Transcript Electophilic Aromatic Substituion

Aromatic Nitration - Mechanism
The combination of nitric acid and sulfuric acid produces
NO2+ (nitronium ion), a powerful electrophile.
The reaction with benzene produces nitrobenzene.
Two Steps
Nu:
E+
cationic intermediate –
draw resonance structures
Aromatic Sulfonation
Add a sulfonic acid group (-SO3H) to the ring
using sulfuric acid and SO3.
.
CF3
CF3
+ H2SO4
SO3
+ H2O
SO3H
.
With –CF3, the new group is added meta.
What is the electrophile?
Aromatic Sulfonation
 Reaction with a mixture of sulfuric acid and SO3 gas
(“fuming sulfuric acid”).
 Electrophile is sulfur trioxide (SO3) or its conjugate acid.
 Reaction occurs via cationic intermediate and is reversible.
Review: EAS Reaction Conditions
Halogenation (Cl2 or Br2):
+
Cl2
Cl
FeCl3
+
HCl
Benzene Nitration:
+
HNO3
NO2
H2SO4
+ H2O
Aromatic Sulfonation:
HO
+
H2SO4
SO3
HO
+ H2O
SO3H
Effect of Substituents
But what if the benzene ring has a substituent?
Q1 Where does the new group go?
NO2
+ HNO3
NO2
H2SO4
or
or
NO2
ortho
meta
para
Q2 Is the reaction faster or slower than benzene?
x
Substituent Effects in Aromatic Rings
Substituents can make a ring more reactive or less reactive:
“activators” vs. “deactivators”
Substituents also control orientation of the reaction:
“ortho/para directors” vs. “meta directors”
least reactive
most reactive
Substituent Effects
Q. How do different substituents influence the rate
of electrophilic aromatic substitution reactions?
activate the ring = faster reaction
deactivate ring = slower reaction
Q. How do different substituents effect
orientation of the substitution reaction?
(ortho/para vs. meta substitution)
Answer:
Inductive Effects
Resonance Effects
Inductive vs. Resonance Effects
Inductive Effect
Resonance Effect
Based on:
electronegativity
conjugation
Electrons Move:
via sigma bonds
pi bonds/lone pairs
short-range
may be long range
electron cloud
is shifted
by electronegativity
pi bonds and
lone pairs move
by resonance
Distance:
Overall Effect:
Example:
CH3
+
..
NH2
H
+
E
.
+
H
E
NH2
H
E
.