Difference Between Nucleophilic and Electrophilic Substitution Reaction

Main Difference – Nucleophilic vs Electrophilic Substitution Reaction

Both nucleophilic and electrophilic substitution reactions are found in organic and inorganic chemistry. These substitution reactions are very important in the synthesis of certain compounds. A substitution reaction is a reaction that involves the replacement of an atom or a group of atoms by another atom or a group of atoms. The main difference between nucleophilic and electrophilic substitution reaction is that nucleophilic substitution reaction involves the displacement of a leaving group by a nucleophile whereas electrophilic substitution reaction involves the displacement of a functional group by an electrophile.

Key Areas Covered

1. What is Nucleophilic Substitution Reaction
      – Definition, Reaction Process, Examples
2. What is Electrophilic Substitution Reaction
      – Definition, Reaction Process, Examples
3. What is the difference between Nucleophilic and Electrophilic Substitution Reaction
      – Comparison of Key Differences

Key Terms: Aliphatic and Aromatic Compounds, Electrophile, Electrophilic Substitution, Leaving Group, Nucleophile, Nucleophilic Substitution, S_E1 Reaction, S_E2 Reaction, S_N1 Reaction, S_N2 Reaction, Substitution Reaction

What is Nucleophilic Substitution Reaction

A nucleophilic substitution reaction is a chemical reaction which involves the displacement of a leaving group by a nucleophile. This leaving group is given that name because it leaves when a nucleophile reacts with the molecule the leaving group is attached to (the whole molecule is called a substrate). The part that the nucleophile is going to be attached is called an electrophile. This electrophile lacks electrons in order to become stable. Therefore, it accepts electrons from a nucleophile. This results in the formation of a covalent bond between nucleophile and electrophile. 

Most of the times, the nucleophile is negatively charged. But it can also be a neutrally charged molecule having a free pair of electrons that is ready to be donated. These nucleophilic substitution reactions take place in aliphatic and aromatic organic compounds.

Figure 1: An Example of Nucleophilic Substitution in Aromatic Compounds

In the above example, the benzene ring is attached to a Chlorine (Cl) atom. It is the leaving group in the presence of NaNH₂. The nucleophile is –NH₂ group. The carbon atom (with a star mark in the above image) is attacked by the nucleophile and Cl atom is displaced by –NH₂ group. This is called a nucleophilic substitution.

Figure 2: An Example of Nucleophilic Substitution in Aromatic Compounds

In the above example, the nucleophile is indicated by symbol “Nuc”. The carbon atom in the center is attacked by the nucleophile and the leaving group “X” is displaced by the nucleophile. It can clearly be seen when considering the difference between the first and the last molecules in the above image.   

There are two main types of Nucleophilic substitution reactions categorized according to their mechanism.

S_N1 Reactions

The symbol “S” refers to “substitution” and “N” refers to “Nucleophilic”. The number (“1” here) indicates the kinetic order of the reaction. These reactions involve the formation of a carbocation intermediate. Therefore, the reaction occurs in two steps.

Figure 3: SN1 Reaction Mechanism

In the above example, N₂⁺ is the leaving group of the initial molecule. As the first step, the leaving group leaves, forming a carbocation intermediate. The intermediate that is formed here is an aryl cation. Since it is a stable ion, this is the rate determining step of this reaction. As the second step, the nucleophile is attached to the carbocation.

S_N2 Reactions

In the S_N2 reaction, a carbocation is not formed. Therefore the reaction occurs through a single step. Therefore, it is the rate-determining step of the reaction.

Figure 4: SN2 Reaction Mechanism

The above example shows the leaving of the leaving group (“X” here) and the substitution of the Nucleophile occurring at the same time. Read more: Difference Between S_N1 and S_N2 Reactions.

What is Electrophilic Substitution Reaction 

Electrophilic substitution is a chemical reaction that involves the displacement of a functional group by an electrophile. Most of the times, hydrogen atoms are displaced in this manner. Electrophilic substitution reactions are also found in aliphatic and aromatic compounds. Electrophilic substitution reactions are especially used to make benzene derivatives.

