Nukleofilnost

Nukleofilnost refers to the ability of a chemical species to donate an electron pair to form a new covalent bond. This species is known as a nucleophile, which literally means "nucleus loving" because it is attracted to positive charges, typically found in atomic nuclei. Nucleophiles are typically electron-rich, possessing lone pairs of electrons or pi bonds. Common nucleophiles include anions such as hydroxide (OH-) and halide ions (Cl-, Br-, I-), as well as neutral molecules with lone pairs like ammonia (NH3) and water (H2O).

The strength of a nucleophile is influenced by several factors. Steric hindrance plays a role; bulky nucleophiles are generally weaker because they have difficulty accessing the electrophilic center. Solvent effects are also significant. Polar protic solvents, which have hydrogen atoms bonded to electronegative atoms, can solvate and stabilize nucleophiles through hydrogen bonding, thus reducing their reactivity. Polar aprotic solvents, lacking such hydrogen atoms, do not solvate nucleophiles as strongly, often leading to increased nucleophilicity.

Factors like charge and electronegativity also impact nucleophilicity. Negatively charged species are generally stronger nucleophiles than their neutral counterparts because they have a higher electron density. Within a group in the periodic table, nucleophilicity generally increases down the group, as larger atoms are more polarizable and their electron clouds are more diffuse, making them more willing to donate electrons. Within a period, nucleophilicity generally decreases across the period as electronegativity increases, meaning atoms hold onto their electrons more tightly.

Nukleofilnost is a fundamental concept in organic chemistry, particularly important in reactions like nucleophilic substitution and addition, where a nucleophile attacks an electron-deficient atom.