The stabilities of carbonium ions

Abstract

An empirical bond energy scheme for carbonium ions is derived which gives good agreement with experimental values for some primary, secondary, and tertiary ions. If the chemical bonds are considered as polarizable, the bond energy scheme, and some reactions of carbonium ions, can be rationalized solely from the application of classical electrodynamics. As example it is deduced that dienylic ions are less stable than cyclopentenylic ions and cyclohexenylic ions. It is also shown that the activation energy for the cyclization of dienylic ions is largely composed of electrostatic repulsion between the equally charged ends. While the results are mainly applicable for the gas phase an approximate discussion of solvation energies shows that, while all C-C bonds stabilize a carbonium ion centre to a greater extent in the gas phase, a methyl will stabilize positive charge to a greater extent than, say, an ethyl in aqueous solution if the positive charge is less than 0.6e. That is, the Baker-Nathan order is predicted in solution if the charge is delocalized. It is concluded that the concept of hyperconjugation is unnecessary for a description of carbonium ions.