Understanding the Basic Strength of Nucleophiles

Electrophiles and nucleophiles are known as attacking reagents that alter the reactivity of the corresponding product formed according to the reactivities of different functional groups.

1. Electrophile: A reagent having an affinity for electrons is called electrophile. They are either neutral or positively charged. Electrophiles are electron-deficient species that accept electron pairs. They are called electron-loving (philic) or electron-seeking (E). According to the Lewis concept of acids and bases, they are known as Lewis acids.

Examples:

Positively charged electrophile: H+, H3O+, Cl+, CH3+, NO2+

Neutral electrophile: AlCl3, BF3, SO3. Here, both Al and B have a total of six electrons, I.e., two less than a complete octet. Hence, by trying to complete their octets, they act as electrophiles.

Let us take up an example of a reaction.

C6H6 + CH3CO+    → C6H5COCH3 

Here, CH3CO+ is an electrophile.

1. Nucleophile: A reagent having an electron pair is called a nucleophile. Such reagents have an atom that has an unshared or lone pair of electrons. A nucleophile is an electron-rich species and has an affinity for electron-deficient sites, hence called nucleus-loving or nucleus-seeking (Nu). According to the Lewis concept of acids and bases, they are known as Lewis bases.

Examples:

Negatively charged nucleophile: X–, OH–, CN–, R-COO–

Neutral nucleophile: H2O, R3N, R-O-R

Let us take up an example of a reaction.

CH3COOH + OH–   → CH3COO– + H2O

Here, OH– is a nucleophile.

Nucleophilicity:

Nucleophilicity is used for determining the nucleophilic strength of a given nucleophile.  It depends on various factors like basicity, steric hindrance, polarity, solvent, charge, etc. The more the negative charge on a nucleophile the more will be its nucleophilicity and basicity. Therefore, the ones having a high negative charge act as excellent nucleophiles and bases.

Nucleophilicity vs Basicity: What’s the Difference?

The properties of nucleophilicity and basicity of a nucleophile may sound similar, but there’s one major difference between the two: Nucleophilicity refers to the tendency to attack electrophilic sites (Lewis base), whereas basicity refers to the tendency to accept acidic hydrogen protons H+ (Bronsted base), in order to neutralise a given chemical reaction.

Working Mechanism of Electrophilic/Nucleophilic Attack:

1. During a polar organic reaction, a nucleophile attacks an electrophile site, which is either a part of the electrophile that is electron-deficient or a specific atom.
2. Similarly, an electrophile attacks a nucleophile site, which is either a part of the nucleophile that is electron-rich or a specific atom.
3. Thus, the electrophile gets the electron pair from the nucleophile, resulting in the formation of a bond. 

      E + :Nu →  E – Nu     

In neutral molecules such as alkyl halides, the carbon atom is bonded to the halogen (X) atom.  Due to the C – X bond polarity, carbon acquires a partial positive charge ( +), and the electronegative halogen atom acquires a partial negative charge ( –). Thus, carbon becomes an electron-deficient or an electrophilic centre at which the nucleophile canattack.            

Conclusion: 

In this article, we saw an overview of electrophiles and nucleophiles. An electrophile (Lewis acid), being electron-deficient, has an affinity for electrons. A nucleophile (Lewis base), on the other hand, being electron-rich, is nucleus-seeking. Moving on, we discussed what nucleophilicity is, as well as its correlation with basicity. Nucleophilicity determines the tendency to attack electrophilic sites (Lewis base), whereas basicity is the tendency to accept acidic hydrogen protons H+ (Bronsted base) in order to neutralise a given chemical reaction. We also saw the general working mechanism of both electrophiles and nucleophiles.