Question

what is the difference between nucleophilic substitution and electrophilic addition?

Answer 1

Hint: In electrophilic reactions, an electrophile is generated, which is a species that accepts electrons, while in nucleophilic reactions the nucleophile is generated, that is the species which donates electrons. The substitution reactions have the incoming species attached in place of the leaving group, while in addition reactions, the reactants are added without any removal of a group.

Complete answer:
We know that nucleophiles are the species that donates the electrons, while electrophiles are the species that accepts electrons. This suggests that nucleophiles are nucleus loving, while electrophiles are electron loving.
The substitution reactions are those which involve the displacement of the group or molecules and attachment of the attacking group to that position. While addition reactions do not have any displacement, as the reactant simply adds the attacking species.
The difference between nucleophilic substitution and electrophilic addition is:

Nucleophilic substitution	Electrophilic addition
Nucleophilic substitution involves a nucleophile attacking the site of the electrophile in the reactant molecule and displacing it to form a product.	Electrophilic addition reaction has an electrophile, which is an electron deficient species that accepts electrons. It is added to the place where there is more electron density in the molecule.
For example,${{C}_{2}}{{H}_{5}}Cl+KOH(aq)\to {{C}_{2}}{{H}_{5}}OH+KCl$The chlorine atom is the leaving group which is replaced by the hydroxide OH group which is the nucleophile	For example, ${{R}_{2}}C=C{{R}_{2}}+B{{r}_{2}}\to {{R}_{2}}BrC-{{C}^{+}}{{R}_{2}}+B{{r}^{-}}\to {{R}_{2}}BrC-CBr{{R}_{2}}$Here the bromine atom acts as electrophile. The cleavage of a double bond provides electrons that are taken up by the bromine atom.
This shows the leaving group is replaced by the attacking group.	This shows that the electrophile is added on the part where there is more electron density.

Hence, nucleophilic substitution involves displacement by the nucleophile to form the product, while electrophilic addition involves the addition of the electron deficient electrophile on the electron dense site of the molecule.

Note:
Nucleophilic substitution are classified as ${{S}_{N}}^{1}$ and ${{S}_{N}}^{2}$reactions, they takes place in one and two steps respectively. ${{S}_{N}}^{2}$ reaction follows inversion of configuration, as the attack of the nucleophile on the site of the leaving group is from backside and in a transition state where the molecule is planar in state. A nucleophile always consists of a lone pair that it can donate, while electrophile does not contain any lone pair of electrons.