Transition-State Barrier for Electrophilic Reactions. Solvation of Charge-Transfer Ion Pairs as the Unifying Factor in Alkene Addition and Aromatic Substitution with Bromine

Alkenes and arenes are known to form 1:1 electron donor-acceptor complexes with molecular bromine. The disappearance of the charge-transfer (CT) absorption bands for these alkene and aromatic complexes coincides with the kinetics of electrophilic addition and electrophilic aromatic substitution, respectively. The rate constants (log k_Br) for both classes of electrophilic brominations follow linear but separate correlations with the CT transition energies (hv_CT). However, a single free energy relationship in eq 29 obtains for both alkene addition and aromatic substitution when the solvation energies of alkene and aromatic cations are specifically included. Solvation energies (ΔG^s) for these transient cations are evaluated from the gas-phase ionization potentials of the alkene and aromatic donors together with their rates of oxidation in solution by a prescribed series of outer-sphere iron(III) oxidants. The theoretical basis of eq 29 is shown to derive directly from Mulliken theory, in which the CT transition hv_CTrelates to the vertical excitation of the donor-acceptor complex to the ion-pair state, i.e., [D Br₂]→[D⁺Br₂⁻] *, where D represents the alkene and aromatic donors. Inclusion of the solvation term ΔG^swith hv_CTcorresponds to the formation of the solvated ion pair [D⁺Br₂⁻]_s. The single, remarkable correlation in Figure 8 indicates that the activation process is equivalent to the formation of solvated ion pairs in both classes of electrophilic brominations. The CT formulation thus unifies the activation processes for electrophilic additions to alkenes and electrophilic aromatic substitution into a single concept readily amenable to physical interpretation. Its significance to the more conventional linear free energy relationships based on the Taft σ* and Brown σ⁺correlations for alkenes and arenes, respectively, is delineated.