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The problem of the non-classical ion

A cornerstone of the classical theory of structural chemistry since the time of Kekulé in the 1860s is that carbon can bind at most four other atoms (tetra-coordination).
    Around 1950 S. Winstein in the USA found a short-lived carbocation that contained penta-coordinated carbon. He named the ion non-classical. This pioneering and controversial discovery initiated a vigorous academic feud. H.C. Brown, who received the 1979 Nobel Prize in Chemistry for his development of boron compounds into important reagents in organic synthesis, questioned Winstein's results.
    Winstein claimed that the norbornyl cation had a structure containing a penta-coordinated carbon i.e. that it was a carbonium ion.
    Brown on the other hand concluded that the norbornyl cation was a tri-coordinated cation (carbenium ion) that rapidly rearranged into an identical tri-coordinated ion in which the positive charge had moved to another carbon.
    Despite very great efforts by many leading physical organic chemists, the problem remained unsolved until Olah's method of preparing long-lived carbocations was applied. He was able to study the norbornyl cation directly with NMR spectroscopy. The ion proved to have the structure that Winstein suggested.

 
 
Penta-coordinated carbocation
(carbonium ion)
Equilibrating tri-coordinated carbocation
(carbenium ion)



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