Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution Mechanism

Electrophilic aromatic substitution mechanism is the method of substitution reaction in aromatic hydrocarbons or compounds. Aromatic compounds hydrocarbons or organic compounds tend to go through this reaction. In which an atom of a compound such as benzene reacts with an electrophile. And it replaces that atom (i.e. attaches to the aromatic ring). In some common reactions such as benzene, the electrophile replaces the hydrogen atom from the aromatic ring. This aromatic reaction helps preserve the aromaticity of an aromatic compound. Now let us discuss some electrophilic aromatic substitution examples. One such case of aromatic stability is the reaction of a benzene ring with chlorine to form iron chloride and hydrochloride. Similarly, sulphur trioxide reacts with benzene to form sulphuric acid. Here sulphur trioxide is the electrophile.

Different Types of Electrophilic Aromatic Substitution Reaction

Although there are multiple types of electrophilic aromatic substitution reaction, Let us discuss a few of them. Also, we will go through some example of electrophilic substitution reaction. They are Nitration, halogenation, sulfonation, Friedel crafts alkylation and acylation. All of them are aromatic reactions, but they are very different from each other. The only thing common between them is the benzene ring.  Some electrophilic aromatic substitution examples are:

Nitration

Aromatic Nitration reactions involve nitro (NO₂) group. Nitro group acts as an electrophile to replace the hydrogen atom. This process also involves the use of a catalyst in the form of sulfuric acid (H₂SO₄). There is another acid used as well called nitric acid that loses a proton to form nitronium ion. By the application of Electrophilic aromatic substitution mechanism, we can process this nitronium ion. A great example of Electrophilic substitution reaction involving the nitro group is TNT or high explosives. Toluene also is known as methylbenzene that goes through this process to create trinitrotoluene.

Halogenation

Aromatic halogenation reactions involve halogen group elements, mainly bromine and chlorine. Benzene goes through a substitution reaction to replace its hydrogen atoms with chlorine or bromine. Since they do not have the strength to complete the reaction on their own, we use acids such as lewis acids as a catalyst to speed up or complete the process. These acids, such as aluminium bromide or iron bromide, transfer a pair of electrons so that their atoms can form permanent bonds (cl-cl or Br-Br). In this reaction, the benzene ring loses its aromaticity and generates activation energy. To overcome that energy Br or cl uses their electrophilic strength due to their positive charge.

Sulfonation

As the name suggests, the aromatic sulfonation reaction involves sulfonic acid (SO₃). We use sulfuric acid (lewis acid) as a catalyst in this reaction. That makes it possible for sulfonic acid to gain a proton and generate a strong electrophile. Subsequently, this electrophile reacts with benzene and replaces its hydrogen atom. Then we use the electrophilic aromatic substitution mechanism to further complete the process. This reaction is quite similar to the aromatic nitration reaction. 

Friedel Crafts Alkylation

Friedel Crafts alkylation reaction involves the use of alkyl group (R). In the previous reactions, we saw the reaction of different molecules with the carbon of benzene, but it is also possible to form a carbon-carbon bond. It requires alkyl halides to react with benzene in the presence of a catalyst such as lewis acids. An example of Electrophilic substitution reaction can be, chloromethane reacts with benzene in the presence of aluminium chloride or iron chloride. The lewis acids make it easy for the chlorine atom to leave the bond by weakening the bond. Although, the product of this reaction has high nucleophilic strength.    

Friedel Crafts Acylation

This reaction is similar to the Friedel Crafts alkylation only it involves the use of acyl group (RC=O) instead of the alkyl group. Presence of lewis acids speeds up the process. For instance, acyl chlorides gain a proton in the presence of Lewis acids to become acyl ions. This ion acts as an electrophile and weakens the carbon chlorine bond. It uses one pair of chlorine while the other fills with the aluminium octet. Generally, aryl ketone comes out as a product of this reaction. Let us go through the steps involving this mechanism. 

What is the Mechanism for Electrophilic Aromatic Substitution

This mechanism mainly involves three fundamentals. There is a formation of a new pi bond from carbon double bond, removal of a proton from the carbon-hydrogen bond, and reformation of carbon double bond. You must understand these two main steps involving electrophilic aromatic substitution reaction mechanism. The first step initiates the attack of an electrophile on the benzene ring. After that, initial attack helps the formation of arenium ion by gaining positive charge or protons. Subsequently, the entire process is slow due to electrophile taking its time attacking the aromatic ring.  

Since the aromatic ring loses its aromaticity, it results in the release of high activation energy.  Several factors, such as steric hindrance, probability, and resonance, play a crucial role in the electrophilic attack. And the second step involves the removal of a proton from the ion by a weak base. This removal occurs due to the attack of a weak base on the formed carbocation. Then the aromaticity is stored again by the formation of the pi bond via electrons.  The entire process is relatively fast. One key thing to remember is that due to the attack of the electrophile, carbocation loses a proton in the process.      

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