Although generations of organic chemists have used rates of nucleophilic substitution reactions as a means of judging the stability of carbocations, this approach is fraught with error.
- Most 2o substrates do not react by the SN1 pathway
- One is never sure of the extent to which cations that do form are differentially stabilized by dipolar or more specific interactions with the solvent
Two methodologies exist for more directly measuring carbocation stabilities.
- Formation of carbocations in hyperacid media, the discovery of which won the Nobel for George Olah a dozen years ago, allows us to study the properties of carbocations.
- Completely eliminating solvent can be accomplished by making use of mass spectrometry, particularly ion cyclotron resonance.
- One can, for example, measure the enthalpy of protonation of an alkene to a carbocation, or the ionization of a free radical to a carbocation.
Your textbook gives brief collections of data obtained in these ways. The table below contains a more extensive selection. It also includes for a few structures the 13C chemical shift of the carbocation carbon in hyperacid media, which is very nearly invariant with structure.
Structures marked with a ** are in fact not the simple structures represented. See the next page for an explanation.
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