ring, and directs further substitutions to meta positions. The ortho and para
electrophilic attacks in trifluoromethyl benzene result in one highly unstable
contributing resonance structure of the arenium ion, but no such highly unstable
resonance structure is formed from meta attack. In the case of ortho and para
attacks, the positive charge in one of the resulting contributing resonance
structures is located on the ring carbon that bears the electron-withdrawing
group. The arenium ion formed from meta attack is the most stable among the
three, and thus the substitution in the meta-position is favoured. Therefore, the
trifluoromethyl group is a meta-directing group.
Why the À ÀCH 3 group is ortho–para directing
The stability of the carbocation intermediate formed in the rate-determining
step is actually the underlying factor for a substituent to direct an incoming
electrophile to a particular position, ortho, meta or para. The methyl group
(À ÀCH 3 ) donates electrons inductively, and in the presence of this electrondonating group the resonance contributors formed from ortho, meta and
para attacks are shown below. In the most stable contributors, arising from
ortho and para attacks, the methyl group is attached directly to the
positively charged carbon, which can be stabilized by donation of electrons
through the inductive effect. From meta attack no such stable contributor is
formed. Thus, the substitutions in ortho and para positions are favoured.
Therefore, the methyl group is an ortho and para directing group.
CH 3
E
H
CH 3
E
H
CH 3
E
H
CH 3
E
H
CH 3
E
H
CH 3
E
H
CH 3
H E
CH 3
H E
CH 3
H E
CH 3
+ E +
Methylbenzene or toluene
−CH 3 is an electron donating group
ortho attack
meta attack
para attack
+
Most stable contributor
+
+
+
+
+
+
+
+
Most stable contributor
Why halogens are ortho–para directing
Halogens are the only deactivating substituents that are ortho–para directors. However, they are the weakest of the deactivators. Halogens withdraw
126
CH4 ORGANIC FUNCTIONAL GROUPS
electrophilic attacks in trifluoromethyl benzene result in one highly unstable
contributing resonance structure of the arenium ion, but no such highly unstable
resonance structure is formed from meta attack. In the case of ortho and para
attacks, the positive charge in one of the resulting contributing resonance
structures is located on the ring carbon that bears the electron-withdrawing
group. The arenium ion formed from meta attack is the most stable among the
three, and thus the substitution in the meta-position is favoured. Therefore, the
trifluoromethyl group is a meta-directing group.
Why the À ÀCH 3 group is ortho–para directing
The stability of the carbocation intermediate formed in the rate-determining
step is actually the underlying factor for a substituent to direct an incoming
electrophile to a particular position, ortho, meta or para. The methyl group
(À ÀCH 3 ) donates electrons inductively, and in the presence of this electrondonating group the resonance contributors formed from ortho, meta and
para attacks are shown below. In the most stable contributors, arising from
ortho and para attacks, the methyl group is attached directly to the
positively charged carbon, which can be stabilized by donation of electrons
through the inductive effect. From meta attack no such stable contributor is
formed. Thus, the substitutions in ortho and para positions are favoured.
Therefore, the methyl group is an ortho and para directing group.
CH 3
E
H
CH 3
E
H
CH 3
E
H
CH 3
E
H
CH 3
E
H
CH 3
E
H
CH 3
H E
CH 3
H E
CH 3
H E
CH 3
+ E +
Methylbenzene or toluene
−CH 3 is an electron donating group
ortho attack
meta attack
para attack
+
Most stable contributor
+
+
+
+
+
+
+
+
Most stable contributor
Why halogens are ortho–para directing
Halogens are the only deactivating substituents that are ortho–para directors. However, they are the weakest of the deactivators. Halogens withdraw
126
CH4 ORGANIC FUNCTIONAL GROUPS