Electrophilic and Aromatic Substitution Reactions

When electrons in the pi orbitals of a conjugated cycloalkene compound can move around or are delocalised, this enhances its stability, and it is termed aromaticity. Aromatic compounds are organic compounds made up of carbon and hydrogen atoms in continuous rings like structures with delocalised pi electrons.

Aromatic substitution reactions are organic reactions that occur in aromatic compounds in which an electrophile takes the position of an atom, usually a hydrogen atom.

Substitution Reaction

Substitution reactions are those reactions in which, as the name suggests, an atom or molecule of the compound is replaced or substituted with another atom or molecule. In other words, a functional group of the chemical compound is substituted by another functional group. For example, when methane chloride(CH3Cl) is reacted with hydroxyl ion (OH–), it forms methanol (CH3OH).

This is an example of a substitution reaction. Here, the chloride functional group is replaced by the hydroxyl functional group, which is hydroxyl ions.

Aromatic Substitution Reaction

We now have a general idea of how a substitution reaction behaves; let us see how that mechanism applies to aromatic compounds. When we react benzene (C6H6) with chloride ion, we will get benzyl chloride 𝛂 – chlorotoluene.

C6H6 + Cl+  => C6H5Cl + H+

In this reaction, the chloride functional group replaces benzene’s hydrogen ion, yielding. This reaction is also called the Halogenation reaction. This is the general working principle of aromatic substitution reaction.

Electrophilic and Nucleophilic Substitution Reaction

Nucleophilic Substitution

Electrophilic Substitution

But a question arises, what makes the substitution reaction work? What is the driving force, and why does substitution occur? To that question, we look at the concept of electrophiles and nucleophiles.

Electrophiles are electron-hungry species and are highly attracted to electrons that hold a negative charge while positively charged. Nucleophiles are proton-hungry species attracted to protons that hold a positive charge while being negatively charged or sometimes neutral.

Due to these electrophiles and nucleophiles, the substitution occurs in compounds. The chloride ion acts as a nucleophile in the previous aromatic substitution example. It attacks the positive hydrogen atom of the compound and replaces it, thus yielding benzyl chloride.

This type of reaction in which a nucleophile initiates the attack is called a nucleophilic substitution reaction. The reaction in which an electrophile initiates the attack is called an electrophilic substitution reaction.

The mechanism for Electrophilic Substitution Reaction for Aromatic Compounds

We now understand the meaning behind electrophiles and nucleophiles and how it plays a role in substitution reactions. We also looked at the definition of electrophilic and nucleophilic substitution reactions. Let us now understand how electrophilic aromatic substitution reaction works.

During electrophilic aromatic substitution reaction, we need to consider these three fundamental ideas that are given below:

1  A sigma bond is formed from the C=C bond during the reaction.

2. By the breaking of the C-H 𝛼 bond, a proton is removed.

3. The C=C covalent bond is reformed such that it restores the aromaticity of the compound.

Now, the mechanism of electrophilic substitution reaction for aromatic compounds is completed in two steps.

Step 1

• The electrophile attacks the pi electrons of the aromatic compound.

• This will result in delocalised cyclohexadienyl cation, which is positively charged yet stabilised due to resonance. This cation is called an arenium ion.

• Aside from the resonance-driven stability, the compound still has an unbalanced charge due to the attack of the electrophile. The loss of aromaticity causes the presence of high activation energy.

• To determine the attack of the electrophile resonance, probability and steric hindrance are the main factors to care for.

• This process is slow and gradual.

Step 2

• In this step, a weak base is added to the equation to deprotonate the arenium ion.

• The reaction between the weak base and the carbocation results in the loss of a proton.

• The electrons reform the pi bond, and aromaticity is restored.

• Unlike the first one, this step is very fast and is usually exergonic.

Examples

Some examples of electrophilic and nucleophilic aromatic substitution reactions are given below:

Aromatic Sulfonation Reaction

The hydrogen is substituted by a sulfonic acid (SO3). The electrophile is formed from the protonation reaction of SO3 and H2SO4. The gained electrophile is strong, and the mechanism of electrophilic substitution reaction is followed.

Aromatic Nitration Reactions

The hydrogen is replaced with a nitro (NO2) functional group using H2SO4 as a catalyst in this reaction.

Conclusion

This article taught us what aromatic compounds are and what substitution reactions are. We learned the definition of electrophiles and nucleophiles and that they help work substitution reactions. We familiarised ourselves with the working mechanism of the electrophilic substitution reactions in aromatic compounds and learned how electrophilic and nucleophilic substitution reactions differ. We delved deep into the world of aromatic compounds and their reactions with other functional groups.