THEORETICAL-STUDY OF THE MECHANISM OF BRANCHING REARRANGEMENT OF CARBENIUM IONS

Owing to the practical interest of the acid catalyzed isomerization re actions of hydrocarbons, the mechanism of the branching rearrangements of C4H9+ and C5H11+ carbenium ions has been studied theoretically usi ng ab initio methods which include electron correlation and extended b asis sets. It has been found that the protonated cyclopropane-type spe cies does not appear as a common intermediate for these reactions, sin ce it is a transition state and not a minimum on the potential energy surfaces studied. In the case of C4H9+ cation, the protonated methyl-c yclopropane ring is the transition state for the carbon scrambling rea ction in the secondary n-butyl cation, while the isomerization of n-bu tyl cation into t-butyl cation occurs via a primary cation. The activa tion energies calculated assuming this mechanism are in very good agre ement with those obtained experimentally. For the branching rearrangem ent of n-pentyl cation two reaction paths have been considered. In the first one the secondary n-pentyl cation is converted through the 1,2- dimethyl-cyclopropane ring into the secondary 3-methyl-2-butyl cation, which is converted into the I-pentyl cation by a 1,2-hydrogen shift. In the second one the secondary n-pentyl cation is directly converted into the t-pentyl cation through a primary monobranched cation. Compar ison of the calculated activation energies for both mechanisms with th e experimental value indicate that this reaction does not occur via th e primary cation as was the case for n-butyl cation, but occurs via th e protonated 1,2-dimethyl-cyclopropane ring.