Aromatics

Question 1

What are aromatics?

Answer 1

Cyclic compounds with unusual stability due to cyclic delocalisation and resonance

Question 2

What is the simplest of the aromatic hydrocarbons?

Answer 2

Benzene

Question 3

Why is the term aromatics used?

Answer 3

Historically many fragrant compounds were found to be the derivatives of Benzene

Question 4

What does the average of the Benzene resonance structures look like/how is it represented?

Answer 4

A circle within a hexagon

Question 5

How might benzenes be named?

Answer 5

Many have common names by which they are known.

Others are named by substituents attached to the aromatic ring. For example:

• Phenol (OH)
• Toluene (CH3)
• Aniline (NH2)
• Nitrobenzene (NO2)
• Benzoic acid (CO2H)

Question 6

How are disubstituted benzenes named?

Answer 6

As derivatives of their primary substituents.

The ortho-meta-para system may be used

Question 7

What is the ortho-meta-para system?

Answer 7

• Ortho substituents are at the second position from the primary substituent.
• Meta substituents are at the third position from the primary substituent.
• Para substituents are at the forth position from the primary substituent

Question 8

If there are more than two substituents on the aromatic ring what naming system is used?

Answer 8

The numbering system

Question 9

What is a benzene called when it is a substituent?

Answer 9

Phenyl or aryl group.

The shorthand for phenyl is Ph.

Toluene without a hydrogen on the methyl substituent is called a benzyl group

Question 10

What is electrophilic aromatic substitution?

Answer 10

An important reaction of aromatic compounds, it occurs with electrophilic reagents.

The reaction is similar to a Sn1 mechanism in that an addition leads to a rearrangement which produces a substitution. However in this case it is the electrophile (not a nucleophile) which substitutes for an atom in the original molecule. Diagram on p39.

It is important to understand that the electrophile used in electrophilic aromatic substitution must always be a powerful electrophile. After all the resonance stabilised aromatic ring is resistant to many types of routine chemical reactions

Question 11

Br+ is an example of what?

Why?

Answer 11

A powerful electrophile.

Br is very electronegative

Question 12

What is bromination?

Answer 12

A reaction where Br2 is used to generate Br+ species which adds to the aromatic ring.

Similar reactions are performed to juice up other potential substituents to become powerful electrophiles to add to the aromatic ring

Question 13

When groups are attached to the aromatic ring, the intermediate charge delocalisation is affected. What are the two classes of substituents?

Answer 13

Ortho-para (o-p) directors and meta directors.

As implied, these groups indicate where most of the electrophile will end up in the reaction

Question 14

When is a substituent group on a benzene said to be an activating group?

Answer 14

If the substituted benzene reacts more rapidly than a benzene alone.

Activating groups can donate electrons to the ring. Thus the ring is more attractive to an electrophile. All activating groups are o/p directors. Some examples are -OH, -NH2, -OR, -NR2 and alkyl groups.

Good stabilisation results with a substituent at the ortho or para positions

Question 15

When there is a substituent at the meta position, the primary -OH can no longer help to delocalise the positive charge. What positions are favoured over the meta?

Answer 15

The o-p positions.

Note that even though the substituents are o-p directors, probability suggests that there will still be a small percentage of the electrophile that will add at the meta position

Question 16

When is a substituent group said to be a deactivating group?

Answer 16

If a substituted benzene reacts more slowly than the benzene alone.

Deactivating groups can withdraw electrons from the ring. Thus the ring is less attractive to an electrophile. All deactivating groups are meta directors, with the exception of the weakly deactivating halides which are o-p directors. Some examples of meta directors are -NO2, -SO2, -CN.

Without any substituents the partial positive charge density will be at the o-p positions. Thus the electrophile avoids the positive charge and favours attack at the meta position