1,2- versus 1,4-Addition to 1,3-Butadiene by a Simple LCAO-MO Method

Abstract

The addition of various reagents such as halogens and hydrogen halides to 1,3‐butadiene by an ionic mechanism is ideally visualized as proceeding in two steps, viz., the electrophilic attack of the 1‐ or 2‐position followed by nucleophilic attack to form either the 1,2‐ or 1,4‐addition product. A simple LCAO—MO approach was applied to a proposed transition state arising from the first step of the reaction. Atom stabilization energies and frontier electron densities were calculated for the various positions of attack involved during the final step of the addition. The effect of the electronegativity of the group adding during the first step relative to the electronegativity of the carbon atoms in the conjugated system was used to alter certain of the LCAO—MO parameters. Agreement of the results with experiment is discussed briefly.