Difference Between Electrophilic and Nucleophilic Aromatic Substitution

Main Difference – Electrophilic vs Nucleophilic Aromatic Substitution

Aromatic compounds are ring structures having alternating single and double bonds. But the pi bond in their double bonds does not exist due to delocalization of electrons. Therefore, aromatic compounds have electron clouds parallel to their planar structure. This property of aromatic compounds causes them to undergo electrophilic and nucleophilic aromatic substitution reactions. The main difference between electrophilic and nucleophilic aromatic substitution is that electrophilic aromatic substitution involves the replacement of an atom of the aromatic compound with an electrophile whereas nucleophilic aromatic substitution involves the replacement of an atom of the aromatic compound with a nucleophile.

Key Areas Covered

1. What is Electrophilic Aromatic Substitution
      – Definition, Types, Mechanism, Examples
2. What is Nucleophilic Aromatic Substitution
      – Definition, Mechanism, Examples
3. Difference Between Electrophilic and Nucleophilic Aromatic Substitution
      – Comparison of Key Differences

Key Terms: Aromatic Compound, Benzene, Electrophile, Electrophilic Aromatic Substitution, Meta Substitution, Nucleophile, Nucleophilic Aromatic Substitution, Ortho Substitution, Para Substitution

What is Electrophilic Aromatic Substitution

Electrophilic aromatic substitution is a chemical reaction that involves the replacement of an atom in an aromatic molecule with an electrophile. An electrophile is an atom or a molecule that does not contain electrons. It can accept electrons from an electron-rich species. This electrophile can either be a positively charged species or a neutrally charged species. A positively charged electrophile attracts electrons in order to neutralize the charge. A neutral species may need electrons to fill the free p-orbitals in order to obey the octet rule.

The mechanism of the electrophilic aromatic substitution reaction can be explained using the most common aromatic molecule, benzene. Benzene is rich in electrons due to the delocalization of the electrons in the pi bond. Therefore, it can donate electrons to an electrophile. Benzene has one hydrogen atom per one carbon atom. Therefore, the electrophile can replace a hydrogen atom. Then the electrophile can make a bond with the carbon atom which the replaced hydrogen atom was bonded to. This substitution reaction is very useful in introducing functional groups to the benzene ring.

According to the position where the electrophile is going to be substituted, there are three types of electrophilic aromatic substitution reactions. The following image shows these substitutions. The initial molecule is Nitrobenzene.

Figure 1: Synthesis of Dinitrobenzene

Types of Substitution

Ortho Substitution

Here, the Electrophile is substituted to the ortho position of the benzene ring.

Meta Substitution

The electrophile is substituted to the meta position.

Para Substitution

The electrophile is substituted to the Para position.

What is Nucleophilic Aromatic Substitution

Nucleophilic aromatic substitution is a type of chemical reaction that involves the substitution of a nucleophile in an aromatic ring. Here, the nucleophile replaces a leaving group of the benzene ring. This nucleophilic aromatic substitution is possible when a strong nucleophilic reagent is used.  If the benzene ring is already substituted with a highly electron attracting species, the adjacent carbon atoms (adjacent to the carbon that the electron attracting species is attached to) get a partial positive charge. Then this positively charged carbon atom can be attacked by a nucleophile.

Figure 2: Nucleophilic Aromatic Substitution

The above image shows the substitution of a Nucleophile (indicated as “Nu” in the above image) to a benzene ring that is already substituted with –NO2 groups and a halogen (indicated by “X” in the above image). There, the–NO₂ groups attract electrons from the benzene ring. Therefore, the carbon atom which the halogen is attached can be attacked by the nucleophile. This causes the replacement of the halogen atom by the Nucleophile.

Difference Between Electrophilic and Nucleophilic Aromatic Substitution

Definition

Electrophilic Aromatic Substitution: Electrophilic aromatic substitution is a chemical reaction that involves the replacement of an atom of an aromatic molecule with an electrophile.

Nucleophilic Aromatic Substitution: Nucleophilic aromatic substitution is a type of chemical reaction that involves the substitution of a nucleophile into an aromatic ring.

Aromatic Ring

Electrophilic Aromatic Substitution: In electrophilic aromatic substitution, the aromatic ring acts as the nucleophile.

Nucleophilic Aromatic Substitution: In nucleophilic aromatic substitution, the aromatic ring acts as the electrophile.

Added Reagent

Electrophilic Aromatic Substitution: In electrophilic aromatic substitution, the added reagent acts as the electrophile.

Nucleophilic Aromatic Substitution: In nucleophilic aromatic substitution, the added reagent acts as the nucleophile.

Conclusion

Electrophilic and nucleophilic aromatic substitution reactions are fundamental chemical reactions in organic chemistry. These reactions are very useful in synthesis and analysis of different organic compounds. The main difference between electrophilic and nucleophilic aromatic substitution is that electrophilic aromatic substitution involves the replacement of an atom of the aromatic compound with an electrophile whereas nucleophilic aromatic substitution involves the replacement of an atom of the aromatic compound with a nucleophile.

Image Courtesy:

1. “Nitration2” By Yikrazuul – Own work (Public Domain) via Commons Wikimedia
2. “Nucleophilic aromatic substitution example” Apcpca (based on copyright claims).(Public Domain) via Commons Wikimedia