On the nature of hydrated aliphatic carbenium ions and on their role in homogeneous acid catalysis

Abstract

The structure of hydrated methyl and t-butyl carbocations is discussed on the basis of non-empirical quantum chemical calculations and literature data from high pressure mass-spectrometry. It is concluded that primary carbocations are transformed by hydration into alkoxonium ions solvated by four or five additional water molecules. The methyl fragment in such a complex is essentially covalent. Therefore any mechanisms of acid-catalyzed reaction in aqueous solution involving the participation of free primary carbenium ions are unrealistic. The t-butyl cations are more weakly solvated, but also bind four water molecules in their hydration sphere. A new explanation for the acid-catalyzed S N1 mechanism based on the hydrophilic properties of iso-butyl fragments is proposed.