On the real nature of aliphatic carbenium ions as active intermediates of homogeneous and heterogeneous acid catalysis

Abstract

The structure and properties of active intermediates of acid-catalyzed homogeneous and heterogeneous transformations of olefins and alcohols are discussed, taking into account the hydration in homogeneous systems or the interaction with the surface of heterogeneous zeolite catalysts on the basis of nonempirical quantum-chemical calculations. In both cases carbenium ions do not represent the actual existing active intermediates, but rather the excited transition states. They could be formed from the more covalent and more stable surface esters in heterogeneous acid catalysis or from hydrated alkoxonium ions in acid catalysis in aqueous solutions. This results in concerted reaction pathways with low activation energies due to almost complete compensation of the energy required for the rupture of the reacting chemical bonds in the initial compounds by the energy released by the formation of the new bonds in the final products.