Nukleofiilicity

Nukleofiilicity, also known as nucleophilicity, is a measure of how readily a species can donate an electron pair to an electrophilic center, thereby forming a chemical bond. It describes the reactivity of nucleophiles in reactions such as substitutions, additions, and condensations. In many contexts, nucleophilicity and basicity are related but not identical; a species can be a strong base yet a relatively weak nucleophile, depending on solvation and the reaction mechanism.

Nukleofiilicity depends on several factors. Charge increases nucleophilic strength because anionic species typically have higher electron density than neutral ones. The nature of the electron pair and the atom bearing it also matter: more polarizable or readily available lone pairs enhance nucleophilicity. Steric hindrance around the nucleophilic center reduces nucleophilicity by restricting approach to the electrophile. Solvent effects are crucial: in protic solvents, small, highly solvated anions (like F−) are heavily hindered, making nucleophilicity increase down a group (I− > Br− > Cl− > F−). In polar aprotic solvents, nucleophilicity often correlates with basicity and can rise up the periodic table (F− becomes highly nucleophilic). Polarizability also plays a role, with soft nucleophiles favoring soft electrophiles under HSAB principles.

Common nucleophiles include halide ions (Cl−, Br−, I−), alkoxides, amines, thiolates, cyanide, azide, and organometallic reagents. Nucleophilicity governs reaction pathways in SN2 and SNAr mechanisms and influences regio- and stereochemistry. Understanding nucleophilicity helps predict reactivity across solvents, substrates, and conditions, alongside considerations of solvent, temperature, and competing processes.

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