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Chapter 12 :
Reactions of Arenes. Electrophilic Aromatic Substitution |
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Substituent Effects
So far we have only seen electrophilic aromatic substitution of benzene
but substituted benzenes can also undergo further substitution to give
polysubstituted systems.
Two issues arise based on this general scheme...
-
How easy is the second substitution compared to the first ? Faster or slower
-
Where does E2 substitute
relative to E1 ? There
are 3 possibilities:
Experimental evidence from nitration experiments is tabulated below:
|
Starting Material
|
Relative rate*
|
ortho
|
meta
|
para
|
Comments
|
|
25
|
63%
|
3%
|
34%
|
activated
ortho / para director
|
|
2.5 x 10-5
|
6%
|
91%
|
3%
|
deactivated
meta director
|
* the relative rate is expressed with respect to
rate of recation of benzene under the same conditions
The rate data and product selectivity show the following:
-
Toluene is more reactive than benzene, i.e. it is said to be activated
-
The methyl group, -CH3, directs the new substituent
to the ortho- and para- positions
In contrast,
-
Trifluoromethylbenzene is less reactive than benzene, i.e. it is
said to be deactivated
-
The trifluromethyl group, -CF3, directs the new
substituent to the meta position.
The origin of these effects and application to substituents in
general is discussed on the following page.
