Electrophiles and Nucleophiles

For any organic reaction to take place, substrate or organic compounds react with the reagent or attack species that directly result in the formation of products. Generally, the attacking reagent is categorised under two types – nucleophiles and electrophiles. In simpler terms, these reagents either donate or accept electrons to establish a bond. The word “electrophiles” is made from the combination of two words – electro means electron, and phile means loving. Alternatively, “nucleophiles” formed from nucleo means nucleus and phile means loving. 

Electrophiles 

As mentioned above, electro means electrons and phile means loving, is referred to as electrophiles. Any ion, molecule or atom that needs electrons is termed electrophile. Being positively charged species, they love to attack the negatively charged species and form a chemical bond. They can accept the electrons to become chemically stable. 

Types Of electrophiles 

Generally, there are two types of electrophiles – positively charged and neutral electrophiles. They both can accept electrons to become chemically stable. 

1. Positively charged electrophiles:

The electrophiles that have a positive charge on them are termed positively charged electrophiles. 

A few examples of such electrophiles are 

H+, SO3H+, NO+, NO2+, X+, R+, C6H5N2+, + CH3, etc. 

2. Neutral electrophiles:

Some atoms that are electron deficient but don’t have any charge on them are termed neutral electrophiles. Mainly lewis acids, neutral atoms, free radicals, carbenes and nitrene and species with electrophilic centres come in the category of neutral electrophiles. 

Here are a few examples of such electrophiles – BF3,  AlCl3,  SO3,  ZnCl2,  BeCl2 FeCl3,  SnCl2, CO2,  SnCl4,  R *COCl,  R – * Mg – X,  *I – Cl, CH3 – *CN,  R*–Cl,  R*–O etc. 

Nucleophiles

Unlike electrophiles, nucleophiles are known to donate electron pairs and establish a strong bond via covalent pairing. A negative charge present on the atom, ion or molecule indicates that it has an excess pair of electrons. Additionally, these species attack the positively charged species and sometimes function as lewis base in the nucleophilic substitution reaction. 

Types of nucleophiles

Similar to electrophiles, nucleophiles are divided into two main categories. 

1. Negatively charged nucleophiles

The species that have negative charge present on it are termed negatively charged nucleophiles. They can donate such a pair and establish a strong covalent bond. 

Example: CH3- , Cl-

2. All Lewis bases that have lone pairs

The lewis base that has lone pairs available comes under nucleophiles. They are capable of donating the electrons to the substrate, resulting in a nucleophilic substitution reaction. 

Example: NH3 , R-NH2

3. Ambident nucleophile

The third type of nucleophile is ambident nucleophiles. It has two sites that are rich in electrons and have the capability of forming a bond. Remember, resonating structures also come in the category of ambident nucleophiles. 

Example: CN- , NO2-

4. Amphiphile nucleophile

The molecule that has multiple bonds between the carbon atom and an electronegative atom can act as both nucleophiles and electrophiles.

Example: R-COH , R-CN

Features of electrophiles 

Here are the features of electrophiles that will help you to differentiate between electrophiles and nucleophiles. 

1. Deficient electron pair, therefore, gets attracted towards the atom that is negatively charged. 

1. Can be either positively charged or neutral.

1. Attack the areas where they can find the electrons. Especially the carbon-carbon double bonds. 

1. Electrons move from high-density areas to low-density areas. 

1. Species can favour both electrophilic addition and electrophilic substitution reaction. 

1. They are known by another name, Lewis acids. The reason is that they can accept electrons. 

Features of nucleophiles 

Here are some of the features of nucleophiles that help us to differentiate between the two. 

1. Being rich in electrons, they can attract electron-deficient species. 

1. They are either negatively or neutrally charged species. 

1. They are known to provide electrons to the species that lack the same. 

1. In this reaction, the electrons are transferred from a low-density area to a high-density area.

1. Favour both nucleophilic addition and nucleophilic substitution reactions.

1. They are known by another name, Lewis base. The reason is that they can easily donate extra electrons. 

Major difference between Electrophile and Nucleophile

Check out a few points that will make you understand the difference between electrophiles and nucleophiles. 

1. Electrophiles can accept extra electron pairs, whereas nucleophiles can donate extra electron pairs. 

1. Electrophiles are either neutral or positively charged species, whereas nucleophiles are either neutral or negatively charged species. 

1. Electrophiles have an extra orbital to receive an electron pair. On the other hand, nucleophiles have a loosely held pair of electrons to donate electrons easily. 

1. Electrophiles are known to attack high electron density areas. Alternatively, nucleophiles are known to attack low electron density areas. 

1. Lewis acids or electrophiles can accept electrons. Nucleophiles are known as lewis bases that can donate electrons. 

Nucleophilic & Electrophilic Substitution Reaction 

Nucleophilic substitution reaction is a reaction that attacks the positively charged species and donates the extra electrons to it. On the other hand, an electrophilic substitution reaction is a reaction that is executed when a functional atom is replaced by an electrophile. 

An example that completely satisfies the reaction and makes it more clear is  – CH₃Br that has Br attached to the positive area of CH₃. When this compound is made to react with CN-, the Br atom gets replaced by the CN-  atom. As a result, the compound gets transferred to CH₃CN, indicating the substitution reaction. 

In simple terms, a nucleophilic substitution reaction takes place when an electron-rich species attacks another compound that is resulting in the replacement of an already attached chemical compound. On the other hand, in the electrophilic substitution reaction, the cation or compound having a vacant valence shell replaces another electrophile already attached to a compound. 

Conclusion 

To recap, both electrophilic and nucleophilic species play a critical role in making or breaking the entire chemical reaction. Being opposite in nature to each other, these two species can get attracted to each other. This, in turn, gives the result of a strong chemical bond.