Nucleophiles are atom or molecule species in a chemical reaction. Rich in electrons, they readily donate electrons to other electron-deficient species. The reactions that proceed through nucleophile attacks are called nucleophilic reactions. Broadly, we can classify nucleophiles in three different types – 

1. Lone pairs
2. Sigma bond
3. Π –bond

What is a chemical reaction?

A chemical reaction is a process that involves breaking and forming chemical bonds in the reactant species to form a product through an endothermic (heat absorbed) or exothermic (heat evolved) pathway using reagents.

What is a chemical reagent?

A chemical reagent in a chemical reaction is a substance or atom, or molecule which has the power to trigger the chemical reaction. It is also used to detect the presence of other substances in a chemical reaction.

Example- electrophile, nucleophile, and radicals

How does a chemical reagent form?

When any reaction occurs fission and fusion of bonds occur, i.e. breaking and forming of chemical bonds. Fission occurs in two ways – Homolytic bond fission and heterolytic bond fission.

Homolytic bond fission-

• As the name suggests Homolytic, homo means the same, and lytic means breaking. It indicates equal breaking of the bond. If we have a bond between two species A and B, and we know that 2 electrons form a single bond, both species will get one – one electron equally when breaking this bond.

• And  are called free radicals. 

• Homolytic bond fission is also known as homolytic cleavage or bond homolysis. The free radicals have one unpaired electron. Homolytic bond fission happens in the non-polar medium in the presence of sunlight or UV rays. It also occurs in the presence of TEL( tetraethyl led)/ peroxide.

                   Heterolytic bond fission –

• As the name suggests Heterolytic, hetero means different, and lytic means breaking. It indicates unequal breaking of bonds. If we have a single bond between two species A and B, and we know that a single bond forms by 2 electrons, so when breaking of this bond occurs, one species will get both the electron, and the other species will get nothing. So one will become negatively charged due to an excess of electrons, and one will become positively charged due to lack of electrons.

• A+  + B– 

• The positive charge species is called electrophile, and the negative charge species are called nucleophiles. 
• The distribution is based upon the difference in electronegativity.

The more electronegative species will get both the electrons and become negatively charged and one with less electronegativity will become positively charged due to deficiency of electrons.

What are  electrophiles?

Electrophiles are a type of reagent that is atom or molecule species in a chemical reaction. These are electron deficient species that attract other species bearing a full or partial positive charge.

Electrophiles are Lewis acid, species with vacant orbitals, and bronsted acid Example – AlCl3, BF3, NO2+

What are Nucleophiles?

Nucleophiles are a type of reagent or species in a chemical reaction that has extra electrons, so they behave as an electron donor for those who are electron deficient. Example – OH,,CN–, NH3 ,H2O

Types of nucleophile-

1. Lone pair – 

Any species which has a lone pair bears a negative charge and is capable of donating an electron to the deficient species. 

Understand it with an example of OH– (hydroxide ion) and H+ (hydrogen ion), Here OH– has a negative charge or lone pair, so it is rich in electron and as we can see that H+ has positive and is electron deficient, so the hydroxide will donate its electron to the positive hydrogen ion as the nucleophile needs nucleus and electrophile needs electron. By forming a bond, they will both fulfil each other’s requirements.

EXAMPLE – OH–, CN–, F– , Br– , Cl–, SH–   

1. Π-BOND 

This category is not only for double bond but also for the triple bond, aromatic, enols, and enolates.

We know that multiple bonds can donate electrons, but that electron pair is shared between two atoms.

Example – Benzene has 3 pi bonds, so it can donate electrons to the CH2+  and they both can form a bond.

3-Sigma bond – 

This is a rare case because it involves breaking a single bond which is difficult and  involves the leaving group.  

Example – 1, 2- hydride shift for giving a more substituted carbocation

Differences between Nucleophiles and Bases

Understand this with an example – 

Take a species R–, if the electron in R is donating its electron to hydrogen cation, it is referred to as base. If the electron in R is donating to a species, say any atom or molecule other than hydrogen cation, it is referred to as nucleophile.

  Conclusion – 

• Nucleophiles and electrophiles are formed by heterolytic fission.
• An electrophile is electron-deficient and attracts electrons.
• A nucleophile is an electron-rich species that donate an electron.
• Nucleophiles can be broadly divided based on bond – lone pair, pi – bond, and sigma bond.