Rates of Reaction

1
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
E
Where will the electrophile go?
These rings therefore will react faster.
x
2
Electron Donation Ring is More Reactive
Electron donation by resonance

• Flow of pi electrons donated to ring is shown by
  resonance structures.

Electron donation by induction
3
Electron Donation Ring is More Reactive
In an electrophilic reaction, where will the
electrophile find the most electron density?

• Aminobenzene is more reactive that benzene.(Rate
  of electrophilic reaction is higher.)
• The -NH2 substituent is an activating group.

4
Electron Withdrawl Ring is Less Reactive
Electron withdrawl by resonance

• Flow of pi electrons removed from ring is shown
  by resonance structures.

Electron withdrawl by induction
Based on electronegativity
5
Electron Withdrawl Ring is Less Reactive
In this electrophilic reaction, where is the
most electron density?

• Trifluoromethylbenzene is less reactive that
  benzene.
• The -CF3 substituent is a deactivating group.

6
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.

7
Summary Substituent Effects

• Activating groups donate electrons to the
  ring, stabilizing the carbocation intermediate
• Deactivating groups withdraw electrons from
  the ring, destabilizing intermediate

slower
faster
x
8
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 Inductive Effects Resonance Effects
x