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http://phs244.wikispaces.com/. Lab 1. Electrophilic substitution. Electrophilic substitution. chemical reactions in which an electrophile displaces a group in a compound, typically but not always hydrogen. Electrophilic substitution. Electrophilic aromatic substitution - PowerPoint PPT PresentationTRANSCRIPT
Electrophilic substitution chemical reactions in which an electrophile
displaces a group in a compound, typically but not always hydrogen.
electrophilic aliphatic substitutionchemical reactions in which an electrophile
displaces a functional group in aliphatic compounds.
Mechanism of reaction: two electrophilic reaction mechanisms SE1 and SE2
electrophilic aliphatic substitutionSE1 course of action the substrate first ionizes into
a carbanion and a positively charged organic residue. The carbanion then quickly recombines with the electrophile.
SE2single transition state in which the old bond
and the newly formed bond are both present.
electrophilic aliphatic substitutionNitrosationKetone halogenationKeto-enol tautomerismaliphatic diazonium couplingcarbene insertion into C-H bonds
Electrophilic aromatic substitutionatom appended to the aromatic ring, usually
hydrogen, is replaced by an electrophile. TypeNitration halogenation sulfonation acylation and alkylating Friedel-Crafts reactions.
X2, FeX3
(x = Cl, Br)
X
HX
R
CR
O
SO3H
NO2
HONO2
H2SO4
SO3
H2SO4
RCl, AlCl3
(R can rearrange)
RC Cl, AlCl3
O
H2O
HCl
HCl
+
+
+
+
Halogination
Nitration
sulfonation
Friedel-Crafts alkylation
Friedel-Crafts acylation
Mechaniism of EAS: arenium ions
E AE
H
E
H
E
H
Step 1
Arenium ions
(nonaromatic cyclohexadienyl cation)
E
H
:A-
EH A+
Step 2
Mechaniism of EAS: arenium ions
The proton is removed by any of the bases, for example by the anion derived from the electrophile
Benzene does not react with Br2 or CL2 unless a Lewis acid is present in mixture
Lewis acid is an electron-pair acceptorLewis acids most commonly used to effect
bromination and chlorination reactions are FeCl3,FeBr3 and AlCl3
Halogenation of benzene
Electrophilic Aromatic Bromination Br Br + FeBr3 Br Br FeBr3
Br Br FeBr3 slow
HBr
HBr
H Br
Step 1
Step 2
Bromine combines with FeBr3 to form acomplex that dissocates to form apostive bromine ion and
FeBr4-
Arenium ions
Postive bromine ion is attacked by benzen to form an arenium ion
Electrophilic Aromatic Bromination
HBr
Br FeBr3Br
+ H Br + FeBr3
Step 3
Proton is removed from the arenium ion to become bromobenzene
Nitration of Benzene
HO3SO H + H O NO
O(H2SO4)
H O
H
N
O
O
+ HSO4-
In this step nitric acid accepts a proton from the stronger acid, sulfuric acid.
H O
H
N
O
O
O
N+
O
H2O +
Nitronium ion
Now that it is protonated, nitric acid can dissociateto form a nitronium ion.
Step 1
Step 2
Nitration of BenzeneH
NO2
H
NO2
O
N
O
slow
H
NO2
H O H
H
NO2
+
O H
H +
H
NO2
Step 3
Step 4
Arenium ions
The nitronium ion is the actual electrophile in nitration ; it reacts with benzene to form a resonance-stabilized arenium ion
The arenium ion then loses a proton to a Lewis base and becomes nitrobenzene
Sulfonation of benzeneBenzene react with fuming sulfuric acid at
room temperature to produce benzensulphonic acid
Fuming sulfuric acid is sulfuric acid that contain added sulfur trioxide (SO3)
Sulfonation can takes place in conc sulfuric acid, alone but more slowly
S
O
O
O H +
O
SOO
25oC
concd H2SO4
Sulfurtrioxide Benzenesulfonic acid
(56%)
In either reaction the electrophile appears to be sulfur trioxide. In concentrated sulfuricacid, sulfur trioxide is produced in an equilibrium in which H2SO4 acts as both as an acidand a base(see step 1 of the following mechanism).
