Nukleophils

Nucleophiles are chemical species that donate an electron pair to an electrophilic center, forming a chemical bond. They are Lewis bases by definition and are attracted to positively polarized atoms in substrates such as alkyl halides or carbonyl groups. Nucleophiles can be anions, such as hydroxide (OH−), alkoxides (RO−), cyanide (CN−), halides (Cl−, Br−), and thiolates (RS−), or neutral molecules with lone pairs or π electrons, such as water, alcohols, ammonia, and amines.

In reactions, nucleophiles attack electrophilic centers via bimolecular nucleophilic substitution (SN2) or unimolecular pathways (SN1), and by nucleophilic addition to carbonyls. SN2 involves backside attack and typically leads to inversion at a stereocenter; SN1 involves carbocation formation and is more influenced by the substrate than by the nucleophile.

Factors affecting nucleophilicity include charge, polarizability, solvent, and steric hindrance. Higher charge and greater polarizability generally increase nucleophilicity. In protic solvents, nucleophilicity decreases down a period due to solvation of small anions; in polar aprotic solvents, nucleophilicity trends align more closely with basicity (for example, F− often emerges as a stronger nucleophile than I− in such media).

Hard and soft nucleophiles, described by HSAB theory, categorize nucleophiles by their tendency to interact with hard or soft electrophiles. Hard nucleophiles prefer hard electrophiles, while soft nucleophiles prefer soft electrophiles.

Nucleophilicity is a kinetic property and is distinct from basicity. It is a central concept in organic synthesis, underpinning reactions such as the Williamson ether synthesis, acyl substitutions, and nucleophilic additions to carbonyl compounds.