The term nucleophile literally translates to "nucleus loving." In chemistry, the nucleus of an atom contains positively charged protons. Therefore, a nucleophile is a molecule or ion that is attracted to regions of positive charge. To function as a nucleophile, a chemical species must possess at least one unshared pair of electrons, often called a lone pair, that it can use to establish a new covalent bond.
Many nucleophiles also carry a full negative electrical charge, which makes them even more highly reactive toward electron-deficient targets.
Nucleophiles are fundamental participants in many important organic reactions, most notably substitution and addition reactions. In a typical substitution reaction, a nucleophile attacks a carbon atom that is bonded to a different group, known as the leaving group. Because the leaving group pulls electron density away from the carbon, the carbon becomes slightly positive, drawing the nucleophile inward.
The nucleophile donates its electron pair to the carbon, forming a new bond and forcing the leaving group to detach and depart.
The strength, or "nucleophilicity," of a nucleophile depends on several different chemical factors. Generally, an ion with a negative charge is a stronger nucleophile than a similar neutral molecule. For instance, the hydroxide ion (OH<sup>-</sup>) is a much better nucleophile than a neutral molecule of water (H2O).
Additionally, larger atoms often make better nucleophiles because their outermost electrons are held less tightly by the nucleus, allowing the electron cloud to easily distort and reach out to form a bond. Understanding how nucleophiles behave is essential for predicting the outcome of complex organic synthesis processes.