Transcript Electophilic Aromatic Substituion

Rates of Reaction
Rates of electrophilic
aromatic substitutions
reactions increase with
electron donating groups
(EDG) on the ring.
Rates decrease
when electron
density is removed
from the ring:
EWG
EDG
E
+
H
EDG = -alkyl, -OH,
-NH2, etc.
These rings therefore
will react faster.
x
E+
Where will the
electrophile go?
O
EWG = -NO2, -C-R,
-halogen, etc.
Electron Donation – Ring is More Reactive
Electron donation by resonance:
..
+
NH2
NH2
+
NH2
+
NH2
..
..
..
ortho
para
Flow of pi electrons donated
resonance structures.
ortho to ring is shown by
Electron donation by induction:
CH3
or
Electron Donation – Ring is More Reactive
In an electrophilic reaction, where will the electrophile
find the most electron density?
..
..
NH
.
2
NH2
+
E+
E
.
Aminobenzene is more reactive that benzene.
(Rate of electrophilic reaction is higher.)
The -NH2 substituent is an activating group.
Electron Withdrawl – Ring is Less Reactive
Electron withdrawl by resonance:
O
O
O
O
+
+
+
Flow of pi electrons removed from ring is shown by resonance structures.
Electron withdrawl by induction:
CF3
Br
or
Based on
electronegativity
Electron Withdrawl – Ring is Less Reactive
In this electrophilic reaction, where is the
most electron density?
.
CF3
+
E+
E
.
Trifluoromethylbenzene is less reactive that benzene.
The -CF3 substituent is a deactivating group.
Substituent Effects in Aromatic Rings
The more electron density a substituent donates, the faster
the rate of AES reaction.
Slowest reactions are seen with groups that withdraw the
most electron density.
least reactive
most reactive
Summary – Substituent Effects
Activating
groups donate
electrons to the
ring, stabilizing
the carbocation
intermediate
Deactivating
groups withdraw
electrons from
the ring,
destabilizing
intermediate
slower
x
faster
Substituent Effects
Q. How do different substituents influence the rate
of electrophilic aromatic substitution reactions?
(activate or deactivate the ring)
Q. How do different substituents effect the
orientation of the substitution reaction?
(ortho/para vs. meta substitution)
Answer:
x
Inductive Effects
Resonance Effects