Compounds with a benzene ring are quite common. Benzene rings are called aromatics, and compounds with a benzene ring are aromatic compounds.

The molecule with the double bond is the benzene ring. However, the benzene ring undergoes a completely different chemical reaction than the alkene ring. Benzene rings are electron-rich like alkenes, but they do not undergo electrophilic addition reactions like alkenes do.

Instead of an addition reaction, a substitution reaction occurs at the benzene ring. The chemical reaction proceeds without loss of aromaticity. This is called the electrophilic aromatic substitution.

The Friedel-Crafts reaction is the most famous substitution reaction at the benzene ring. We will explain how electrophilic aromatic substitution proceed, including these reaction mechanisms.

The Benzene Ring Is Stable and No Addition Reaction to the Double Bond Occurs

Benzene rings are known to be very stable molecules. In the case of an alkyl chain, an addition reaction to an alkene occurs and the double bond becomes a single bond. On the other hand, although the benzene ring contains a double bond, it does not undergo an addition reaction to the benzene ring.

For example, an alkane can be synthesized from an alkene by the addition reaction to a double bond as follows.

In contrast, the addition of hydrogen bromide (HBr) or bromine (Br2) to the benzene ring, for example, does not give the following compounds.

The aromatic rings are very stable, and in the figure above, the compound after the reaction has lost its aromaticity. The activation energy is too high for the synthetic reaction to proceed, as we synthesize unstable compounds from stable compounds.

The Reaction Mechanism of the Benzene Ring Is to React Without Loss of Aromaticity

So, what kind of synthetic reaction does an aromatic compound undergo? In the case of the benzene ring, the synthetic reaction proceeds without loss of aromaticity. In other words, even if a substituent is attached to the benzene ring and the aromaticity is momentarily lost, the next reaction takes place so that the aromaticity is restored.

In short, understand that in electrophilic aromatic substitution reactions, the hydrogen atoms attached to the benzene ring are replaced by other substituent group. For example, it is as follows.

It is called electrophilic aromatic substitution reactions because the hydrogen atoms attached to the benzene ring are replaced by other substituents.

So what is the reaction mechanism of electrophilic aromatic substitution reactions? In all electrophilic aromatic substitution, the synthesis proceeds that aromaticity is restored as described above. The reaction mechanism is as follows.

Intermediates that undergo electrophilic aromatic substitution lose their aromaticity. However, because of a resonance structural that can be written, the intermediate is unstable, but to some extent stable. The hydrogen atoms (protons) are then pulled out to regain aromaticity and complete the substitution reaction.

All electrophilic aromatic substitution proceed in this way. Although the functional groups that bind to the benzene ring are different, the basic reaction mechanism is the same for all of them.

Nitration and Sulfonation Are Examples of Electrophilic Aromatic Substitution Reactions

So what are some examples of electrophilic aromatic substitution? Typical electrophilic aromatic substitution include nitration and sulfonation.

As the name implies, electrophilic aromatic substitution are caused by the presence of a strong electrophilic agent, which leads to the substitution of the benzene ring. Therefore, in nitration and sulfonation, strong acids such as sulfuric acid are used in the reaction.

-Nitration to Benzene Ring

Nitrobenzene is formed when concentrated nitric acid and concentrated sulfuric acid are mixed together and benzene is added. When concentrated nitric acid and concentrated sulfuric acid are added, nitronium ions are formed. Nitronium ion is a powerful electrophilic agent.

Therefore, nitration occurs by the following reaction mechanism.

-Sulfonation to Benzene Ring

Sulfonation of the benzene ring is also a typical electrophilic aromatic substitution. Sulfonation can be achieved by reacting the benzene ring with oleum (a liquid in which sulfur trioxide is absorbed into concentrated sulfuric acid).

Strong electrophilic agents are produced by the reaction between concentrated sulfuric acid and sulfur trioxide. It is as follows.

The electrophiles then react with the benzene ring as follows.

For reference, sulfonation of the benzene ring is a reversible reaction. Therefore, when it reacts with water under high temperature conditions, the sulfo group is removed to form a benzene. The synthetic reaction of benzene sulfonic acid with water is also one of the electrophilic aromatic substitution reactions.

Halogenation of the Aromatic Rings

In addition to nitration and sulfonation, there is another important reaction in electrophilic aromatic substitution, which is the Friedel-Crafts reaction. In the synthesis of substituents into benzene rings, we must study Friedel-Crafts reactions.

There are two types of Friedel-Crafts reactions.

  • Friedel-Crafts Alkylation
  • Friedel-Crafts Acylation

In order to understand these two reaction mechanisms, you must learn about the halogenation of the benzene ring beforehand. Once you understand the halogenation to aromatic rings, the reaction mechanism is the same for both alkylation and acylation.

As mentioned above, the benzene ring is so stable in structure that it does not react with hydrogen chloride (HCl) or hydrogen bromide (HBr) as alkenes do. Instead, the benzene ring can be halogenated by the addition of Cl2 (chlorine) or Br2 (bromine) using Lewis acids such as FeCl3 (iron chloride) or FeBr3 (iron bromide).

In Lewis acid catalysts such as FeCl3 and FeBr3, there is an empty orbit. Chlorine or bromine atoms are attached to this empty orbital to produce an electrophilic agent.

Subsequently, the benzene ring attacks the electrophiles, resulting in halogenation. The reaction mechanism is as follows.

