Elementary addition-substitution reactions. H�ckel approach for the description of aromatic and antiaromatic arylmethyl cations

Elementary addition–substitution reactions have been described based on reactivity of aromatic and antiaromatic arylmethyl cations. Geometries, charge densities, and reaction profiles have been calculated with a bond-length-dependent resonance integral in a Hückel molecular orbital (HMO) description with limited configuration interaction. A valence bond (VB) formalism has been used to visualize the predictions. For the antiaromatic arylmethyl cations these descriptions resulted in asymmetric and symmetric geometries depending on the geometry of the precursor. Methylation of aromatic arylmethyl cations as the pentamethylbenzyl cation results in localization of the positive charge as methylated pentadienyl cation, as the dominant VB contribution, leading to an exclusive reactivity.With the assistance of MO symmetry arguments a reevaluation is given for the primary step in the basic halogen addition to ethylenic compounds.