Nukleophil

Nukleophil (nucleophile) is a species that donates an electron pair to form a chemical bond. In organic chemistry, nucleophiles attack electron-deficient carbons in substrates such as alkyl halides or carbonyl compounds, or they can attack positively charged centers in other reagents. Most nucleophiles are negatively charged ions or neutral molecules bearing lone pairs or π electrons.

Factors: The strength of nucleophilicity depends on charge, polarizability, and steric hindrance; anionic nucleophiles are generally more reactive than neutral ones, but reactivity is modulated by solvent and temperature. Polarizability, especially for sulfur and halogens, enhances nucleophilicity toward soft electrophiles. Nucleophilicity is related but not identical to basicity; a species can be a strong nucleophile but only a weak base, depending on the substrate and solvent.

Solvent effects: In polar protic solvents, solvation diminishes nucleophilicity of small, hard anions, making larger, more polarizable nucleophiles (e.g., I-, RS-) more nucleophilic in such media. In polar aprotic solvents, nucleophilicity correlates more closely with basicity, giving F-, HO- strong nucleophiles.

Common nucleophiles include hydroxide (OH-), alkoxides (RO-), cyanide (CN-), thiolates (RS-), amines (R3N), and halide ions (X-). They participate in SN2 reactions with primary and secondary substrates, SN1 pathways with stable carbocations under suitable conditions, and additions to carbonyl groups or to multiple bonds. In synthesis, nucleophiles are chosen for reactivity, selectivity, and compatibility with leaving groups.

Hard and soft nucleophiles: The hard/soft acid-base (HSAB) concept helps rationalize selectivity toward different electrophiles; hard nucleophiles favor ionic centers, soft nucleophiles favor covalent, polarizable centers. Nucleophilicity is a central concept in planning organic synthesis and related fields.