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Chemical species that donates an electron pair

In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are Lewis bases. Nucleophilic describes the affinity of a nucleophile to bond with positively charged atomic nuclei. Nucleophilicity, sometimes referred to as nucleophile strength, refers to a substance's nucleophilic character and is often used to compare the affinity of atoms. Neutral nucleophilic reactions with solvents such as alcohols and water are named solvolysis. Nucleophiles may take part in nucleophilic substitution, whereby a nucleophile becomes attracted to a full or partial positive charge, and nucleophilic addition. Nucleophilicity is closely related to basicity. The difference between the two is, that basicity is a thermodynamic property (i. e. relates to an equilibrium state), but nucleophilicity is a kinetic property, which relates to rates of certain chemical reactions.

The terms nucleophile and electrophile were introduced by Christopher Kelk Ingold in 1933, replacing the terms anionoid and cationoid proposed earlier by A. J. Lapworth in 1925. The word nucleophile is derived from nucleus and the Greek word φιλος, philos, meaning friend.

In general, in a group 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. Within a series of nucleophiles with the same attacking element (e. g. oxygen), the order of nucleophilicity will follow basicity. Sulfur is in general a better nucleophile than oxygen.

Many schemes attempting to quantify relative nucleophilic strength have been devised. The following empirical data have been obtained by measuring reaction rates for many reactions involving many nucleophiles and electrophiles. Nucleophiles displaying the so called alpha effect are usually omitted in this type of treatment.

Carbon nucleophiles are often organometallic reagents such as those found in the Grignard reaction, Blaise reaction, Reformatsky reaction, and Barbier reaction or reactions involving organolithium reagents and acetylides. These reagents are often used to perform nucleophilic additions. Enols are also carbon nucleophiles. The formation of an enol is catalyzed by acid or base. Enols are ambident nucleophiles, but, in general, nucleophilic at the alpha carbon atom. Enols are commonly used in condensation reactions, including the Claisen condensation and the aldol condensation reactions.

Examples of oxygen nucleophiles are water (H2O), hydroxide anion, alcohols, alkoxide anions, hydrogen peroxide, and carboxylate anions. Nucleophilic attack does not take place during intermolecular hydrogen bonding.

Of sulfur nucleophiles, hydrogen sulfide and its salts, thiols (RSH), thiolate anions (RS−), anions of thiolcarboxylic acids (RC(O) S−), and anions of dithiocarbonates (RO C(S) S−) and dithiocarbamates (R2N C(S) S−) are used most often. In general, sulfur is very nucleophilic because of its large size, which makes it readily polarizable, and its lone pairs of electrons are readily accessible.

Nitrogen nucleophiles include ammonia, azide, amines, nitrites, hydroxylamine, hydrazine, carbazide, phenylhydrazine, semicarbazide, and amide.

Although metal centers (e. g., Li+, Zn2+, Sc3+, etc.) are most commonly cationic and electrophilic (Lewis acidic) in nature, certain metal centers (particularly ones in a low oxidation state and/or carrying a negative charge) are among the strongest recorded nucleophiles and are sometimes referred to as "supernucleophiles." For instance, using methyl iodide as the reference electrophile, Ph3Sn– is about 10000 times more nucleophilic than I–, while the Co(I) form of vitamin B12 (vitamin B12s) is about 107 times more nucleophilic. Other supernucleophilic metal centers include low oxidation state carbonyl metalate anions (e. g., CpFe(CO)2–).