In chemistry, the term “nucleophilic” describes a chemical species attracted to positive charge. The word derives from “nucleus” and the Greek word “philos,” meaning “loving,” so a nucleophile is a “nucleus-lover.” A nucleophile is a reactant that provides a pair of electrons to form a new covalent bond with another atom. Because they donate electrons, all nucleophiles are also classified as Lewis bases.
Identifying a Nucleophile
A chemical species is identified as a nucleophile by its electronic structure. The defining feature is a region of high electron density, which manifests as either a lone pair of electrons or a formal negative charge. This makes the species ready to interact with an area of electron deficiency.
A common example of a neutral nucleophile is the water molecule (H₂O). The oxygen atom in water possesses two lone pairs of non-bonding electrons. Although the molecule is neutral, these lone pairs represent a localized source of electron density that allows water to act as a nucleophile.
A more potent type of nucleophile carries a net negative charge. The hydroxide ion (OH⁻), for instance, is a strong nucleophile that is formed when a water molecule loses a proton. This process leaves the oxygen atom with three lone pairs and a formal negative charge, making it significantly more electron-rich. This principle applies broadly, with other examples including anions like the chloride ion (Cl⁻) and neutral molecules like ammonia (NH₃).
Factors Influencing Nucleophilic Strength
The effectiveness, or strength, of a nucleophile is influenced by several factors. Nucleophilicity is a measure of how readily a species donates its electron pair to form a bond. This strength is determined by electrical charge, electronegativity, and the molecule’s physical shape.
Charge is a direct influence on nucleophilic strength. A species with a negative charge is a more powerful nucleophile than its neutral counterpart. For example, the negatively charged hydroxide ion (OH⁻) is a stronger nucleophile than a neutral water molecule (H₂O) because its excess electron density makes it more reactive.
Electronegativity also plays a part when comparing atoms within the same row of the periodic table. Electronegativity is the measure of an atom’s ability to hold onto electrons. An atom with lower electronegativity holds its electrons less tightly, so as electronegativity decreases across a period, nucleophilicity increases.
The physical structure of a molecule can impact its ability to act as a nucleophile through steric hindrance. If the atom with the donatable electron pair is surrounded by large, bulky groups, it may be physically blocked from approaching another molecule. This obstruction reduces the nucleophile’s effectiveness, meaning a bulkier nucleophile will have a slower reaction rate than a smaller, less hindered one.
Nucleophiles in Chemical Reactions
In a chemical reaction, a nucleophile seeks a partner known as an electrophile. The term electrophile means “electron-loving” and describes an electron-deficient species that can accept an electron pair. Electrophiles have a full or partial positive charge, making them attractive targets for the electron-rich nucleophile.
The interaction between these two species is described as a “nucleophilic attack.” During this process, the nucleophile donates its electron pair to the electrophilic center, forming a new covalent bond between them. This action is the basis for many chemical transformations, particularly in organic chemistry.
This electron flow from the nucleophile to the electrophile drives many reaction types. For instance, in nucleophilic substitution reactions, the nucleophile attacks an electrophile and displaces another group, known as a leaving group. In nucleophilic addition reactions, the nucleophile adds to an electrophile, breaking a double or triple bond in the process.
Distinguishing Nucleophilicity from Basicity
The concepts of nucleophilicity and basicity are closely related, as both describe electron-pair donors. Many strong nucleophiles are also strong bases, which can be a source of confusion. However, the terms describe two distinct chemical functions, with the primary difference being what the electron pair attacks.
Basicity is a thermodynamic concept describing a species’ ability to donate its electron pair to a proton (H⁺). It is measured by an equilibrium constant, reflecting the position of an acid-base equilibrium. A strong base readily accepts a proton.
Nucleophilicity, in contrast, is a kinetic concept. It describes how fast a species donates its electron pair to an electrophilic atom, which in organic chemistry is a carbon atom. Nucleophilicity is about the reaction rate, not the final equilibrium.
While the trends can overlap, they can also diverge. Steric hindrance provides a clear example. A large, bulky molecule might be a strong base because it can still access a small, exposed proton. However, that same molecule could be a weak nucleophile because it is physically blocked from attacking a more crowded electrophilic carbon atom.