Free Radical Substitution, Electrophilic Substitution and Nucleophilic Substitution Reactions Notes

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What is substitution reaction?

“The chemical reactions in which an atom or group of atoms directly attached to the carbon in the substrate molecule is replaced by another more reactive atom or group of atoms are called substitution reactions”.

Substrate:

The molecule in which substitution takes place is called a substrate.

Example:

CH4 + Cl2 —> CH3 Cl + HCl

Substitution Reactions:

Substitution reactions are the main property of alkanes.

Types of Substitution Reactions:

Substitution reactions are of three types.

Free Radical Substitution Reactions:

In this substitution, the substituent is a free radical i.e. an atom or group of atoms from the substrate is replaced by a free radical. A free radical is a species which has a single electron. e.g.

CH4        +            Cl        →          CH3Cl    +            H

In substitution two types of sigma bond fission take place.

  • Homolytic fission or Homolysis:

The bond fission in which each of the two bonded atoms acquires one of the bonding electrons is called hemolytic fission or homolysis.

                   

This type of bond fission takes place in free radical substitution reactions.

  • Heterolytic fission or Heterolysis:

The bond fission in which only one of the two bonded atoms acquires both of the bonding electrons is called heterolytic fission or heterolysis. In it positive and negative ions are formed.

This type of bond fission takes place in nucleophilic or electrophilic substitution reactions.

Electrophilic Substitution:

The substitution in which an atom or group of atoms from the substrate is replaced by an electrophile is called an electrophilic substitution reaction.

An electrophile is an electron loving species i.e it is electron-deficient and it takes electrons. It is either a positive ion or a neutral molecule which is electron deficient e.g. H+, Cl+, Br+, NO2+, HSO3+, R3C+, CH3+, AlCl3, BF3.

Nucleophilic Substitution:

The substitution in which an atom or group of atoms from the substrate is replaced by a nucleophile is called a nucleophilic substitution reaction.

A nucleophile is an electron-donating species i.e it donates electrons. It is either a negative ion or a neutral molecule which possesses lone pair of electrons e.g. Cl, Br, I, CN, OH, NH3, R – NH2.


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