3.1: Introduction
After completing this section, you should be able to identify electrophilic substitution as the single most important reaction of aromatic compounds.
In the preceding chapter, we looked at aromaticity—the stability associated with benzene and related compounds that contain a cyclic conjugated system of 4 n + 2 \(\pi\) electrons. In this chapter, we’ll look at some of the unique reactions that aromatic molecules undergo.
The six pi electrons obey Huckel's rule so benzene is especially stable. This means that the aromatic ring wants to be retained during reactions. Because of this, benzene does not undergo addition like other unsaturated hydrocarbons.
The most common reaction of aromatic compounds is electrophilic aromatic substitution , in which an electrophile (E + ) reacts with an aromatic ring and substitutes for one of the hydrogens. The reaction is characteristic of all aromatic rings, not just benzene and substituted benzene, because aromaticity is maintained. In fact, the ability of a compound to undergo electrophilic substitution is a good test of aromaticity.
Other Examples of Electrophilic Aromatic Substitution
Many different substituents can be introduced onto an aromatic ring through electrophilic substitution. To list the five most useful reactions, an aromatic ring can be substituted by a halogen (–Cl, –Br, –I), a nitro group (–NO 2 ), a sulfonic acid group (–SO 3 H), a hydroxyl group (–OH), an alkyl group (–R), or an acyl group (–COR). Starting from only a few simple materials, it’s possible to prepare many thousands of substituted aromatic compounds.
Since the reagents and conditions employed in these reactions are electrophilic, these reactions are commonly referred to as Electrophilic Aromatic Substitution . The catalysts and co-reagents serve to generate the strong electrophilic species needed to effect the initial step of the substitution. The specific electrophile believed to function in each type of reaction is listed in the right hand column.
| Reaction Type | Typical Equation | Electrophile E (+) | |||
|---|---|---|---|---|---|
| Halogenation: | C 6 H 6 |
+ Cl
2
&
heat
FeCl 3 catalyst |
—— > |
C
6
H
5
Cl + HCl
Chlorobenzene |
Cl (+) or Br (+) |
| Nitration: | C 6 H 6 |
+ HNO
3
&
heat
H 2 SO 4 catalyst |
—— > |
C
6
H
5
NO
2
+ H
2
O
Nitrobenzene |
NO 2 (+) |
| Sulfonation: | C 6 H 6 |
+ H
2
SO
4
+ SO
3
& heat |
—— > |
C
6
H
5
SO
3
H + H
2
O
Benzenesulfonic acid |
SO 3 H (+) |
|
Alkylation:
Friedel-Crafts |
C 6 H 6 |
+ R-Cl &
heat
AlCl 3 catalyst |
—— > |
C
6
H
5
-R + HCl
An Arene |
R (+) |
|
Acylation:
Friedel-Crafts |
C 6 H 6 |
+ RCOCl &
heat
AlCl 3 catalyst |
—— > |
C
6
H
5
COR + HCl
An Aryl Ketone |
RCO (+) |
Make certain that you can define, and use in context, the key terms below.
- acylation
- alkylation
- electrophilic substitution
- halogenation
- hydroxylation
- nitration
- sulfonation
In this chapter, you will study all of the reactions shown in the Reaction Type table. In addition to these five reaction types, we also add a sixth common electrophilic substitution known as hydroxylation.
You must recognize the similarities between these reactions to minimize the material you must memorize.