group (RCO-) in the benzene
ring. the product of an alkylation reaction is an aryl ketone. Acylation is carried out in the
presence of a solvent such as carbon disulfide or nitrobenzene .Friedel-Crafts Acylation
Mechanism,
Step=1
Generation of electrophile (acylium ion)
The carbonyl oxygen of acid chloride from complx with AlCl3. This complex forms corresponding
acylium ion that behaves as strong electrophile,
STEP=2
Attack of acylium ion benzene 22/7-electron system (formation of arenium ion).The attack
of acylium ion on benzene ring result in the formation of arenium ion whch is stabilized
through delocalization.
Step=3
Loss of proton (regeneration of catalyst and formation of ketone) in order to regain aromaticity,
the arenium ion undergoes loss of proton that is captured by -AlCl4(Step1) from AlCl3 and simultaneous
removal of HCl . The formation of aryl ketone is an exotherm process.
Example,
Friedel-Crafts acylation versus alkylation,
Quantity of catalyst,
Acylationa reaction requires a larger quantity of catalyst (AlCl3) compared to alkylation. in acylation
reaction, inittially one mole of AlCl3 is utilized in forming complex with the oxygen of the reagent
(acid chloried)and thus.more then one mole of catalysyis required per mole of the reagent .The
product of friedel0-crafts acylation is an ayl ketone. the carbonyl oxygen of ketone also form complex
with AlCl3 catalyst and infact this is the actuala product of acylation reaction. Some of the catalyst
is used up in this complex formation . Hydroltysts of complex yeield free ketone.
Thus, for carrying out successful acylation reaction using acid chlorides,2 moles of catalysts AlCl3 is
required wheras in case of acid anhydrides 3 moles of catalysts is required (because acid parts of the
anhydrides as well as product ketone form complex).
Polysubstitution,
An advantage of friedel-crafts acylation over alkylation is that polyacylation does not occur.
The acyl group is electron-withdrawing in nature once an acyl group is introduced into the rimg.
it deactivates the ring for further substitution.
Synthetic Application of friedel Crafts acylation.
Acylation is used frequently for preparation of alkylbenzene which cannot be prepared by Friedel-Crafts alkylation
due to carbocation rearrangement .The clemmensen reduction of the ketone, formed by acylation, results in
formation of alkylbenzene.