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

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 the 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 relative CT transition energies Shvct, using benzene as the reference arene. This remarkable correlation thus relates the transition state for electrophilic aromatic substitution to the CT excited state [Ar+ E-] * of the arene-electrophile pair. Such a direct relationship requires that the solvation energies remain constant for the 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 the gas-phase ionization potentials support the constancy of the solvation term. 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 electrophilic aromatic substitution are highly solvent 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 p 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 σ and the π basicities 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.