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The Friedel-Crafts Reactions
Friedel-Crafts Alkylation (RCl + AlCl3)
R3 C
H
H
H
Cl AlCl3
H R
H
R
CR3
- H+
R
benzene
arenium
ion
alkyl halide
Lewis acid
complex
alkylbenzene
Friedel-Crafts Acylation (RCOCl + AlCl3)
O
O
CH3
C
+ AlCl3
Cl
CH3
C
Cl AlCl3
CH3
acetyl
chloride
C
O
+
acyllium
ion
O
H
C
H
CH3
H
H
H O
- H+
O
CH3
CH3
benzene
acetophenone
AlCl4
Problems with Friedel-Crafts Alkylation
Carbocation rearrangements
H
H
H
H
H
H
R
R
primary alkyl
halide Lewis acid
complex
secondary
carbenium
ion
H
H
H
H
Cl AlCl3
H
H
H
Cl AlCl3
H
R
R
R
R
primary alkyl
halide Lewis acid
complex
tertiary
carbenium
ion
Friedel-Crafts Alkylation with a primary halide
H
AlCl3
Cl
Cl3Al
H
Cl
H
H
H
H
H
2° carbocation
1-chlorohexane
H 3C
H
H
H
H
H
H CH3
H
- H+
H
benzene
2° cation
2-phenylhexane
Carbocation stability
R
R
R
R
R
R
benzyl
cations
R
> R
≅
>
cyclopropylmethyl
carbocations
R
R
H
>
H
H
R
R
R
3°
cations
2°
cations
1°
cations
R
allyl
cations
>
An alternative method of cation formation
H
AlCl3
H
alkene
H Cl3Al
H
AlCl3
H
H+
two
separate
steps
- AlCl3
H
phenylcyclohexane
2° cation
A Small Problem with Friedel Crafts Acylation
A practical difficulty, acyl benzenes are Lewis bases
H
O
H
R
AlCl3
O
AlCl3
R
Therefore use at least one equivalent of Lewis acid
in Freidel-Crafts acylation reactions.
Reduction of Aryl Ketones
Clemmensen Reduction
H
H
O
Zn(Hg), HCl
R
R
or
Catalytic Hydrogenation
H
O
R
H
H2, Pd or Pt
or other metal
R
Wolff-Kishner
H
H
O
H2NNH2
R
R
NaOH
Reduction of Nitroaromatics
O
N
H
O
Sn
HCl
nitrobenzene
N
H
H
H
NaHCO3
N
Cl
anilinium chloride
or
aniline hydrochloride
aniline
H
Mechanism of Wolff-Kishner
(will not be on an exam)
H
O
H
H
O
H
H2NNH2
R
H HO N
NH2
R
hydrazine
I
H
O
H
H
OH
H
H
H HO N
N
R
II
a hydrazone
I
OH
H
N
N
H
H
N
H
II
N
O
N
H
H
H
O
H
III
OH
H
N
R
R
III
N
R
R
H
H
NH2
R
H
N
H
N
Phenyl Diazonium Ions
Diazotization of anilines
HONO
NaNO2 + HCl
H
N
N2 Cl
HONO
H
aniline
benzene diazonium
chloride
Mechanism of diazotization
H
H
H
N
H
H
O
N
O
O
N
N
O
H
H
O
H
H
N
N
H
N
Cl
Cl
benzene diazonium
chloride
aniline
Diazo Coupling
N
N
H
N
Cl
N
H
N
H
-HCl
N
N
H
Sandmeyer Reaction
Sandmeyer Reaction
N
N
C
Cu2CN2
N
KCN
X
benzonitrile
benzene
diazonium
halide
Reactions closely related to the Sandmeyer Reaction
Br
N
Cu2Br2
N
Cl
Cu2Cl2
X
KI
bromobenzene
chlorobenzene
I
iodobenzene
Replacement of diazo group by fluorine
H
N
N
NaNO2
H
Replacement of diazo group by hydroxyl
Cl
Br
Br
N
N
Br
fluorobenzene
benzene diazonium
fluoroborate
Replacement of diazo group by hydrogen
Br
F
∆
BF4
HBF4
aniline
N
H
H3PO2
N
N
H 2O
Cl
Br
Br
H2SO4
∆
OH