Nukleophilie

Nukleophilie, or nucleophilicity, describes the tendency of a chemical species to donate an electron pair to an electrophilic center, thereby forming a covalent bond. A nucleophile is typically an anion or a neutral molecule that possesses lone pairs or a pi bond capable of electron donation.

Although related to basicity, nucleophilicity is not identical to basic strength. In a given solvent, stronger bases are often more nucleophilic, but solvent effects and substrate structure strongly modulate reactivity. Nucleophilicity governs the rate of many substitution (SN) and addition reactions and is especially important in SN2 mechanisms, where a nucleophile directly attacks an electrophilic carbon.

Several factors influence nucleophilicity: charge (negative species are generally more nucleophilic), polarizability (more easily distorted electron clouds are better at forming bonds with soft electrophiles), steric hindrance (bulky nucleophiles are less reactive), and solvent. Solvent effects are particularly important: protic solvents solvate anions via hydrogen bonding, diminishing nucleophilicity, whereas polar aprotic solvents solvate cations but leave anions relatively unsolvated, enhancing nucleophilicity.

Trends also depend on the nucleophile and solvent. In protic media, nucleophilicity tends to increase down a group (I− is more nucleophilic than Br−, Cl−, F−). In polar aprotic media, the order often reverses, with smaller, less polarizable ions like F− showing high nucleophilicity. Hard nucleophiles prefer hard electrophiles; soft nucleophiles favor soft electrophiles, in line with HSAB theory.

Common nucleophiles include hydroxide, alkoxides, cyanide, thiolates, amines, and halide ions; organometallic and enolate species also act as nucleophiles in appropriate contexts. Nucleophilicity is commonly assessed by reaction rates, with rate laws depending on [Nu−] and substrate.