Reaction Type 4 - C-shift

Molecular models are very useful when looking at these reactions and I would recommend that every chemist interested in terpenoids should possess a set. Ball and stick models or skeletal models are more useful for following carbocation rearrangements than are space filling ones. There are many types of models available any of which will serve the purpose. The choice is up to the reader s personal preferences and budget. Models are particularly useful for those who have difficulty in visualising structures in three dimensions. 

The phenomenon of changing of carbon ring structures by means of carbon shifts, is known as the Wagner-Meerwein rearrangement, after its discoverers. 

For clarity in the following discussion, structure (5.11) shows the numbering system of the 2,2,1-bicycloheptyl skeleton. This ring system nomenclature is based on the bridgehead carbon atoms. To name a bicyclic compound, one first identifies the two bridgehead carbon atoms and then counts the number of atoms in each bridge. Thus in structure 

For instance, structure (5.23) shows one canonical form with the positive charge located on the opposite end of the benzene ring. Note that the cyclopropane based carbocations (5.20) and (5.23) are both examples of non-classical carbocations, i.e. carbocations in which the positive charge is spread across a number of atoms rather than being localised on one. 

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