Electrophilic Aromatic Substitution (EAS)Reactions
Overall reactionOverall reaction
General Mechanism for Electrophilic Aromatic Substitution
Summary of ERS Reactions
Halogenation of Benzene
+ X2
F eX 3
c a ta lys t
X
+ HX
General Equation:General Equation:
Electrophile = X+Electrophile = X+
X X + FeX3X FeX3 + X+ electrophile
EX:EX:
Nitration of Benzene
Electrophile = +NO2 (nitronium ion)Electrophile = +NO2 (nitronium ion)
+ HNO3NO2H 2SO 4
c a ta lys t
General Equation:General Equation:
N+
O
O-
OH + H2SO4N
+
O
O-
OH2
+O N
+O
-H 2O
Sulfonation of Benzene
+ SO3
SO3HH 2SO 4
c a ta lys t
General Equation:General Equation:
S
O
O
O + H2SO4 S+
O
O
OH
Electrophile = +SO3H(protonated sulfer trioxide)Electrophile = +SO3H(protonated sulfer trioxide)
Friedel–Crafts Alkylation
Electrophile = R(carbocation) Electrophile = R(carbocation)
General Equation:General Equation:
+ ClA lC l3
c a ta lys tEX:EX:
Friedel–Crafts Acylation
Acyl group:
Electrophile is R–C≡O⊕ (acylium ion)
General Equation:General Equation:
Substituents effect on reactivity and orientation
EDG (electron-donating group): (R, OH, OR, NR2,)
orthoortho
para
EDG
Reactivity: Ring is electron rich; Ring reacts faster than benzene with E+ (Activation)
Reactivity: Ring is electron rich; Ring reacts faster than benzene with E+ (Activation)
Orientation: Ortho (o), Para (p) position
EWG (electron-withdrawing group):
(CF3 , NO2, SO3H,CN, )
Substituents effect on reactivity and orientation
EWG
metameta
Reactivity: Ring is electron poor; Ring reacts slower than benzene with E+ (Deactivation)
Orientation: Meta (m) position
C
O
R
Xorthoortho
para
Reactivity: Ring reacts slower than benzene with E+ (Deactivation)
Orientation: Ortho (o), Para (p) position
Substituents effect on reactivity and orientation
Halogens (X): (F, Cl, Br, I)
Reactivity towards electrophilic aromatic substitution
then reaction is faster than with benzene
EffectEffect of of Electron-Donating Electron-Donating (releasing) and (releasing) and Electron-Withdrawing GroupsElectron-Withdrawing Groups
arenium ionstabilized
If G is electron-donating group
If G is an electron-withdrawingthen reaction is slower than with benzene
arenium iondestabilized
(1) Mesomeric (+M) Effects:Mesomeric (+M) Effects:
Activate the ring by resonance
(2) ) Inductive (+I): Inductive (+I):
Activate the inductive (through σ bond)
EDG activate the ring by:EDG activate the ring by:
Note: Mesomeric effect stronger than Inductive effect
Deactivate the ring by resonance effect
Deactivate the ring by negative inductive effect
EWG deactivate the ring by:EWG deactivate the ring by:
(1) Mesomeric (-M) Effects:Mesomeric (-M) Effects:
(2) ) Inductive (-I): Inductive (-I):
Note: Mesomeric effect stronger than Inductive effect
Ex1: Write the formulas for benzyl chloride, toluene, and benzoic acid, ortho-xylene, 4-ethyl-2,6-difluorotoluene,
Ex2: Draw the important resonance contributors for the benzenoniumintermediate in the bromination of aniline
Ex3: Devise a synthesis for each of the following, starting with benzene:a. m-bromobenzenesulfonic acidb. p-nitrotoluene
Ex4: Which compound is more reactive toward electrophilic substitution
Exercises
OCH3
OHOCl
CH3
Ex5: using benzene or toluene as the only aromatic starting material, devise a synthesis of each compound.a.2-bromo-4-nitrotoluene b.p-nitroethylbenzene