Electrophilic Substitution Reaction

Electrophilic substitution reaction

An electrophilic substitution reaction is a type of reaction in which a functional group is replaced by an electrophile. In this reaction, hydrogen from the compound is replaced by an electrophile.

An electrophile is an electron-deficient species or electron-loving. Electro means electron and phile mean loving. The general reaction of E.S Rx is given:

A-B + E+ A-E + B+

Is it necessary for every nucleophile to have a positive charge? The answer is no. Benzene and its derivatives (nitrobenzene, toluene, benzaldehyde, phenol, etc.) give electrophilic substitution reactions. Benzene has pi electron-rich species and pi electrons are loosely bounded due to these weakly bounded pi-electrons, the electrophile attracted toward benzene. Generally, the compounds having double bond gives an addition reaction but why benzene gives a substitution reaction?

Benzene gives a substitution reaction not an addition reaction because when benzene undergoes an addition reaction, the product formed is non-aromatic. When benzene gives a substitution reaction, the product remains aromatic and in chemistry, aromatic compounds are more stable. Benzene gives an addition reaction but at critical conditions (at high temperature, pressure, catalyst) but generally, benzene and its derivatives give a substitution reaction.

Steps in Electrophilic Substitution Reaction

Electrophilic substitution reaction generally proceeds by the mechanism which contains three steps:

  1. Generation of Electrophile
  2. Attack of Electrophile
  3. Removal of old Electrophile

Now we discussed a particular example with a detailed mechanism.

Halogenation (Chlorination) of Benzene

Chlorination of Benzene
Electrophilic Substitution Reaction

Mechanism

1. Generation of Electrophile
Generation of Electrophile
2. Attack of Electrophile
Attack of Electrophile
Electrophilic Substitution Reaction

The question is that the sigma complex is not aromatic so why it is formed? It is not aromatic but it has a little bit stable due to resonance. There is no formation of a carbocation in E.S Rx of benzene. In E.S Rx of benzene, a sigma complex is formed.

3. Removal of old Electrophile
Attack of electrophile
Can we use another catalyst in the halogenation of benzene except for FeCl3?

Yes, we can! FeCl3 is a lewis acid so we used another lewis acid as a catalyst like AlCl3, AlBr3, ZnCl2, etc. The purpose of the catalyst is to generate the electrophile and remove the old electrophile.

Which of the following compounds gives a more electrophilic substitution reaction? A) Benzyl cyanide B) Phenol

Properties Of Benzyl Cyanide:

The following are the properties of Benzyl Cyanide:

  • They are colorless only liquids
  • They have a density of 1.015 g/cm3
  • They have a melting point of −24 °C (−11 °F; 249 K).
  • Also, they possess a boiling point of 233 to 234 °C (451 to 453 °F; 506 to 507 K).

Properties of Phenol:

  • Phenol is an aromatic compound with a benzene ring in it.
  • Phenol possesses a peculiar smell.
  • These are water-soluble crystalline solids that have definite shapes and volumes in the 3-dimensional pattern.
  • They have low melting and high boiling point.

What is the Mesomeric Effect?

The mesomeric effect is the effect that enables a ring to become stable. It may increase or decrease the electron density of the ring. So, it may enhance the electrophilic substitution reaction. There are some compounds that are attached to the benzene ring so that they may be activated or deactivated groups. If the ring is electron deficient then the +M effect dominated and they give electrons to the ring the stabilize charge density. If the ring is electron enriched the -M effect dominated and withdraws the electron from the ring to stabilize the charge density.

Which is more stable Benzyl Cyanide or Phenol?

Structures of Benzyl Cyanide and Phenol:

The first structure shown is Benzyl chloride and the second one is phenol. Both stabilities are defined by the mesomeric effect.

Benzyl cyanide
Phenol

The mesomeric effect contains two types of mesomeric effects that are +M effect and the -M effect. +M effect is shown by those compounds having lone pair on them and is directly attached to the benzene ring however on the other hand -M effect is shown by compounds that are directly attached to the ring and with the other compound that is more electronegative than that of compound attached to the benzene ring with pie bond.

In the structure of Phenol, oxygen contains lone pairs on it so that it gives its lone pair to the ring yet the charge density of the overall ring increases. Therefore, it gives a substitution reaction. While in the structure of Benzyl cyanide there is no increase in charge density because carbon takes an electron from the benzene ring and nitrogen takes an electron from carbon so electronic charge density decreases.

By the following relation, it is clear that (Electron charge density is directly proportional to substitution reaction). Therefore, the increase in electronic charge density in phenol gives more substitution reactions.

So, B > A (Conclusion of the question)

Frequently Asked Questions

What are electrophilic substitution reactions give example?

An electrophilic substitution reaction occurs when an electrophile (an electron pair acceptor) replaces the functional group connected to a compound. Alkylation, acylation, halogenations, nitration, sulphonation, and other processes are electrophilic substitution reactions of benzene

What are 5 electrophilic substitution reactions?

  1. Alkylation
  2. Acylation
  3. Halogenations
  4. Nitration
  5. Sulphonation

What are electrophilic and nucleophilic substitution reactions?

Nucleophilic substitution reaction:

It is a chemical reaction in which a nucleophile displaces a leaving group. The donor of the electron is a nucleophile.

Electrophilic substitution reaction:

It is a chemical reaction in which an electrophile displaces a functional group. Electrophiles take electrons in.

What is the four most common electrophilic aromatic substitution reaction?

Nitration:

Nitration is the process of replacing H with NO2, utilising sulphuric acid (H2SO4) as the Lewis acid and nitric acid (HNO3) as the source of NO2:

Sulfonation:

Sulfur trioxide (SO3) can be used to execute it when sulphuric acid (H2SO4) is present as the Lewis acid:

Friedel-Crafts Alkylation:

We first add a Lewis acid like AlCl3 or FeCl3 to an alkyl halide “R-X” such as CH3CH2Cl. Similar to Cl2, the Lewis acid quickens the reaction by coordinating with the halogen, weakening the C-Cl bond, improving its leaving group properties, and facilitating the nucleophilic attack on the attached carbon.

Friedel-Crafts Acylation:

With an acyl halide, we begin. The addition of our Lewis acid causes the breakdown of C-H and the production of C-C.

Why benzene is an electrophilic substitution?

The benzene ring’s resonance causes the delocalized electron to traverse the carbon atoms there effectively. Along with stabilizing the arenium ion somewhat. Because of the arenium ion’s partial stability, electrophilic substitution reactions in benzene are very common.

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