Monosubstituted Benzenes
72.4.2 Inductive Effects
Now let’s look at the inductive effects of deactivating substituents. Let’s imagine that,
instead of a hydroxyl group, we instead have a carbonyl group attached to the ring in
its place. When a carbonyl is attached, the ring is bonded to a carbon which in turn, is
double-bonded to an oxygen, the double-bonded oxygen is withdrawing electrons and this
inductive effect is felt on the ring, strongly deactivating its pi-bond nature and putting
a positive dipole on the carbon. Looking at the resonance structures, this carbon, which
already has some positive nature is now given the added resonance of a positive charge, in
the case of ortho and para attacks. Positive plus positive equals more positive and thus,
less stable. There’s no negative charge or negative electron pair to stabilize this positive
charge.
So in this case, not only is the entire ring less activated, but the ortho and para attacks
result in much more unstable carbocation resonance forms. Hence, meta is the preferred
position, but the overall reaction is less active than plain benzene.
72.4.3 Halides as the Exception
Notice that in the list of activating vs. deactivating substituents, the activating ones are
all ortho/para directing. In the deactivating substituents, all but the halides, are meta
directing. Why are halides an exception?
Because halides are more electronegative than carbon, they induce a positive dipole on
the attached carbon and a negative dipole on their own atom(inductive effect), and in
accordance to the previous logic of activating/deactivating substituents, deactivate the ring.
However, halides also possess lone pair electrons in their outer shell to share with the ring,
allowing the resonance structures with favored ortho/para attacks versus meta attacks due
to their poor resonance forms. In essence, although halides do deactivate the ring to some
extent, they provide major resonance contributors due to the availability of their lone pairs.
Resonance structures usually trump the inductive effect.
72.5 Detailed Effects of Substituents.
We’ve discussed some generalities about the effects of substituents and even some specifics
about certain ones, but let’s look more closely at the substituents and try to understand
the details of what makes them activating vs. deactivating.
-NH 2 , -NHR, and -NRR are all very strongly activating. Though nitrogen is more elec-
tronegative than carbon, its ability to share a pair of electrons greatly outweighs its electron
withdrawing effect.
-OH and -O-is similar in that it is even more electronegative than nitrogen, but it has two
pairs of electrons to share, which also greatly outweighs its electon withdrawing effect.
-NHCOCH 3 and-NHCORarealsostronglyactivating, buttheinductiveeffectofthedouble-
bonded oxygen acts to make the nitrogen more electron withdrawing, so they’re not quite
as activating as the other -N subsituents above.