Friedel-Crafts Reaction
What is a Friedel-Crafts Reaction?
A Friedel-Crafts reaction is
an organic coupling reaction involving an electrophilic aromatic substitution
that is used for the attachment of substituents to aromatic rings. The two
primary types of Friedel-Crafts reactions are the alkylation and acylation
reactions. These reactions were developed in the year 1877 by the French
chemist Charles Friedel and the American chemist James Crafts.
An illustration describing both the Friedel-Crafts reactions undergone by benzene is provided below.
It can be noted that both
these reactions involve the replacement of a hydrogen atom (initially attached
to the aromatic ring) with an electrophile. Aluminium trichloride (AlCl3) is
often used as a catalyst in Friedel-Crafts reactions since it acts as a Lewis
acid and coordinates with the halogens, generating an electrophile in the
process.
Friedel-Crafts Alkylation
Friedel-Crafts Alkylation
refers to the replacement of an aromatic proton with an alkyl group. This is
done through an electrophilic attack on the aromatic ring with the help of a
carbocation. The Friedel-Crafts alkylation reaction is a method of generating
alkylbenzenes by using alkyl halides as reactants.
The Friedel-Crafts
alkylation reaction of benzene is illustrated below.
A Lewis acid catalyst such
as FeCl3 or AlCl3 is employed in this reaction in order to form a
carbocation by facilitating the removal of the halide. The resulting
carbocation undergoes a rearrangement before proceeding with the alkylation
reaction.
Mechanism
The Friedel-Crafts
alkylation reaction proceeds via a three-step mechanism.
Step 1
The Lewis acid catalyst
(AlCl3) undergoes reaction with the alkyl halide, resulting in the formation of
an electrophilic carbocation.
Step 2
The carbocation proceeds to
attack the aromatic ring, forming a cyclohexadienyl cation as an intermediate.
The aromaticity of the arene is temporarily lost due to the breakage of the
carbon-carbon double bond.
Step 3
The deprotonation of the
intermediate leads to the reformation of the carbon-carbon double bond,
restoring aromaticity to the compound. This proton goes on to form hydrochloric
acid, regenerating the AlCl3 catalyst.
What are the Limitations of the Friedel-Crafts Alkylation Reaction?
Some important limitations of Friedel-Crafts alkylation are listed below.
- Since the carbocations formed by aryl and vinyl halides are extremely unstable, they cannot be used in this reaction.
- The presence of a deactivating group on the aromatic ring (such as an NH2 group) can lead to the deactivation of the catalyst due to the formation of complexes.
- An excess of the aromatic compound must be used in these reactions in order to avoid polyalkylation (addition of more than one alkyl group to the aromatic compound).
- Aromatic compounds that are less reactive than mono-halobenzenes do not participate in the Friedel-Crafts alkylation reaction.
It is important to note that
this reaction is prone to carbocation rearrangements, as is the case with any
reaction involving carbocations.
Friedel-Crafts Acylation
The Friedel-Crafts acylation
reaction involves the addition of an acyl group to an aromatic ring. Typically,
this is done by employing an acid chloride (R-(C=O)-Cl) and a Lewis acid
catalyst such as AlCl3. In a Friedel-Crafts acylation reaction, the aromatic
ring is transformed into a ketone. The reaction between benzene and an
acyl chloride under these conditions is illustrated below.
An acid anhydride can be
used as an alternative to the acyl halide in Friedel-Crafts acylations. The
halogen belonging to the acyl halide forms a complex with the Lewis acid,
generating a highly electrophilic acylium ion, which has a general formula of RCO+ and
is stabilized by resonance.
Mechanism
Friedel-Crafts acylations
proceed through a four-step mechanism.
Step 1
A reaction occurs between
the Lewis acid catalyst (AlCl3) and the acyl halide. A complex is formed and
the acyl halide loses a halide ion, forming an acylium ion which is stabilized
by resonance.
Step 2
The acylium ion (RCO+) goes
on to execute an electrophilic attack on the aromatic ring. The aromaticity of
the ring is temporarily lost as a complex is formed.
Step 3
The intermediate complex is
now deprotonated, restoring the aromaticity to the ring. This proton attaches
itself to a chloride ion (from the complexed Lewis acid), forming HCl. The AlCl3 catalyst
is now regenerated.
Thus, the required acyl
benzene product is obtained via the Friedel-Crafts acylation reaction.
Limitations
Despite overcoming some limitations of the related alkylation reaction (such as carbocation rearrangement and polyalkylation), the Friedel-Crafts acylation reaction has a few shortcomings.
- The acylation reaction only yields ketones. This is because formyl chloride (H(C=O)Cl) decomposes into CO and HCl when exposed to these conditions.
- The aromatic compound cannot participate in this reaction if it is less reactive than a mono-halobenzene.
- Aryl amines cannot be used in this reaction because they form highly unreactive complexes with the Lewis acid catalyst.
The acylations can take
place on the nitrogen or oxygen atoms when amine or alcohols are used. Thus,
the reaction details, mechanisms, and limitations of both Friedel-Crafts
reactions are briefly discussed.
0 Comments