Degenerate Carbonium Ions

Carbonium ions are called “degenerate” when they rearrange through finite energy barriers to give products whose gross structures are identical with those of the starting ions. In such degenerate rearrangements (“automerizations”), individual atoms and groups become mixed but the overall structure is conserved. Automerizations can be observed by noting the dispersal of isotopic labels, the loss of optical activity, or the temperature dependence of NMR spectra. In certain cases, a combination of well‐known rearrangement processes results in complete mixing of all atoms, C or H. The 2‐norbornyl and 9‐barbaralyl cations are examples of fully degenerate systems; conditions can be found where both give but single‐line NMR spectra.