Identifying Electrophile and Nucleophile

• Eastern Eye Staff
• 25 November, 2020

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By: Eastern Eye Staff

In an article published more than half a century back, Arthur Lapworth recognised that polar reagents come under two classes which he termed: “Cationoid” and “Anionoid.” 

A molecule is considered polar, or to have a molecular polarity when it has an overall imbalance of charge — a region with a partial positive charge and a region with a partial negative charge. 

Later Sir Christopher Ingold, who fundamentally changed the approach of chemists towards organic chemistry, proposed that the two classes be designated- “electrophilic,” defined as electron seeking, and “nucleophilic,” defined as nucleus seeking. 

So, let’s understand the concepts and Electrophile and Nucleophile differences.

Definition:

Electrophiles (labelled Nu / Nu: / Nu- in reaction mechanisms) are electron-deficient species (or that can accept an electron pair from an electron-rich species and form covalent bonds) attracted to an electron-rich centre. 

Electrophiles are positively or neutral charged atoms or molecules having free orbitals for incoming electrons.

Electrophilic substances are Lewis acids (compounds that accept electron pairs) and some Brønsted acids (compounds that donate protons). 

Examples: Hydronium ion (H3O, from Brønsted acids), aluminium chloride (AlCl3)

boron trifluoride (BF3), the halogen molecules, viz. Chlorine (Cl2), Fluorine (F2), Bromine (Br2), and Iodine (I2).

Nucleophile (labelled E / E in reaction mechanisms) is a chemical species that seeks a positive centre in a chemical reaction. It is like the nucleus of an atom since a nucleophile contains an electron pair available for bonding.

Examples: The hydroxide ion (OH-), Halogen anions (Cl-, I-, Br-), the cyanide ion (CN-), ammonia (NH3), etc.

Some key points- Electrophile:

• Electrophile consists of two words: “electro” derived from electron and “phile”, meaning loving. Therefore, any molecule, ion or atom which has deficient electrons can act as an electrophile. They usually are positively charged or are neutral species (electron-deficient molecules) having empty orbitals.
• Since they are electron deficient, they tend to attack electron-rich areas such as carbon-carbon double bonds.
• Electrophiles are often positively charged since they possess an atom with a positive charge or an atom that does not have an octet of electrons. 
• The majority of organic reactions can be classified as Lewis acid-base reactions.  However, chemists usually love to call Lewis acid an electrophile (which is electron deficient) and a Lewis base as a nucleophile (electron-rich). 
• They may be electrically neutral chemical species. 
• The strength of an electrophile is determined by its electrophilicity. Electrophilicity indicates the electrophilic nature of an electrophile and depends on factors like the charge of the electrophile.
• Electrophiles normally either have electron-withdrawing groups or groups containing electronegative elements, which pull the electron towards themselves. Alternatively, electrophiles may also possess polarisable π-bonds like C=N or C=O.
• Positive Electrophiles: H , Cl , NO, etc.
• Neutral Electrophiles: AlCl3 :R, SO3, FeCl3, etc. 

Electrophilic substitution reactions involve the substitution of an electrophile by replacing a functional group of a molecule. Most common electrophilic substitution reactions can be seen with benzene.

Electrophilic substitution of an electrophile to benzene, replacing a hydrogen atom.

Some key points- Nucleophiles:

A Nucleophile is a reagent that brings an electron pair, or they are an electron-rich reagent. Molecules possessing pi bonds or atoms or molecules having free electron pairs act as nucleophiles. Nucleophiles generally fall into three main categories. 

• Lone Pair: If we travel across the periodic table from right to left, Nucleophilicity increases, which implies nucleophilicity increases as basicity increases. 

Nucleophilicity also moves up while going down the periodic table. So by comparing the halides we get this hierarchy: I-  > Br- > Cl-  > -. But there are exceptions too. 

In polar protic solvents, nucleophilicity takes an upward trajectory with polarisability since hydrogen bonds form a shell around the less polarizable atoms to decrease their nucleophilicity. However, in the case of polar aprotic solvents, this doesn’t happen.

