Nonclassical ion controversy

The Brown-Winstein nonclassical ion controversy can be summed up as differing explanations of the same experimental facts (which were obtained repeatedly and have not been questioned) of the observed significantly higher rate of the hydrolysis of the 1-exo over the 2-endo-norbornyl esters. As suggested by Winstein, the reason for this is participation of the Ci-Q single bond leading to delocalization in the bridged nonclassical ion. In contrast. Brown maintained that the... 

The most studied hypercoordinate carbocation is the 2-norbornyl cation, around which the nonclassical ion controversy centered (Chapter 9). 

In the nonclassical ion controversy discussed in Chapter 9, there was never any question on either side of the debate about the validity of the observed data, only about their interpretation. Had any of the experimental data been questioned or found to be incorrect, this would have been soon found out because so many people repeated and rechecked the data. This is the strength of science (in contrast to politics, economics, etc.), i.e., that we deal with reproducible experimental observation and data. Nevertheless, interpretation can still result in heated discussions or controversies, but science eventually will sort these out based on new results and data. 

The classical-nonclassical ion controversy also frequently included the question of the so-called ethylenephenonium ions. 

Due to its complexity, the addition of acids to alkenes does not encourage straightforward mechanistic conclusions. The studies with norbornene appear to add a new dimension to the nonclassical ion controversy. 

While only an ancillary topic in this discussion, the nonclassical ion controversy, extensively reviewed elsewhere,has a foundation in the historical context of the Wagner-Meerwein rearrangement. After completing a solvolytic study of structurally similar exo- and e cto-2-norbomyl brosylates, Winstein and Trifan suggested that the reaction s cationic intermediate was instead a o-delocalized, symmetrically bridged norbomyl ion 9. This concept deviated from the accepted classical cation structure proposed by Meerwein as the equilibrium between 7 and 8, where the positive charge was considered to be localized on a single atom. 

The isotopic perturbation method is very useful for studying fractional bonding and hypercoordination in coordinatively unsaturated and electron deficient compounds such as transition metal complexes or carbocations. The 2-norbornyl cation and the bicyclobutonium cation are the most prominent examples of carbocations whose structures have led to much controversial discussion (the so-called classical-nonclassical ion controversy). The application of the isotopic perturbation methed is likely to be the most decisive piece of nmr evidence for the hypercoordinated structure of these two cations in solution. 

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