Electrophilic Substitution & Aromatic Hydrocarbons

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson we will learn what happens when arenes and electrophiles react, and how electrophilic substitutions occur. We will also look at an example of electrophilic substitution.

Electrophiles and Arenes

Aromatic compounds are important in many of our everyday lives. They are a part of rubber, hormones in our body, and medications such as aspirin.

In order to make these compounds we often start with an arene, which is an aromatic hydrocarbon. But it is difficult to make that arene react in order to make other compounds. Aromatic compounds are extremely stable, and if the aromaticity is broken then a huge increase in energy is needed. So how are these compounds made?

One way is through electrophilic substitution, which removes a hydrogen and substitutes it for an electrophile. An electrophile is an electron deficient compound. This can be a compound with a full positive charge (so there are more protons than electrons) or there can simply be a partial positive charge through resonance.

The arene has several pi bonds (the double bonds) making it highly electron rich, so it is referred to as a nucleophile.

Substitution vs Addition

Typically when an electrophilic compound reacts with a double bond we see a process called electrophilic addition. The electrophilic compound is added and the double bond is broken. But the double bond in arenes don't want to break because the aromaticity will then be broken. So instead, electrophilic substitution is used.

Since electrophilic substitution removes the hydrogen from the arene and replaces it with the electrophilic compound, the double bond can remain and the arene keeps its aromaticity.

General Mechanism

Often a pre-step needs to occur before electrophilic substitution occurs. In this step the electrophile is formed, where it disassociates to form a positive charge.

X refers to the electrophile and Y is the weak base which disassociates from the electrophile

In general this reaction occurs by the following steps:

  • Step 1: pi electrons from one of the double bonds attack the electrophile forming a new bond between the arene and the electrophile. Since the aromaticity is momentarily broken in order for this reaction to occur, it takes a lot of energy, making this the slowest step.

The arene attacks the electrophile
Arene attacks electrophile

  • Step 2: The positive charge can resonate to three different locations.

The positive charge can resonate in three different places on the ring
Step 2 resonance

  • Step 3: The extra hydrogen is removed with a weak base.

The hydrogen is removed
Step 4 hydrogen removed

With the extra hydrogen removed, the pi bond can reform, re-creating the aromaticity while replacing the hydrogen with the electrophile. Although the aromaticity is broken for part of this reaction, it still occurs because in the end the aromaticity is returned, and the molecule is at a lower energy level than before.

So overall the mechanism looks like this:

Electrophilic general mechanism

In this general mechanism we see that the weak base can come from the ionization of the electrophile, but this isn't always the case.

Types of Electrophilic Substitution Reactions

The simplest arene is benzene (C6 H6 ), so we'll use it as an example, but the same principles are applied to any arene.

There are four main types of electrophilic substitution reactions. They differ based on what type of electrophile is being added. These are:

  • Halogenation
  • Nitration
  • Sulfonation
  • Friedel-Crafts

Halogenation reactions add a halogen (such as chlorine or bromine) to the arene. The halogen complex (X2 ) ionizes into individual halogens, one with a positive charge and another with a negative charge. Iron (III) chloride is used as a catalyst.

Nitration reactions add a nitro group (NO2 ) to the arene. The nitric acid ionizes into a nitro group, which is an electrophile. Sulfuric acid (H2SO4 ) is used as a catalyst.

Sulfonation reactions add a sulfonic acid to the arene. It occurs quickly when there is a second sulfonic acid to act as a catalyst.

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