Nucleophilic

Nucleophilic is an adjective describing a species that tends to donate an electron pair to an electrophilic center. Nucleophilicity, the rate at which a species reacts with electrophiles, is a kinetic property that often reflects basicity but is not identical to it. Nucleophiles are typically Lewis bases and can be anions (such as halides, hydroxide, or thiolates) or neutral molecules (such as ammonia or amines).

Nucleophiles attack positively polarized or electron-deficient centers in substrates, most commonly carbon atoms in alkyl halides (SN2 and related substitutions), carbonyl carbons in nucleophilic additions or acyl substitutions, and conjugated systems in Michael-type additions. The strength of a nucleophile influences reaction rate and outcome, but the reaction medium, temperature, and substrate structure also play crucial roles.

Factors affecting nucleophilicity include charge, polarizability, electronegativity, and solvation. Anions are generally more nucleophilic than neutral species. More polarizable nucleophiles (for example iodine or sulfur-containing species) can be more effective in many reactions. Solvent effects are significant: in protic solvents, nucleophilicity often increases down a group (I− > Br− > Cl− > F−) because larger ions are less strongly solvated; in polar aprotic solvents, nucleophilicity more closely tracks basicity, and smaller ions can be stronger nucleophiles.

Hard/soft acid–base considerations (HSAB) also help predict matchups: hard nucleophiles prefer hard electrophiles, soft nucleophiles prefer soft electrophiles.

Overall, nucleophilicity is a central concept in organic and inorganic chemistry, guiding the design of substitution and addition reactions and influencing selectivity and mechanism.