Meerwein and equilibrating carbocations

Fifty-seven years ago, Hans Meerwein published a paper, “uber die Gleichgewichts-Isomerie zwischen Bornylchlorid, Isobornylchlorid and Camphenchlorhydrat 1)”, which had immense consequences for the development of physical organic chemistry. The paper contains the first proposal of carbonium ions as intermediates in molecular rearrangements. It also provided the basis for the later proposal of a nonclassical ion as an intermediate2, in place of the pair of rapidly equilibrating classical cations utilized by Meerwein1). Consequently, this publication opened the door to major research efforts by numerous chemists throughout the world. It is the purpose of this review to discuss the question of whether it is the Meerwein concept of equilibrating cations or the Wilson proposal of a mesomeric nonclassical cation that best accounts for the fascinating behavior of 2-norbornyl cations.