Nucleophilic and Electrophilic Reagents

reagents that participate in substitution reactions (seeSUBSTITUTION REACTIONS).

Nucleophilic reagents, or nucleophiles, replace the leaving group of a molecule and provide a pair of electrons for the formation of a new bond. The leaving group departs with the pair of electrons that had formed the old bond. Such reactions are called nucleophilic substitutions. In the following examples of nucleophilic substitution, Y is the nucleophile, R is an organic radical, and X is the leaving group:

Y:^– + RX → RY + X^–

Y: + RX → RY⁺ + X^–

Nucleophilic reagents comprise negatively charged ions, including OH^–, CN^–, NO₂^–, OR^–, RS^–, NH₂^–, RCO₂^–, and halogen ions (designated Hal^–); neutral molecules with a free pair of electrons, for example, H₂O, NH₃, R₃N, R₂S, R₃P, ROH, and RCO₂H; and those organometallic compounds (designated R—Me), that are capable of forming carbanions—that is, those in which a bond between a carbon atom and the metal is sufficiently polarized. Nucleophilic substitution is characteristic mainly of aliphatic compounds. Examples of nucleophilic substitution include hydrolysis, with the nucleophiles OH^– and H₂O; alcoholysis, in which the nucleophiles are RO^– and ROH; acidolysis, with the nucleophiles RCOO^–; and RCOOH; amination, involving such nucleophiles as NH₂^–, NH₃, and RNH₂; and cyanation, with the nucleophile CN^–.

Electrophilic reagents, or electrophiles, replace the leaving group of a molecule and act as electron-pair acceptors in the formation of a new bond. The leaving group departs as a positively charged species. Electrophilic reagents include positively charged ions, for example, H⁺ and NO₂⁺; neutral molecules with an electron deficiency, for example, SO₃; and highly polarized molecules, for example CH₃CO₂^–Br⁺. Such polarization is achieved efficiently by complexing with Lewis acids, for example, Hal⁺—Hal^–·A, R⁺—CL^–A, and RCO⁺—Cl^–·A, where A = AlCl₃, SbCl₅, or BF₃. Substitution reactions that involve electrophiles are called electrophilic substitutions. These include the most important reactions of aromatic hydrocarbons, for example, nitration, halogenation, sulfonation, and Friedel-Crafts alkylation. In the following general formula for electrophilic substitution, E⁺ = Hal⁺, NO₂⁺, RCO⁺, R⁺, or some other electrophile.

Under certain conditions, nucleophilic substitution can occur in aromatic compounds, and electrophilic substitution can occur in aliphatic compounds—usually organometallic aliphatic compounds.