Mythical Carbocations?

About the first case, we can be definitive: primary carbocations do not exist. That is, species with the full postive charge localized on a primary carbon are not formed as intermediates in chemical reactions, and cannot be found even in the gas phase in an icr spectrometer.

So far as can be judged by computation, all of these species are hydrogen bridged. A hydrogen from a neighboring carbon becomes partially bonded to the cation center, so the charge is distributed over both carbons and the hydrogen.

For example, here is CH₃CH₂⁺ as described by a 6-31G*/MP2 ab initio calculation:

The ion is symmetrical; the hydrogen is exactly between the two carbons. The picture looks very much like ethylene with a proton sitting in the middle of the p-bond.

Here is another way of seing the same thing. The picture shows the LUMO of the cation, the locus of the positive charge. Again, the symmetry is evident.

The simple secondary cations, 2-butyl and cyclopentyl, probably are not intermediates in most solvolytic reactions.
- Both undergo S_N2 substitution more readily than S_N1.
- Theoretical evidence is mixed, but leans toward hydrogen-bridged structures for the gas-phase ions.
  - Relevant to this point is the observation that the ¹H nmr spectrum of cyclopentyl carbocation in hyperacid media is a single line, suggesting scrambling of the expected three types of hydrogen by rapid 1,2- migrations.
The 2-norbornyl cation, the famous "non-classical" carbocation, sparked one of the fiercest debates ever seen in organic chemistry. The debate now seems to be resolved in favor of the non-classical structure for the unsubstituted ion, but not for most of its substituted analogs.

Here is the norbornyl ion, the result of a B3LYP/6-311G* calculation, oriented as in the right-hand line drawing:

2-Norbornyl Cation LUMO of the Norbornyl Cation

Don't be misled by the position of the C6-C2 bond; SPARTAN leaves bonds where you draw them in the initial structure. The C6-C2 and C6-C1 bonds are both the same length, 1.89 Â.
Like simple secondary ions, the benzyl cation probably is not an intermediate in solvolytic reactions. The conjugation that would be present in the carbocation also is present in the S_N2 transition state, greatly facilitating the bimolecular reaction. In the gas phase, the benzyl cation rearranges instantaneously to tropylium.

Can you write a mechanism for this remarkable transformation?
References for this brief tour of carbocation structure and stability are on the next page.

This page last modified 12:01 PM on Tuesday July 5th, 2011.
Webmaster, Department of Chemistry, University of Maine, Orono, ME 04469


2-Norbornyl Cation	LUMO of the Norbornyl Cation