nukleophilicity

Nucleophilicity is the tendency of a chemical species to donate an electron pair to an electrophile, thereby forming a bond. It is a kinetic property describing the rate of bond formation, and it is not identical to basicity, which concerns proton transfer equilibria. In many organic reactions, nucleophilicity governs the rate of nucleophilic substitution (SN) and addition steps.

Key determinants include charge and electron density, polarizability, solvent, and steric hindrance. More negative charge usually increases nucleophilicity, and higher electron density facilitates attack. Polarizable atoms (for example, I−, S-based nucleophiles) can be highly nucleophilic in certain solvents because their electron cloud adapts during bond formation.

Solvent effects are pronounced. In protic solvents, extensive hydrogen-bonding stabilizes anions, and nucleophilicity trends often increase down a group (I− > Br− > Cl− > F−) due to weaker solvation of larger anions. In polar aprotic solvents, solvation is reduced and nucleophilicity tends to track basicity more closely, often yielding the opposite trend (F− > Cl− > Br− > I−). Steric hindrance lowers nucleophilicity by impeding approach to the electrophilic center.

Hard-soft acid-base considerations explain preferences for different electrophiles. Hard nucleophiles prefer hard centers; soft nucleophiles react more readily with soft electrophiles. Common nucleophiles include halide ions, alkoxides, hydroxide, amines, and thiolates, and nucleophilicity generally increases with charge and decreases with steric bulk, though solvent can reverse or modify these trends.

Nucleophilicity is typically measured by reaction rate constants in SN2 or related processes and is context-dependent, varying with solvent, temperature, and substrate.