ORGANIC CHEMISTRY I CHEM 2323

ELECTROPHILIC AROMATIC SUBSTITUTION

Introduction

The pi bonds are more involved in holding the benzene ring together than are the pi bonds of a carbon-carbon double bond. But compared to a sigma electrons, these pi electrons are loosely held and are available to a reagent that is seeking electrons. The benzene ring serves as a source of electrons (base). Therefore, the typical reactions of a benzene ring are electrophilic substitution reactions.

Reactions of Benzene

1. Nitration

2. Sulfonation

3. Halogenation

4. Friedel-Craft Alkylation

5. Oxidation

Effect of Substituent groups

Any group already attached to the benzene affects the reactivity of the ring (slows or speeds up the rate at which the second group attaches) and the orientation (where the second groups goes in relationship to the group already present .... ortho, meta, para).

• Reactivity
  • A group which makes the ring more reactive than benzene is called an activating group.
  • A group which makes the ring less reactive is called a deactivating group.
• Orientation
  • A groups which causes the attack on the second group to occur on the meta position is call a meta director.
  • A groups which causes the attack on the second group to occur on the ortho and para position is call a ortho,para director.

Determination of relative reactivity

• Time required
• Severity of conditions
• Competitive conditions

Classification of substituent groups

Orientation in disubstituted benzenes

The two substituent groups may be located so that the directive influence of one reinforces that of the other.

When the directive effect of one groups opposes the that of the other, it may be difficult to predict the major products and complex mixtures may be produced. But predictions can be made with the following generalizations:

• Strongly activating groups generally win out over deactivating or weakly activating groups.
• There is often little substitution between two groups meta to each other.

Orientation and Synthesis

A laboratory synthesis is generally aimed at obtaining a single, pure substance. The synthesis may be done in a series of steps. But the order in which we introduce the various substituents onto the ring must be considered due to a previous groups orientation activities.