Everything about The Friedel-crafts totally explained
The
Friedel-Crafts reactions are a set of
reactions developed by
Charles Friedel and
James Crafts in
1877. There are two main types of Friedel-Crafts reactions:
alkylation reactions and
acylation reactions. This reaction type is part of
electrophilic aromatic substitution.
Several reviews have been written.
Friedel-Crafts alkylation
Friedel-Crafts alkylation involves the alkylation of an
aromatic ring and an
alkyl halide using a strong
Lewis acid catalyst. With anhydrous
ferric chloride as a
catalyst, the alkyl group attaches at the former site of the chloride ion.
This reaction has one big disadvantage, namely that the product is more
nucleophilic than the reactant due to the electron donating alkyl-chain. Therefore, another hydrogen is substituted with an alkyl-chain, which leads to overalkylation of the molecule. Also, if the chlorine isn't on a
tertiary carbon,
carbocation rearrangement reaction will occur. This is due to the relative stability of the tertiary
carbocation over the secondary and primary carbocations.
Steric hindrance can be exploited to limit the number of alkylations, as in the t-butylation of 1,4-dimethoxybenzene.
Alkylations are not limited to alkyl halides: Friedel-Crafts reactions are possible with any
carbocationic intermediate such as those derived from
alkenes and a
protic acid or
lewis acid,
enones and
epoxides. In one study the electrophile is a
bromonium ion derived from an alkene and
NBS:
In this reaction
samarium(III) triflate is believed to activate the NBS halogen donor in halonium ion formation.
Friedel-Crafts dealkylation
Friedel-Crafts alkylation is a
reversible reaction. In a
reversed Friedel-Crafts reaction or
Friedel-Crafts dealkylation, alkyl groups can be removed in the presence of protons and a
Lewis acid.
For example, in a multiple addition of
ethyl bromide to
benzene,
ortho and para substitution is expected after the first monosubstitution step because an alkyl group is an
activating group. However, the actual reaction product is 1,3,5-triethylbenzene with all alkyl groups as a
meta substituent.
Thermodynamic reaction control makes sure that thermodynamically favored
meta substitution with
steric hindrance minimized takes prevalence over less favorable
ortho and
para substitution by
chemical equilibration. The ultimate reaction product is thus the result of a series of alkylations and dealkylations.
Friedel-Crafts acylation
Friedel-Crafts acylation is the
acylation of aromatic rings with an
acyl chloride using a strong
Lewis acid catalyst. Friedel-Crafts acylation is also possible with
acid anhydrides. Reaction conditions are similar to the Friedel-Crafts alkylation mentioned above. This reaction has several advantages over the alkylation reaction. Due to the electron-withdrawing effect of the
carbonyl group, the
ketone product is always less reactive than the original molecule, so multiple acylations don't occur. Also, there are no
carbocation rearrangements, as the carbonium ion is stabilized by a resonance structure in which the positive charge is on the oxygen.
The viability of the Friedel-Crafts acylation depends on the stability of the acyl chloride reagent. Formyl chloride, for example, is too unstable to be isolated. Thus, synthesis of
benzaldehyde via the Friedel-Crafts pathway requires that formyl chloride be synthesized
in situ. This is accomplished via the
Gatterman-Koch Synthesis, accomplished by reacting
benzene with
carbon monoxide and
hydrogen chloride under high pressure, catalyzed by a mixture of
aluminium chloride and
cuprous chloride.
Reaction mechanism
In a simple mechanistic view, the first step consists of dissociation of a chlorine atom to form an
acyl cation:
»
This is followed by nucleophilic attack of the arene toward the acyl group:
»
Finally, a chlorine atom reacts to form HCl, and the AlCl
3 catalyst is regenerated:
»
Friedel-Crafts hydroxyalkylation
Arenes react with certain
aldehydes and
ketones to the hydroxyalkylated product for example in the reaction of the
mesityl derivative of
glyoxal with benzene to form a
benzoin with an
alcohol rather than a
carbonyl group:
Scope & variations
This reaction is related to several classic named reactions:
Dyes
Friedel-Crafts reactions have been used in the synthesis of several
triarylmethane and
xanthene dyes . Examples are the synthesis of
thymolphthalein (a pH indicator) from two equivalents of
thymol and
phthalic anhydride:
»
A reaction of phthalic anhydride with
resorcinol in the presence of
zinc chloride gives the fluorophore
Fluorescein. Replacing resorcinol by N,N-diethylaminophenol in this reaction gives
rhodamine B:
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