Chemistry Reference
In-Depth Information
reacts readily with bromine in water, without any
need for a catalyst, giving 2,4,6-tribromoaniline in
nearly quantitative yield. The same is true for phenol,
which rapidly gives 2,4,6-tribromophenol, because of
the powerful activation provided by the phenol group.
The activation is so great that all three positions are
brominated.
NH
2
NH
2
OH
OH
Br
Br
Br
Br
Br
2
Br
2
H
2
O
H
2
O
Br
Br
aniline
phenol
2,4,6-tribromoaniline
2,4,6-tribromophenol
Groups such as amides and esters, where the
heteroatom is bonded to the aromatic ring, might
be expected to behave similarly; this is true, but the
level of activation is markedly less than for amines
and phenols. We can understand this because of
two conflicting types of resonance behaviour in such
molecules. These groups do activate the ring towards
electrophilic attack, and are
ortho
and
para
directing,
but activation is considerably less than with simple
amino and phenolic groups.
O
O
O
O
H
H
O
N
N
R
O
R
R
R
stabilizing
destabilizing
stabilizing
destabilizing
H
H
H
H
E
E
E
E
For most substituents, electron-donating ones
activate the ring towards electrophilic attack and
also direct
ortho
and
para
. Conversely, electron-
withdrawing substituents are deactivating and direct
substitution
meta
. This appears so straightforward
a concept that we must have an exception; this is
found with halogen substituents. Thus, chloroben-
zene is nitrated about 50 times more slowly than
benzene, but yields
o
-and
p
-nitro products. How-
ever, the explanation is simple, and does not alter
our reasoning. It turns out that, because of the
high electronegativity of halogen atoms, we have
a very strong electron-withdrawing inductive effect
and, consequently, significant deactivation towards
electrophilic substitution. However, because there is
an electron-donating resonance effect we get
ortho
and
para
substitution. This lone pair donation is not
nearly as effective as with oxygen and nitrogen, how-
ever, because in the larger atom the orbitals are less
able to overlap effectively. So we have the con-
flicting trends, deactivation from a strong inductive
effect through
bonds, but
ortho
and
para
directing
because of a weak resonance effect through the
σ
π
bond system.
Cl
Cl
strong electron-
withdrawing inductive
effect deactivates
weak electron-releasing
resonance effect favours
ortho and para substitution
E
E
H
chlorobenzene
