By James Ashenhurst

Last updated: December 20th, 2019 |

Factors That Determine Whether A Species Is A Good Nucleophile

If you read the last post, you’ll recall that a nucleophile is a species that donates a pair of electrons to form a new covalent bond. Nucleophilicity is measured by comparing reaction rates; the faster the reaction, the better (or, “stronger”) the nucleophile.

Table of Contents

1. Reminder: Nucleophilicity Is Measured By Reaction Rate

When discussing nucleophilicity we’re specifically talking about donating a pair of electrons to an atom other than hydrogen (usually carbon). When a species is donating a pair of electrons to a hydrogen (more specifically, a proton, H+) we call it a base.

This post attempts to address one of the most vexing question to students of organic chemistry. What are the factors that make a good nucleophile?

For our purposes, there are at least four key factors contributing to nucleophilicity.

Charge Electronegativity Solvent Steric hindrance

The first two should hopefully be familiar from the discussion of what makes something a strong base. After all, basicity and nucleophilicity essentially describe the same phenomenon, except basicity concerns donation of lone pairs to hydrogen, and nucleophilicity concerns donations of lone pairs to all other atoms. It’s the third and fourth points where extra factors come into play.

2. The Role Of Charge: Nucleophilicity Increases As An Atom’s Electron Density Increases

Since a nucleophile is a species that is donating a pair of electrons, it’s reasonable to expect that its ability to donate electrons will increase as it becomes more electron rich, and decrease as it becomes more electron poor, right? So as electron density increases, so does nucleophilicity.

A handy rule to remember for this purpose is the following: the conjugate base is always a better nucleophile.

3. Electronegativity: Across The Periodic Table, Nucleophilicity Increases With Decreasing Electronegativity

Assuming an atom has a pair of electrons to donate, the ability of a species to donate that pair should be inversely proportional to how “tightly held” it is. The key factor for determining how “tightly held” an electron pair is bound is the familiar concept of electronegativity. Bottom line: as electronegativity increases, nucleophilicity decreases. Note: It’s important to restrict application of this trend to atoms in the same row of the periodic table; for instance, C N O F, or Si P S Cl. Going down the periodic table, another factor comes into play (next)

4. The Choice Of Solvent (Polar Protic vs. Polar Aprotic) Can Drastically Affect Nucleophilicity Trends

Nucleophilicity is not a property inherent to a given species; it can be affected by the medium it’s in (otherwise known as “the solvent”). [For an introduction to the different classes of solvents, click here]

A polar protic solvent can participate in hydrogen bonding with a nucleophile, creating a “shell” of solvent molecules around it like mobs of screaming teenage fans swarming the Beatles in 1962. In so doing, the nucleophile is considerably less reactive; everywhere it goes, its lone pairs of electrons are interacting with the electron-poor hydrogen atoms of the solvent.

The ability of nucleophiles to participate in hydrogen bonding decreases as we go down the periodic table. Hence fluoride is the strongest hydrogen bond acceptor, and iodide is the weakest. This means that the lone pairs of iodide ion will be considerably more “free” than those of fluoride, resulting in higher rates (and greater nucleophilicity).