Core Concepts
In this tutorial, you will learn what determines the nucleophilicity of a molecule. This will be done by familiarizing yourself with various concepts that decide whether a nucleophile is strong or weak. Additionally, you will learn a list of strong and weak nucleophiles.
Topics Covered in Other Articles
Vocabulary
- Anion: a negatively charged ion
- Covalent Bond: the sharing of electrons between two nonmetals or metal and metalloid
- Electron: the negatively charged subatomic particle located outside the nucleus
- Electronegativity: the measure of electron attraction of a molecule
- Nucleophilicity: the ability of a compound to act as a nucleophile
- Solvent: the main liquid in a solution; usually dissolves a solute
- Polar Aprotic Solvent: solvent lacking O-H or N-H bonds
- Polar Protic Solvent: solvent containing O-H or N-H bonds
- Steric Hindrance: the effect that the physical size and structure of a molecule has on its reactivity
Introduction to Nucleophiles
A nucleophile is an electron-rich molecule that can form covalent bonds by donating electrons to electron-poor sites. Many molecules can act as a nucleophile in a chemical reaction, though some are stronger than others. Learn more about what constitutes a strong or weak nucleophile below!
What Makes a Strong Nucleophile?
Charge of Nucleophile
Nucleophilicity depends on a molecule’s ability to be used as an electron source for other molecules. Since an anion has extra electrons creating a negative charge, it is a stronger nucleophile than a neutral molecule. In contrast, a cation is not an electron source and therefore cannot be an adequate nucleophile.
Electronegativity of Nucleophile
The electronegativity and strength of a nucleophile have an inverse relationship. For instance, if electronegativity increases, it is less likely that the molecule will act as an electron source, so the nucleophilicity will decrease. A quick way to determine the strength of a nucleophile is to use the periodic table. Electronegativity increases across the rows and decreases down the columns of the periodic table. Meanwhile, nucleophilicity decreases across the rows and increases down the columns of the periodic table.
Steric Hindrance of Nucleophile
The more crowded a potential bond site is, the less likely it is to efficiently share its electrons. This is known as steric hindrance and it increases when the number of bonds adjacent to the nucleophilic site increases. Ultimately, this decreases a molecule’s ability to act as a strong nucleophile.
Read more about steric hindrance here!
The Solvent Present
In polar protic solvents, the strength of a nucleophile increases down the periodic table. Conversely, in polar aprotic solvents, the strength decreases down the periodic table.
If the solvent is a polar protic solvent, it is able to use the nucleophile to create hydrogen bonds. These bonds will physically surround all the nucleophiles, forming a barrier and decreasing the likelihood of proper electron donations. Furthermore, as you go down the periodic table, the molecules are less likely to interact with the solvent and will be stronger nucleophiles.
On the other hand, a polar aprotic solvent does the opposite because it is incapable of performing hydrogen bonding. Due to this, nucleophiles can more easily bond to the desired molecule, which reverses the protic solvent periodic table trend.
Examples of Nucleophiles
Strong Nucleophiles
- Halides – Br–, I–, Cl–, F–
- Hydroxide Ion – HO–
- Nitrile Ion – CN–
- Sulfide Ion – RS–, HS–
Weak Nucleophiles
- Water – H2O
- Alcohols – CH3OH
- Carboxylic Acids – RCOOH
- Bulky Bases – tBuOK, DBU