What is a Nucleophile?

The basic question of this section will be “what is a nucleophile?” followed by a discussion of nucleophile strength. A nucleophile has at least one lone pair of electrons (often even more) that it uses to react with other atoms and molecules. Because nucleophiles tend to be electron-rich, it’s fairly common for them to have a negative charge.

 

What is a nucleophile? In this image, 2 examples are shown. Note that both have lone pairs of electrons on the nucleophilic atom.

 

Nucleophile Strength

We can assess the nucleophile stength by looking at (1) its polarizability and (2) its general stability. We will examine each of these two factors more closely.

 

1. Polarizability

Polarizability is the ability of an atom’s electron cloud to change shape to better attack another atom/molecule. This can therefore be viewed as the flexibility of the atom to distort its electron cloud. Critically, the more polarizable (or “flexible”) an atom or molecule is, the greater the nucleophile strength.

Polarizability is directly related to the size of the atom: the larger the atom, the more polarizable it is. This is because larger atoms have their outer valence electrons further away from the central nucleus, so these electrons experience more freedom to roam around and change shape. Polarizability therefore increases as you move down the periodic table.

Furthermore, polarizability is the most important factor in determining the strength of a nucleophile. Therefore, it is the first factor to assess in these types of questions.

As polarizability increases, nucleophile strength increases as well.

 

2. The general stability of the molecule

Stability is used as a tiebreaker when polarizability alone doesn’t give enough information on the nucleophilicity of an atom. To assess the general stability of the molecule, use the same method presented in the acid base chapter.

 

 

If a negative charge is on a highly electronegative atom or if the charge is spread out throughout the molecule, then the molecule is more stable. If a molecule is very stable, then it will be a weak nucleophile as it won’t want to react due to its high stability.

Furthermore, if the nucleophile has a negative charge, it is a stronger nucleophile than the same nucleophile in neutral form. This is because a neutral molecule is more stable than a charged molecule. As a result, the neutral molecule is less reactive (making it a weaker nucleophile), while the less stable charged molecule is more reactive (and therefore a better nucleophile).

Note that this is not as high priority in determining the strength of a nucleophile as polarizability and should only be used as a tiebreaker if polarizability alone doesn’t give enough information to determine the strength of the nucleophile.

Example
Which of the following is the strongest nucleophile?

Picture36

Answer
Molecule 1

There are two factors we must look at to assess the strength of a nucleophile: the polarizability and the stability.

First, let’s look at polarizability as this is the most important factor in determining the strength of a nucleophile. Molecule 1 and 2 are more polarizable than molecule 3 because sulfur is more polarizable than oxygen. Therefore, we can eliminate molecule 3.

We now look at the stability of each of the 2 molecules as the tie breaker to determine which is the stronger nucleophile. Since molecule 1 has a negative charge while molecule 2 is neutral, molecule 1 is less stable making it the stronger nucleophile.

 

In summary, this section described what is a nucleophile and how to determine nucleophile strength.