Nucleophilic and Electrophilic Reagents

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.

REFERENCE

Cram, D., and G. Hammond. Organicheskaia khimiia. Moscow, 1964. (Translated from English.)

I. P. BELETSKAIA