electrophilic

Electrophilic is an adjective used in chemistry to describe reagents or molecular sites that are electron-poor and seek electron density from other molecules. In most contexts, electrophiles are electron-pair acceptors and react with nucleophiles, which donate electrons. Electrophilicity is the tendency of a species to accept electrons and can be discussed qualitatively or quantified with theoretical indices in physical organic chemistry.

Common electrophiles include cationic species such as carbocations and protons (H+); and Lewis acids like aluminum chloride (AlCl3), boron trifluoride (BF3), and iron(III) chloride (FeCl3). Halogen reagents such as Br2 and I2 under polar conditions also act as electrophiles. Strong electrophiles such as the nitronium ion NO2+ are central to nitration reactions. In many substrates, the carbonyl carbon of aldehydes and ketones, as well as acyl chlorides and sulfonyl derivatives, presents an electrophilic center that accepts nucleophiles in addition or substitution reactions. In aromatic chemistry, electrophiles such as NO2+ and SO3 drive electrophilic aromatic substitution.

The concept underpins mechanisms such as electrophilic addition to alkenes, nitration, sulfonation, and Friedel-Crafts alkylation or acylation. The hard–soft acid–base (HSAB) theory classifies electrophiles as hard or soft, guiding which nucleophiles they react with most readily. Quantitative measures, such as the electrophilicity index ω, describe a species’ tendency to accept electrons. In practice, many electrophiles are generated in situ by acidic or catalytic conditions, making their identity dependent on reaction context.