As with nitration and sulfonation, electrophilic aromatic substitution proceed by the generation of electrophilic agents. The difference is that FeCl3 and FeBr3 are used as Lewis acid catalysts instead of sulfuric acid.

Alkylation by Friedel-Crafts Reaction.

An important fact is that the use of a Lewis acid catalyst allows chlorine and bromine atoms to be incorporated into the catalyst, creating a strong electrophilic agent. This property allows for alkylation to the benzene ring. This is called a Friedel-Crafts alkylation reaction.

In the Friedel-Crafts alkylation reaction, AlCl3 or AlBr3 is used as a Lewis acid catalyst. By using these catalysts and alkyl halides as reagents, the benzene ring can be alkylated.

The reaction mechanism is almost identical to that of halogenation. First, a Lewis acid catalyst reacts with an alkyl halide to form a carbocation (electrophilic agent) as shown below.

The benzene ring then attacks the electrophiles to complete the alkylation.

The reaction mechanism that produces an electrophilic agent is the same as for halogenation. The reaction mechanism for electrophilic aromatic substitution reactions is also the same. The difference is that the benzene ring can be alkylated using a Lewis acid catalyst.

Rearrangement Reactions in Friedel-Crafts Alkylation

There is a note of caution in the Friedel-Crafts alkylation reaction, which is a rearrangement reaction. Since the reaction passes through a carbocation as an intermediate, a rearrangement reaction may occur.

The intermediates of the carbocation have a order of stability. The order is as follows.

Therefore, after the carbocation is formed, a more stable carbocation is formed by the transfer of the hydrogen atom to the neighboring carbon. For example, it looks like the following.

By reacting with a Lewis acid catalyst, the primary carbocation is initially formed. However, the intermediate is more stable than this state if the hydrogen atom is transferred to a neighboring carbon atom to form a secondary carbocation. Therefore, the hydrogen atoms are transferred to become more stable carbocations.

Because of the carbocation rearrangement, a different product from the expected compound may be obtained. This is related to the carbocation rearrangement.

-Formation of Polysubstituted Compounds Is Likely to Be a Problem

It should be noted that in Friedel-Crafts alkylation reactions, polysubstituted benzene rings can be easily synthesized.

Carbon is known to push out electrons. Therefore, when an alkyl chain is attached to a benzene ring, the electron density of the aromatic ring increases. As a result, the increased reactivity of the benzene ring frequently results in polysubstituted compounds as well as compounds with a single substituent attached. It is as follows.

The alkyl chain of the benzene ring is ortho-para oriented. This leads to the formation of compounds with alkyl chain substituents in the ortho or para position.

This situation can be avoided by increasing the amount of benzene. If the benzene is present in excess rather than the compound formed by alkylation, the reagent is more likely to react with the benzene. As a result, the synthesis of polysubstituted benzene is avoided.

Acylation to Benzene Rings: Friedel-Crafts Acylation

In addition to alkylation, the Friedel-Crafts reaction is also used for acylation. This is called the Friedel-Crafts acylation reaction.

In the Friedel-Crafts acylation, acyl halides are used as reagents. The addition of a Lewis acid, AlCl3, allows the Friedel-Crafts reaction to proceed. The reaction mechanism by which the electrophilic agent is formed is as follows.

Acyl cations are formed as electrophiles. Thereafter, the electrophilic aromatic substitution proceeds as follows.

For Friedel-Crafts acylation reactions, the reaction mechanism is simple if you understand what we have discussed so far. The reaction mechanism is the same, with the only difference being the use of acyl chloride compounds as reagents.

-Friedel-Crafts Acylation with Acid Anhydride

In the Friedel-Crafts acylation reaction, not only acyl halides but also acid anhydrides undergo Friedel-Crafts acylation.

Acyl chloride reacts with aluminum chloride (AlCl3) to produce acyl cations. In other words, when the acyl cation, which is an electrophilic agent, is formed, the Friedel-Crafts acylation proceeds. When the acid anhydride reacts with AlCl3, the acyl cations are produced as shown below.

In this way, an acyl cation is created and the Friedel-Crafts acylation just described occurs. Since the reaction mechanism is the same, we omit the explanation.

-There Cannot Be Two Acyl Groups

Earlier, we explained that Friedel-Crafts alkylation reactions produce polysubstituted compounds. On the other hand, in Friedel-Crafts acylation, no two acyl groups are attached to the benzene ring.

When an acyl group is attached to an aromatic ring, the acyl group becomes an electrophilic functional group. This reduces the electron density of the benzene ring and makes the aromatic ring less reactive. This is why we do not have to worry about the formation of polysubstituted compounds in Friedel-Crafts acylation reactions.

Electrophilic Aromatic Substitution of Benzene Ring Has the Same Reaction Mechanism

One of the most important molecules is the benzene ring. Many organic compounds contain benzene rings, and it is important to understand how substituents can be added to aromatic rings.

However, aromatic rings are different in nature from the double bonds of alkenes. An electrophilic substitution reaction occurs, not an addition reaction. The aromaticity is not lost, and after the chemical reaction, the benzene ring is restored to give the product.

The reaction mechanism is the same for all electrophilic aromatic substitution. Therefore, the reaction mechanism is easy to understand, although there are different types of nitration, sulfonation, halogenation, Friedel-Crafts alkylation and Friedel-Crafts acylation.

Just be careful about rearrangement reactions, orientation and the formation of polysubstituted compounds. In this way, substituents can be put into the benzene ring.