Activation vs. Deactivation and ortho/para vs. meta directing72.4.1 Resonance Effects
Let’s first look at resonance effects. Resonance effects are the ability or inability of a
substituent to provide electrons to the ring and enhance its resonance stability. To see
this, we must first get a basic understanding of the mechanism of Electrophilic Aromatic
Subsitution. We’ll discuss EAS in more detail in the next section, but some basics are called
for here.
Figure 167 Basic Mechanism of Electrophilic Aromatic Substitution
As you can see in the image above, the electrophile is attacked by pi electrons in the ring.
The same carbon is now bonded to both the hydrogen that was bonded to it and the
electrophile. This in turn creates a carbocation on the adjacent carbon, making the ring
non-aromatic. But aromatic rings like to remain aromatic. The nucleophile which was
previously bonded to the electrophile now attacks the hydrogen, abstracting it from the
ring and allowing the pi-bond to re-form and returning the ring to its aromatic nature.
As we’ve seen before in some other reactions, when a carbocation is created as an intermedi-
ate, stability of that carbocation is crucial to the reaction. This is the case in Electrophilic
Aromatic Subsitution as well.
So what is the effect of substituents on the ring?
Figure 168 Resonance Stability of ortho, para, and meta attacks.
Let’s look at the situation above. In this case we have Phenol, a benzene ring with an -OH
(hydroxyl) group attached. When we nitrate the ring with nitric acid in sulfuric acid (a
reaction we’ll discuss in the next section), a nitro group is attached to the benzene ring.
There are 3 possible places for the nitro group to attach: An ortho, meta, or para position.
To understand the stability of the carbocation, we need to look at the resonance structures
for a given attack and see what the results are.
The first resonance structure of the ortho attack results in a positive charge on the carbon
with the hydroxyl group. This happens to be the most stable of the 3 resonance structures
for an ortho attack because the two negative electron pairs in the oxygen act to stabilize
the positive charge on the carbon. The other two resonance forms leave a carbon with a
hydrogen attached, to hold the positive charge. Hydrogen can do nothing to stabilize the
charge and thus, these are less stable forms.
In the para attack situation, notice that the second resonance form also puts a positive
charge on the carbon with the hydroxyl group. This provides for stability just as it does in
the case of an ortho attack and thus, the middle resonance form is very stable.
Finally, in the meta attack situation, all of the resonance forms result in a positive charge
on a carbon with only a hydrogen attached. None of these is stable, and thus, meta attack
with a hydroxyl group attached, is a very small percentage of the product.
So the electron pairs in the oxygen act to stabilize the ortho and para attacks.