Electrophiles are molecules that are either positively charged or neutrally charged but lacks electrons. Electrophiles accept electrons from nucleophiles in order to neutralize its charge or to obey the octet rule and become stable.

Figure 5: An Example of Electrophilic Substitution Reaction in Aromatic Compounds

In the above example, one hydrogen atom of the benzene ring is displaced by NO₂⁺ ion. In this case, NO₂⁺ is the electrophile. There is a positive charge in the nitrogen atom. The benzene ring is rich with electrons due to the presence of pi-bonds. Therefore, electrophile attacks the benzene ring and attach with it, making a hydrogen atom the “leaving group”.  

The Electrophilic substitution reactions are mainly found in two types of mechanisms.

S_E1 Reactions

These S_E1 reactions involve the formation of a carbocation that is stable. Therefore, the rate determining step is the step of the carbocation formation. This indicates that the S_E1 reactions occur in two steps.  The attachment of electrophile to the carbocation can also be observed here. But the leaving group is still attached to the carbocation. As the second step, the departure of the leaving group occurs.  

Figure 6: SE1 Reaction Mechanism

S_E2 Reactions

The S_E2 reactions involve only one step. A carbocation is not formed. Therefore the rate determining step is the formation of substituted molecule.

Figure 7: SE2 Reaction Mechanism

Similarities Between Nucleophilic  and Electrophilic Substitution Reaction

• Both types of reactions are related to electron sharing.

• Both reactions result in covalent bonds.

• Both reactions result in a displacement of a group present in the substrate molecule.

• They produce leaving groups.

• Both reaction types are found in chemical reactions related to aliphatic and aromatic compounds.

Difference Between Nucleophilic  and Electrophilic Substitution Reaction

Definition

Nucleophilic Substitution Reaction: Nucleophilic substitution reaction is a chemical reaction which involves the displacement of a leaving group by a nucleophile.

Electrophilic Substitution Reaction: Electrophilic substitution is a chemical reaction that involves the displacement of a functional group by an electrophile.

Electron Sharing

Nucleophilic Substitution Reaction: In nucleophilic substitution reaction, nucleophile donates its electrons.

Electrophilic Substitution Reaction: In electrophilic substitution reaction, electrophile accepts electrons.

Electrical Charge

Nucleophilic Substitution Reaction: In nucleophilic substitution reactions, the nucleophile is either negatively charged or neutrally charged and the electron accepting molecule is positively charged or neutrally charged.

Electrophilic Substitution Reaction: In electrophilic substitution reaction, the electrophile is either positively charged or neutrally charged and the electron donating molecule is either negatively charged or neutrally charged.

Conclusion

Nucleophilic and electrophilic substitution reactions are fundamental reactions in organic and inorganic chemistry. The main difference between nucleophilic and electrophilic substitution reaction is that the nucleophilic substitution reaction involves the displacement of a leaving group by a nucleophile whereas the electrophilic substitution reaction involves the displacement of a functional group by an electrophile.

Image Courtesy:

1. “Substitution via benzyne” (Public Domain) via Commons Wikimedia
2. “General Scheme for Base Catalyzed Nucleophilc Acyl Substitution” By Ckalnmals – Own work (CC BY-SA 3.0) via Commons Wikimedia
3. “Benzene-nitration-mechanism” By Benjah-bmm27 – Own work (Public Domain) via Commons Wikimedia
4. “Aromatic SN1 mechanism” (Public Domain)  via Commons Wikimedia
5. “Mechanismus der Sn2 Reaktion-Seite001” By Poyraz 72 – Own work (CC BY-SA 3.0) via Commons Wikimedia
6. “Arenium ion mechanism” (Public Domain) via Commons Wikimedia

Reference:

1.” Electrophilic substitution.” What is electrophilic substitution? N.p., n.d. Web. Available here. 27 June 2017. 
2.Hunt, Dr Ian R. “Nucleophilic substitution.” Ch 8: Nucleophilic Substitution. N.p., n.d. Web. Available here. 27 June 2017. 
3.”B. What is Nucleophilic Substitution?” Chemistry LibreTexts. Libretexts, 24 June 2016. Web. Available here. 27 June 2017.