Chem318
Electrophilic Aromatic Substitution
Bromination of Toluene
02/06/07
The electron-rich !-systems of aromatic rings make them susceptible to electrophilic
aromatic substitution. In such cases, electrophiles are electron deficient species that carry
either a positive charge or a partial positive charge.
H
H
!+
E
H
H
H
H
A
H
H
H
H
H
H
H
E
H
H
H
H
H
H
!+
E
H
H
E
H
H
H
H
A-
H
E
H
H
H
H
H
H
+
H
Ho
Hm
Hp
CH3
Br
Ho
Hm
Hm
CH3
Ho
Ho
Br
Hm
Hp
Ho
Hm
Br
H
As seen in the above figure, toluene has five hydrogen atoms (sites for bormination), two
are ortho to the methyl group, two are meta and the fifth is para. Assuming that all
positions were equally favored, we would expect a product mixture where 40% of the
product was ortho, 40% meta and 20% para (or a product ratio of 2:2:1, o/m/p).
However, this may not be the case. It is possible that not all of these products are
equally favored. Therefore, we may not observe this ideal ratio of isomers. We will
evaluate the isomeric distribution of products based on IR spectra. Each isomer has a
characteristic absorption band between 700 and 810 cm-1 (o-bromotoluene 747 cm-1,
m-bromotoluene 770 cm-1 and p-bromotoluene 801 cm-1). In absorption spectra, the
peak area for each isomer is proportional to its relative abundance.
H
!+
H
H !+
H
Hm
Br2
FeBr3
Bromination of toluene and possible substitution products.
Arenium Ion
E
CH3
Ho
Hp
H
H
CH3
Ho
Hm
!+
H
H !+
In the case of toluene you have a methyl substitutent already on the ring. Therefore the
bromine must add at one of the other positions. This would provide for ortho (o), meta
(m) and para (p) substitution products.
A
H
Electrophilic aromatic substitution.
In halogenation reactions, benzene (toluene) does not react with bromine or chlorine by
themselves. However, the presence of a Lewis acid such as FeBr3, AlCl3 or FeCl3 can
serve to catalyze the reaction. (Fluorine is much more reactive than either chlorine or
bromine and iodine is much less reactive)
h
Step 1 (formation of Br+)
Br
Br
FeBr3
+
Br
Step 2 (electrophillic attack)
H
Br
H
H
H
H
H
H
H
H
H
Br
FeBr3
Br
H
Br
H
H
H
H
H
H
Arenium Ion
Step 3 (deprotonation of arenium ion)
H
H
Br
H
FeBr3
H
Br
H
Br
H
H
H
+
H
H
Bromination of benzene.
H
FeBr3
Br
FeBr3
H
Br
H
H
H
H
Br
H
H
w1/2
Peak Area = h x w1/2