Electrophiles

An electrophile is a chemical species that seeks electrons and accepts an electron pair from a nucleophile to form a covalent bond. In Lewis theory, electrophiles are electron-pair acceptors. They are typically electron-poor or positively charged and include ions such as protons (H+), metal cations, and carbocations, as well as neutral, polar molecules with an electron-deficient center such as carbonyl carbon in aldehydes and ketones, iminium ions, and activated alkyl halides.

Reactivity of electrophiles is influenced by charge, polarizability, leaving-group ability, and the development of negative charge on the nucleophile side. According to hard-soft acid-base (HSAB) theory, hard electrophiles prefer hard nucleophiles and soft electrophiles prefer soft nucleophiles. Common electrophiles encountered in organic chemistry include H+, NO2+, AlCl3, BF3, carbocations, iminium ions, and activated alkenes or carbonyl compounds that undergo nucleophilic addition. Michael acceptors (such as enones) are classic examples where a carbon-carbon double bond acts as an electrophile toward nucleophiles.

In reactions, electrophiles are attacked by nucleophiles. They enable many transformations, including SN1 and SN2 substitutions, electrophilic aromatic substitution, and Friedel–Crafts acylation or alkylation. Electrophiles also drive polymerization in certain ionic or cationic processes and are central to many catalytic cycles that generate reactive intermediates.

In biochemistry and chemical biology, electrophiles can covalently modify nucleophilic sites in biomolecules, such as amino groups or thiols, using alkylating agents, sulfonyl fluorides, or NHS esters. Because electrophilicity governs selectivity and rate, reaction conditions, solvent, and temperature are important considerations for both laboratory synthesis and practical applications.