Nucleophile

In the field of chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. The term comes from the word nucleus and the Greek word *philos*, meaning friend. This name describes the strong affinity these species have for positively charged atomic nuclei. Because nucleophiles donate electrons, they are also classified as Lewis bases. Any molecule or ion possessing a free pair of electrons or at least one pi bond can act as a nucleophile. These particles are essential for driving many chemical transformations in both organic and inorganic chemistry.

Nucleophiles participate in several types of chemical processes. In nucleophilic substitution, a nucleophile is attracted to a full or partial positive charge and replaces another group. A common example is the SN2 reaction, which involves a backside attack. In this mechanism, the nucleophile attacks the target carbon atom from the side exactly opposite the leaving group.

This specific movement causes an inversion of the configuration of the electrophile. If the molecule is chiral, its absolute configuration will be flipped. Another major process is nucleophilic addition, where the nucleophile adds to a molecule to form a new structure.

Nucleophiles can be categorized by the specific atoms they use to bond. Oxygen nucleophiles include water (H2O), hydroxide ions, alcohols, and carboxylate anions. Sulfur nucleophiles, such as hydrogen sulfide and thiols, are often very strong. Sulfur is highly nucleophilic because its large size makes it readily polarizable. Nitrogen nucleophiles include ammonia, amines, and azides. Some species are even called ambident nucleophiles. These can attack from two or more different locations, which often results in a mixture of different chemical products.

The history of these terms traces back to the early 20th century. In 1925, A. J. Lapworth proposed the terms anionoid and cationoid. Later, in 1933, Christopher Kelk Ingold introduced the terms nucleophile and electrophile to replace them. Since then, scientists have sought to quantify nucleophilicity, which is a kinetic property. This refers to the rate at which a chemical reaction occurs. This is distinct from basicity, which is a thermodynamic property relating to an equilibrium state.

To compare the strength of different nucleophiles, scientists use several mathematical equations. The Swain–Scott equation, derived in 1953, uses a nucleophilic constant ($n$) and a substrate constant ($s$). For example, in a reaction with benzyl chloride, the azide anion reacts 3000 times faster than water. The Ritchie equation, developed in 1972, provides a different way to measure reactivity. It uses a parameter called $N+$ to show that some nucleophiles react with similar relative reactivity regardless of the electrophile.

In methanol, the thiophenol anion has a high $N+$ value of 10.7.

A more complex model is the Mayr–Patz equation from 1994. This equation relates the reaction rate constant ($k$) to a nucleophilicity parameter ($N$) and an electrophilicity parameter ($E$).

Researchers use these values to predict how substances will behave in different environments. For instance, the Mayr equation can also describe SN2 reactions. This helps chemists understand how nucleophilicity changes across different types of molecular centers. An internet database maintained by the Mayr group helps scientists access these specific reactivity parameters.

Some nucleophiles are exceptionally powerful, earning the name "supernucleophiles." While most metal centers are electrophilic, certain metals in low oxidation states can be incredibly strong nucleophiles. For example, the Co(I) form of vitamin B12 is approximately $10^7$ times more nucleophilic than the iodide ion.

Other examples include certain carbonyl metalate anions. Understanding these extreme cases helps scientists master complex chemical reactions. These principles connect to many broader fields, including the study of how medicines interact with the body and how new materials are synthesized in a lab.