One of the challenges of understanding nucleophilicity is because it is highly dependent on the electrophile.

• π bonds: They are often compounded by having an atom with π bonds.

π bonds donate a pair of electrons, but in this case, the pair is shared between two atoms. This happens in double bonds and the case of triple bonds (alkynes: R – CH = CH2, Benzene, etc.) as well as enols, aromatics, and enolates. 

The key factor determining the nucleophilicity of π bonds is the presence of donor groups or atoms that can share electrons with the double bond to help stabilize it (after it has donated its pair of electrons to the electrophile). When a double bond reacts with an electrophile, the result is a carbocation.

• Sigma bonds: The pair of electrons in sigma bonds can, on certain occasions, also act as nucleophiles. For a sigma bond to act as a nucleophile, it has to be broken. 

While Sigma bonds are formed due to head-to-head overlapping of atomic orbitals, pi bonds are formed by the lateral overlap of two atomic orbitals.

Identifying a good nucleophile 

For this, we can use the concept of electronegativity. For example, let us compare NH3 with H2O. 

Incidentally, both the molecules by dint of their lone pairs can behave like a nucleophile by forming bonds, but the question remains, which one is better of the two? Let us find out!

Note since NH3 has a lone pair on nitrogen, it is automatically less electronegative than oxygen. H2O, on the other hand, is more electronegative than the former since it has two lone pairs of oxygen; more tightly.

Since nitrogen holds its lone pair more loosely than oxygen, it has a better chance of forming new bonds with another molecule by donating the electron pair.

Conclusion: Ammonia (NH3), therefore, is a better nucleophile than water (H2O). Simple, isn’t it?

Typical cases of Nucleophile and Electrophile:

Typical electrophiles normally have good leaving groups like sulfonate esters or halides. They may also have polarizable C=O bonds like aldehydes, ketones, or carboxylic – acid – derivatives.

On the other hand, Nucleophiles can be some smaller negatively charged species or molecules with N, P, or S atoms.

1. What is the difference between an Electrophile and a Nucleophile? 

Ans. Electrophiles and nucleophiles can be considered as the derivatives of atoms or molecules.

As a corollary to that, chemical reactions between organic and inorganic chemical species mostly happen through electrophiles and nucleophiles. 

Therefore, the main difference lies in the acceptance or donation of electron pairs. E.g., Electrophiles are atoms or molecules which can accept electron pairs, while Nucleophiles are atoms or molecules that can donate electron pairs. 

A more detailed view on the differences between Electrophiles and Nucleophiles are tabulated below:

1. Why are Electrophiles called Lewis acids? 

Ans: Electrophiles are called Lewis acids because of their property to accept electrons. 

An electrophile is formed when an atom or a molecule has secondary electrons and obeys the octet rule or has a positive charge that is required to be neutralized to become stable.

For example, Hydronium ion (H3O ) is an electrophile — it has a positive charge, and the hydrogen atom has free space for incoming electrons. Therefore, it can accept electron pairs from Lewis bases such as –OH to form H3O molecules.

Fig: Alkene and Bromine Addition

In organic chemistry, it is observed that electrophiles undergo addition and substitution reactions. Eg. The addition of halogens to alkenes takes place via electrophilic addition reactions.

1. Why are Nucleophiles called Lewis’ Base?

Ans: We know nucleophile is an atom or a molecule that can donate electron pairs, and due to this function, it is also called Lewis base. Simply put: Nucleophiles can donate electrons to electrophiles.

1. What is Nucleophilicity?

Ans: Nucleophilicity indicates the strength of a particular nucleophile, and it depends on many factors such as charge, polarisability, basicity, etc. 

E.g. When the negative charge of the nucleophile is raised, the nucleophilicity also increases, which implies that nucleophiles bearing a high negative charge will act as excellent nucleophiles.

Conclusion

Since any organic reaction can be thought of as a nucleophile ‘attacking’ an electrophile to form a new bond, identifying nucleophiles and electrophiles is INCREDIBLY important in predicting the outcomes of organic reactions. Inorganic chemistry and in inorganic chemistry, electrophiles and nucleophiles play a significant primary role.