An electrophilic substitution reaction is where the functional group attached to a compound is replaced with an electrophile. In simple words, the functional group of the compound is displaced with an electrophile. While the electrophile can be any electron pair accepting group, the functional group is most often a hydrogen atom.
We can understand the mechanism behind an electrophilic substitution reaction with the help of the following steps:-
1. The generation of the electrophile
As we know that the electrophile is deficient in electron pairs. Thus, it forms a bond only after accepting an electron pair. Therefore, to generate electrophile substances such as anhydrous chloride are used as they leverage the process of chlorination, alkylation, or acylation for an aromatic compound and generate the corresponding electrophile. The reagent attacks the electron center and generates electrophiles such as Cl+, R+, and RC+O.
2. Formation of an intermediate carbocation
Now, the electrophile generated from the initial step forms a covalent sigma bond with the aromatic compound. This leads to the formation of an intermediate carbocation. The initially generated electrophile attacks the ring of the aromatic compounds. The complex formed due to this reaction is called the arenium ion or sigma complex. It has to be noted that the complex contains an sp3 hybridized carbon. Due to this property, the aromatic compound loses its aromaticity.
3. Removal of the proton from the intermediate carbocation
Finally, the proton is released from the sigma or the arenium ion complex. This takes place when a reagent such as AlCl4 attacks the sp3 hybridized carbon. Due to this, the compound's aromatic character is restored, and the electrophile finally replaces the hydrogen from the aromatic compound.
Many electrophiles are not sufficiently electrophilic to react independently, or instead, the reaction is prolonged. Thus, we can also increase the rate of electrophilic substitution reaction with the help of lewis acids. For example, the electrophilic substitution reaction of benzene with Cl₂ can be carried out at a much faster rate with the presence of AlCl₃ and FeCl₃. Now, we know that alcohol groups form conjugate acid whenever they undergo substitution or elimination types of reactions.
Moreover, it is a known fact that H2O is a weaker base, which in turn makes it a much better-leaving group as compared to HO–. Alcohols like Ethanol itself seem to never react with solutions of compounds like NaCl to give ethyl chloride. This is because the resulting leaving group HO- is too strong to a base relative to Cl- for the chemical reaction to proceed to any extent further.
However, if we were to convert the alcohol into its conjugate acid R-OH2 + with a strong acid such as HCl, the reaction can then proceed at a much faster rate. This is because Cl– is displacing the much weaker base H2O. Thus, the carbon is attached to the electrophile much more efficiently as the H+ weakens the C-O bond. The exact mechanism is applied to Lewis acids being introduced in electrophilic substitution reactions from the electrophile. This weakens the bond between the electrophilic molecule and makes it an even better nucleophile compound.
We can also understand this efficiently with the help of an example. In chemical reactions such as chlorination and bromination, elements such as chlorine or bromine(Cl2, Br2) are introduced in the presence of the related lewis acids such as iron halide (FeCl3 or FeBr3) as a catalyst. However, since iron halides can be easily deactivated by water from the air, it is most efficient to use iron metal powder since this reacts quickly with Cl2 or Br2 to form FeCl3 or FeBr3, respectively.
There exist different types of electrophilic substitution reactions. They are as follows:-
1. Electrophilic Substitution Reaction on Aromatic compounds
The reaction where the aromatic ring itself is replaced or displaced by an electrophile is known as an electrophilic substitution reaction with aromatic compounds. In such reactions, the aromaticity of the compounds is maintained. Electrophilic aromatic substitution reactions can include chemical reactions such as Friedel-Crafts reactions., sulfonation, and nitration in aromatic compounds.
2. Electrophilic Substitution Reaction in aliphatic compounds
The chemical reactions where the electrophile displaces the functional group in aliphatic compounds are called electrophilic aliphatic substitution reactions. In these reactions, the electrophile attacks the aliphatic compound, which results in an inversion, i.e., 180⁰ inversion.
The following reactions are considered as the type of electrophilic aliphatic substitution reaction:-
1. Reaction of halogenation of ketones
2. Nitrosation reaction
3. Reaction displaying Keto-Enol tautomerism
4. Reaction where the insertion of a carbene into a carbon-hydrogen bond
5. Diazonium coupling reaction