4.6 Directing Effects

4.6 Directing Effects

The effect of the nitro group can be evaluated using two factors.

It's simple.
The carbon atom has a positive charge, while the nitrogen atom has a negative charge.
The ring should be less nucleophilic because the nitro group withdraws electron density from it.
We still have to think about resonance.

There is a positive charge spread throughout the ring, rather than a negative charge, as we saw in the case of phenol.

There is a partial positive charge spread throughout the ring.

There is no competition between the two.

That is what we see in the laboratory.

Let's look at aromatic substitution reactions with benzene rings.

We must first review important terminology that we will use frequently throughout the rest of the chapter in order to explore this topic.

Let's consider the products obtained when toluene or nitrobenzene undergoes bromination.

The first reaction with toluene is quicker because the methyl group activated the ring.

Consider the difference in regiochemical outcome.

There is an exception to the second rule.

halogens are the exception.

In the previous section, we looked at the effect of an OH group on an aromatic ring and found that there were two competing effects.

They are the exception.

Let's take a closer look.

OH is a deactivator.

We need to explore the effect of a halogen on the electron density of the aromatic ring.
We have seen in the previous section that we need to focus on two factors.
Let's start with that.

We need to consider resonance effects.

Again, we see that a halogen is very similar to an OH group.

Let's take a look at the net effect of a halogen.

There is a competition between the two.

In the case of the OH group, we used the argument that resonance beats induction, which we said was a general rule that holds true most of the time.

The OH group's net effect was to donate electron density to the ring.
resonance does not beat induction with a halogen.
This is one of the rare cases where resonance is actually beaten by insturment.
The resonance structures of chlorobenzene have a positive charge.
Positive charges are not easy to bear.
The resonance structures don't contribute much to the overall structure of the compound.
In this case, resonance is a weak effect.

Predicting some directing effects is something we should try to do.

There is a monosubstituted benzene ring.

Predict where the incoming substituent would be installed if this compound undergoes an aromatic substitution reaction.

The halogens are F, Cl, Br, and I.

The general rules do not include the exception of halogens.

If each of the following compounds underwent an aromatic substitution reaction, the expected directing effects would be observed.

The group is a deactivator.

This group is active.

The group is a deactivator.

This group is active.

You can't predict where the substitution will take place if you don't know the group's identity.

In the next section, we will learn how to determine if a group is an activator or deactivator.
Let's get some practice predicting products.

We can determine what kind of reaction is taking place by looking at the reagents.

There are two reagents, nitric acid and sulfuric acid.

The result is to put a group on a ring.

Predicting the directing effects of the group already present on the ring is what we must do to answer this question.

There is a ring with a group on it.

The only exception was the halogens.

Let's say we try to destroy this compound.

Let's look at the effect of the group.

The comes off.

Let's look at the effect of the group.

The nitro group is a powerful deactivator.

The nitro group and the methyl group are following the same path.

There is no conflict between the directing effects of the two groups.

The directing effects of the methyl group are more powerful than those of the nitro group.

It is common to see a situation where the directing effects of two groups are competing with each other.

Rules are needed for determining which group wins.

meta directors are always beaten by ortho-para directors.

The example we just saw is a perfect example of this rule.

Weak activators are always beaten by strong activators.

The first rule always prevails over the second rule.

If you have a weak activator against a strong deactivator, the weak one will win.

The nitro group is a strong deactivator.

Predict the effects of an aromatic substitution reaction involving a disubstituted aromatic ring.

There are two groups.

There is a competition for directing effects.

This is an example of a reaction.

There are two substituents in the aromatic ring.

The OH group is a strong deactivator.

Predict the effects of an aromatic substitution reaction involving different types of aromatic rings.

Unless otherwise stated, assume that anything labeled as a deactivator is not a halogen.