What makes a Good Nucleophile?

What is a Nucleophile?

A nucleophile is an electron-rich species that donates electron pairs to an electron-deficient species and forms a covalent bond.

Ammonia, Water, Carbanions, and Cyanide ion are a few examples of nucleophiles.

Table of Content

• Nucleophile and Base
• What makes a Good Nucleophile?
• Charge
• Electronegativity
• Solvent
• Steric hindrance
• Frequently Asked Questions – FAQs

Nucleophile and Base

A nucleophile is an electron-rich species that donates electron pairs to an electron-deficient species and forms a covalent bond.

Ammonia, Water, Carbanions, and Cyanide ion are a few examples of nucleophiles.

To measure the nucleophilic strength, the reaction rate is calculated. The kinetically favoured reaction has a good nucleophile.

A base is an electron-rich species that donates electron pairs to a hydrogen atom and forms a covalent bond.

Nucleophilicity and Basicity are often correlated. The primary difference between a nucleophile and a base is that a nucleophile donates electron pair to an electron-deficient species except for hydrogen and forms a covalent bond. In contrast, a base donates electrons pairs to the hydrogen atom and forms a covalent bond.

Moreover, nucleophilic strength is calculated by estimating the rate of reaction. In contrast, a base strength is calculated by estimating the value of pKb. Further, a nucleophile favours a substitution reaction, whereas a base favours an elimination reaction.

What makes a Good Nucleophile?

The four primary factors that affect the nucleophilicity of a nucleophile are;

• Negative Charge
• Electronegativity
• Solvent
• Steric Hindrance

Negative Charge

With an increase in the number of electrons, the ability of a nucleophile to donate electrons pairs increases. The more the negative charge, the more quickly a nucleophile will lose its electron pair to acquire stability and form a bond. Thus, better will be nucleophiles.

Nucleophile ∝ Negative Charge

A conjugate base will always be a better nucleophile. For example, OH^– is a better nucleophile than H₂O.

Electronegativity

Electronegativity increases as we move left to the right in a period. Electronegativity is the tendency of an atom to attract shared pairs of electrons. Thus, the ability to donate electrons pairs decreases with increasing electronegativity.

Thus, the lesser the electronegativity better would be a nucleophile.

Nucleophile ∝ 1 / Electronegativity

Solvent

Solvent medium plays a crucial role in affecting nucleophilicity. A polar protic solvent forms hydrogen bonding with the nucleophile. Thus, decreasing the activity of the nucleophile.

Nucleophile ∝ 1 / Polar Protic Solvent

The solvent’s ability to partake in hydrogen bonding decreases as we go down in a group. Fluoride forms the strongest hydrogen bonds, while iodide forms the weakest. Thus lone pairs of iodide would be more accessible than fluoride. Hence, iodide would be a better nucleophile than fluoride in a polar protic solvent.

In contrast, the polar aprotic solvent doesn’t form hydrogen bonding with the nucleophile. Thus, it doesn’t affect the activity of the nucleophile.

Nucleophile ∝ Polar Aprotic Solvent

Under polar aprotic solvent, fluoride would be the most unstable halide, thus will lose electrons quickly, forming a covalent bond. Hence, fluoride would be a better nucleophile than iodide in a polar aprotic solvent.

Steric Hindrance

The bulkier the nucleophile, the slower the reaction would proceed. Thus, the poor the nucleophile will be.

Nucleophile ∝ 1 / Steric Hindrance

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