In chemistry, a nucleophile (from nucleus and the Greek φιλος, phile, love) is a reagent that forms a chemical bond to its reaction partner (the electrophile) by donating both bonding electrons.  Because nucleophiles donate electrons, they are by definition Lewis bases (see acid-base reaction theories). All molecules or ions with a free pair of electrons can act as nucleophiles. Neutral nucleophilic reactions with solvents such as alcohols and water are named solvolysis.
A nucleophile is an electron-rich chemical reactant that is attracted to electron-deficient compounds. Examples of nucleophiles are anions such as Cl−, or a compound with a lone pair of electrons such as NH3 (ammonia).
Nucleophiles may take part in nucleophilic substitution, whereby a nucleophile becomes attracted to a full or partial positive charge.
Nucleophilic is an adjective that describes the affinity of a nucleophile to the nuclei, whereas nucleophilicity or nucleophile strength refers to the nucleophilic character. Nucleophilicity is often used to compare an atom's relative affinity to another's.
In general, in a row across the periodic table, the more basic the ion (the higher the pKa of the conjugate acid) the more reactive it is as a nucleophile. In a given group, polarizability is more important in the determination of the nucleophilicity: The easier it is to distort the electron cloud around an atom or molecule the more readily it will react; e.g., the iodide ion (I−) is more nucleophilic than the fluoride ion (F−).
An ambident nucleophile is one that can attack from two or more places, resulting in two or more products. For example, the thiocyanate ion (SCN−) may attack from either the S or the N. For this reason, the SN2 reaction of an alkyl halide with SCN− often leads to a mixture of RSCN (an alkyl thiocyanate) and RNCS (an alkyl isothiocyanate). Similar considerations apply in the Kolbe nitrile synthesis.
The terms nucleophile and electrophile were introduced by Christopher Kelk Ingold in 1929, replacing the terms cationoid and anionoid proposed earlier by A. J. Lapworth in 1925.
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