Key Difference – Electrophilic vs Nucleophilic Substitution
Electrophilic and nucleophilic substitution reactions are two types of substitution reactions in chemistry. Both electrophilic substitution and nucleophilic substitution reactions involve in the breaking of an existing bond and formation of a new bond replacing the previous bond; however, that is done through two different mechanisms. In electrophilic substitution reactions, an electrophile (a positive ion or partially positive end of a polar molecule) attacks the electrophilic centre of a molecule whereas, in nucleophilic substitution reaction, a nucleophile (electron rich molecular species) attacks the nucleophilic centre of a molecule to remove the leaving group. This is the key difference between Electrophilic and Nucleophilic Substitution.
What is Electrophic Substitution?
They are a general type of a chemical reaction in which a functional group in a compound is displaced by an electrophile. Generally, hydrogen atoms act as electrophiles in many chemical reactions. These reactions can be further divided into two groups; electrophilic aromatic substitution reactions and electrophilic aliphatic substitution reactions. Electrophilic aromatic substitution reactions occur in aromatic compounds and are used to introduce functional groups onto benzene rings. It is a very important method in synthesizing new chemical compounds.
What is Nucleophilic Substitution?
Nucleophilic substitution reactions are a primary class of reaction in which an electron-rich nucleophile selectively attack the positively or partially positively charged atom or a group of atoms to form a bond by displacing the attached group or atom. The previously attached group, which is leaving the molecule, is called the “leaving group” and the positive or partially positive atom is called an electrophile. The entire molecular entity including the electrophile and the leaving group are called the “substrate“.
General chemical formula:
Nu: + R-LG → R-Nu + LG:
Nu-Nucleophile LG-Leaving group
What is the difference between Electrophilic and Nucleophilic Substitution?
Mechanism of Electrophilic and Nucleophilic Substitution
Electrophic substitution: Most of the electrophilic substitution reactions occur in the benzene ring in the presence of an electrophile (a positive ion). The mechanism may contain several steps. An example is given below.
Hydronium ion H 3O + (from Bronsted acids)
Boron trifluoride BF 3
Aluminum chloride AlCl 3
Halogen molecules F 2, Cl 2, Br 2, I 2
Nucleophilic Substitution: It involves the reaction between an electron pair donor (the nucleophile) and an electron pair acceptor (the electrophile). The electrophile must have a leaving group for the reaction to take place.
The reaction mechanism occurs in two ways: SN2 reactions and SN1 reactions. In SN2 reactions, the removal of the leaving group and the backside attack by the nucleophile occurs simultaneously. In SN1 reactions, a planar carbenium ion is formed first and then it is further reacted with the nucleophile. The nucleophile has the freedom to attack from either side, and this reaction is associated with racemization.
Examples of Electrophilic Substitution and Nucleophilic Substitution
The substitution reactions in the benzene ring are examples of electrophilic substitution reactions.
Hydrolysis of alkylbromide is an example of Nucleophilic Substitution.
R-Br, under basic conditions, where the attacking nucleophile is the OH− and the leaving group is Br−.
R-Br + OH− → R-OH + Br−
Recemization: racemization is the of an optically active substance into an optically inactive mixture of equal amounts of the dextrorotatory and levorotatory forms.
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“Electrophilic Aromatic Substitution Ortho Directors” By V8rik at English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
“General Scheme for Acid Catalyzed Nucleophilic Acyl Substitution” by Ckalnmals – Own work (CC BY-SA 3.0) via Commons Wikimedia
“Nitration of Benzene” by Yikrazuul – Own work (Public Domain) via Commons Wikimedia