EFFECT OF AROMATIC STABILIZATION ON REACTIONS
OF BENZENE (and other aromatic compounds)
ADDITION
Br
Br
Br2
No reaction
p bond
would be
endothermic
Br-Br bond
also would
loose AROMATIC
STABILIZATION
INSTEAD - SUBSTITUTION
FeBr3 /Br2
p bond
(65 kcal/mol))
Br-Br
(50 kcal/mol)
exothermic
Br2
Bonds
broken
Br
Bonds
made
2 C-Br
(2 x 70
kcal/mol)
2 C-Br
Aromatic compounds like benzene undergo SUBSTITUTION
Instead of ADDITION
For bromination E+ = Br-Br + FeBr3
Energy Diagram for Bromination
ELECTROPHILIC SPECIES IN EAS
OR
for
ELECTROPHILE
Br2/FeBr3
Br
HNO3/H2SO4
NO2
Cl2/AlCl3
Cl
Br Br FeBr3
O N O
nitronium ion
D2O/D2SO4
D
D
deuterium ion
Cl
AlCl3
carbocation
O
O
R
Cl
AlCl3
O
R
R
O
R
acylium ion
+
Nitronium O=N=O
Benzene
+
Acylium CH3C=O
Summary of Directing Effects
Four categories:
I. Strongly activating, ortho/para directing (eg. -OMe)
II. Weakly activating, ortho/para directing (eg. -CH3)
III. Weakly deactivating, ortho/para directing (eg. -Cl,)
IV. Deactivating, meta directing (eg. -NO2)
I. Strongly activating, ortho/para directing (LP electrons)
(eg. -OMe, Inductive and Resonance effects)
NH2
Aniline
OMe
Anisole
I. Strongly activating, ortho/para directing
(eg. -OMe, Consider inductive and resonance effects)
II. Weakly activating, ortho/para directing (eg. -CH3 Consider inductive effects)
Stability of Intermediate defines outcome
Intermediate is
more stable if
nitration occurs
at the ortho
or para position.
Carbocation
stability!
II. Weakly activating, ortho/para directing (eg. -CH3 inductive)
III. Weakly deactivating, ortho/para directing
(eg. -Cl, Inductive/Resonance)
III. Weakly deactivating, ortho/para directing
(eg. -Cl,Consider inductive and resonance effects)
IV. Deactivating, meta directing (eg. -NO2, inductive effects)
NO2
O2N
NO2
IV. Deactivating, meta directing (eg. -NO2, inductive)
SYNTHETIC STRATEGIES WITH AROMATIC COMPOUNDS
STERIC EFFECTS IN ELECTROPHILIC AROMATIC
SUBSTITUTION