the overlap of a p orbital on the substituent with a p orbital on the aromatic
ring. The presence of a substituent may increase or decrease the resonance
stabilization of the intermediate arenium ion complex.
H
O
H
O
H
O
O
H
:
:
..
+
:
..
+
+
Electron withdrawing effect by an aldehyde (CHO) group
:
..
..
O H
O H
O H
O H
..
..
..
_
_
_
+
Electron donating phenolic hydroxyl group (OH)
..
..
..
+
..
+
..
The electron-donating resonance effect applies with decreasing strength in
the following order:
NH 2
NR 2
OH
OR
X
..
..
..
..
..
..
..
..
:
>
>
Least
electron
donating
Most
electron
donating
Why the À ÀCF 3 group is meta directing
All meta-directing groups have either a partial positive charge or a full positive
charge on the atom directly linked to the benzene ring. In the trifluoromethyl
group (CF 3 ), there are three electronegative fluorine atoms, which make this
group strongly electron withdrawing. As a result À ÀCF 3 deactivates the benzene
CF 3
E
H
CF 3
E
H
CF 3
E
H
CF 3
E
H
CF 3
E
H
CF 3
E
H
CF 3
CF 3
E H
E H
CF 3
E H
CF 3
+ E +
Trifluoromethylbenzene
−CF 3 is an electron-withdrawing group
ortho attack
meta attack
para attack
+
+
+
Highly unstable
contributor
+
+
+
Highly unstable contributor
+
+
+
4.6 AROMATIC COMPOUNDS AND THEIR DERIVATIVES
125