Sulfonation of benzene
Sulfonation of benzene
2 H2SO4 SO3 + H3O+ + HSO4-
This equilibrium produces SO3 inconcentrated H2SO4
O
SO O
+slow S
H
O -O
O other resonance structures
Step 1
Step 2
SO3,is the actual electrophile that reacts with benzene to form an arenium ion
HSO4- +
S
H
O
O O
fastS O
O
O
+ H2SO4
A proton is removed from the arenium ion to form the benzenesulfonate ion.
S
O
O
O + H O H
H
S
O
O
O H + H2O
The benzenesulfonate ion accepts a proton to become benzenesulfonic acid.
Sulfonation of benzeneStep 3
Step 4
Friedel-Crafts AlkylationMechanism for reaction start with formation
of carbocation (step 1 )The carbocation then act as an electrophile
and attack benzene ring to form an arenium ion (step 2)
The arenium ion then loses a proton to generate alkylbenzene (step 3)
Cl
Al
ClCl
Cl Al Cl
Cl
Cl
Cl Cl Al - Cl
Cl
Cl
++
This is a Lewis acid-basereaction.
The complex dissociate to form a carbocation and AlCl4-.
H
Cl
Al -Cl
Cl
Cl HCl + +
Cl
Al
ClCl
The carbocation, actingas an electrophile, reactswith benzene to producean arenium ion.
A proton is removed from the areniumion to form isopropylbenzene. This stepalso regenerates the AlCl3 and liberatesHCl.
Friedel-Crafts AlkylationStep 1
Step 2
+
Friedel-Crafts Acylation
acetyl groupbenzoyl group
Friedel-Crafts Acylation reaction is mean introducting an acyl group into aromatic ring
Two common acyl groups are acetyl group and benzoyl group
Friedel-Crafts AcylationThe reaction is carried out by treating the
aromatic compound with acyl halide( often an acyl chloride)
The reaction requires the addition of a Lewis acid (such as AlCl3 )
The product of reaction is an aryl ketone.
O
CClR + AlCl3
O
CClR AlCl3
O
CClR AlCl3 R C O R C O + AlCl4
Friedel-Crafts AcylationStep 1
Step 2
Acyl halide
Acylium ion
C
H
R
OC
O
Rother resonance structures(draw them for practice)
CH
R
OC
R
OCl Al Cl
Cl
Cl +
+ HCl +
Cl
AlCl Cl
Friedel-Crafts AcylationStep 3
Step 4
A proton is removed from the arenium ion, forming the aryl ketone
The acylium ion, acting as an electrophile, reacts with benzene to form the arenium ion
C
R
O Al
Cl
Cl
Cl + 3H2O
C
R
O +
OH
Al OHHO + 3HCL
Friedel-Crafts Acylation
Step 6
Treating the complex with water liberates the ketone and hydrolyzes the Lewis acid
C
R
OC
R
O Al Cl
Cl
Cl
Cl
AlCl Cl +
Step 5
The ketone acting as lewis base ,reacts with alaminum chloride (Lewis acid) to form a complex
Preparation of nitrobenzene
+ HNO3 + H2SO450-55 oC
NO2
+ H3O+ + HSO4-
(85%)
By nitration of benzene in pesent of sulphoric acid replace hydrogen atom by (NO2) groups
benzene nitrobenzene
Preparation of nitrobenzeneNitrations are usually carried out at
comparatively low temperatures At high temp may loss of material b/c of
oxidaising action of nitric acid
Preparation of nitrobenzeneSuphoric acid furnish astrong acid.
medium to convert nitric acid into highly reactive nitronium ion NO2
+
HNO3 + 2H2SO4 NO2+ + H3O+ + 2HSO4
-
Procedure:Add 9ml of nitric acid and add, in
portion,with shaking conc. Sulphuric acid(10ml) to round-bottemed flask
Place the mixture in cold water and keep the temp between 50◦-55◦ use thermometer
Add slowly (1-2ml) pure benzene 7ml shake after each addition
When addition is complete warm the flask in water bath 60° for 10-15 min with continued shaking.