Electrophilicity

Electrophilicity is the tendency of a chemical species to accept electrons from a nucleophile during a chemical reaction. In practice, electrophiles are electron-poor or positively polarized species, such as Lewis acids, carbocations, polarized carbonyls, or activated alkenes. The concept is central to reaction mechanisms, helping to explain reaction rates and selectivity in many organic transformations.

Qualitative factors that increase electrophilicity include substitution patterns and resonance that lower the energy of the species’ lowest unoccupied molecular orbital (LUMO) or increase the driving force for electron uptake. Environmental factors such as solvent polarity and temperature can also affect the effective electrophilicity by stabilizing charge development in the transition state.

Quantitatively, electrophilicity is often described by the global electrophilicity index ω, defined in conceptual density functional theory as ω = μ^2/(2η), where μ is the electronic chemical potential and η is chemical hardness. These quantities can be estimated from ionization potentials and electron affinities or from HOMO/LUMO energies. A larger ω indicates a stronger electrophile. Local electrophilicity indices, derived from the Fukui function, can map the most receptive sites within a molecule.

Electrophilicity overlaps with the concept of Lewis acidity but is not identical; electrophilicity emphasizes the tendency to accept electrons and is a useful descriptor across organic, inorganic, and organometallic chemistry. In practice, evaluating electrophilicity guides predictions of nucleophile choice and reaction rates, aiding synthesis planning and mechanism understanding.