The importance of orbital orientation as a rate‐controlling factor in intramolecular reactions of carbonium ions

In carbonium ions the relative orientation of the vacant p ‐orbital of the electron‐deficient carbon atom and the C‐H and C‐R bonds at the adjacent carbon atom (“orbital orientation”) is a very important factor for the rates of 1,2‐hydride and alkyl shifts and of cleavage of β C‐C bonds. A large number of examples of this effect can be found among the reactions of directly observable carbonium ions. In some cases it causes the rate of a 1,2‐shift or a β‐cleavage in a cyclic carbonium ion to be several orders of magnitude lower than in acyclic carbonium ions.