Electrophilic Aromatic Substitution. Charge-Transfer Excited States and the Nature of the Activated Complex

Article abstract of DOI:10.1021/ja00414a034

Transient charge-transfer (CT) absorption bands are observed for various benzene derivatives interacting with typical electrophiles, such as the halogens and mercuric trifluoroacetate.The second-order rate constants k for the kinetics of the disappearance of these spectral bands coincide with the rate constants for electrophilic halogenation and mercuration of the aromatic ring.The relative reactivities (log k/k₀) of the arenes in electrophilic aromatic substitution are linearly related to the reactive CT transition energies Δhν_CT, using benzene as the reference arene.This remarkable correlation thus relates the transition state for electrophilic aromatic substitution to the CT excited state +E^->^* of the arene-electrophile pair.Such a direct relationship requires that the solvation energies remain constant for various aromatic cations, since the transition state is attained by an adiabatic process whereas the CT excitation involves a vertical (Franck-Condon) process.Indeed, independent measurements of the cyclic voltammetric peak potentials in acetonitrile and trifluoroacetic acid in comparison with tha gas-phase ionization potentials support the constancy of the solvation terms.The latter provides a ready explanation for the previously puzzling observations that the relative reactivities log k/k₀ are insensitive to solvent polarity, yet the absolute rates of electrophilc aromatic substitution are highly dependent.The CT formulation for electrophilic aromatic substitution also provides a physical interpretation of the linear free energy relationship (LFER) which has been established between the ?⁺ substituent constants and the relative reactivities of arenes.The slope of the LFER correlation ρ is shown to be a measure of the mean separation in the transition state.Other LFER's found between electrophilic aromatic substitution and such parameters as the proton affinities and the ? and the ? basisities of arenes are similarly interpreted.The prediction of the isomeric product distribution resulting from electron-releasing X and electron-withdrawing Y substituents is an important, natural consequence of the CT formulation.It provides unusual insight into the transition from the electron-poor arenes (Ph-Y) producing predominantly meta-substituted products to those (Ph-X) affording the para isomers.