musings on music and life

July 8, 2013

2-Norbornyl cation crystal structure revealed!

Filed under: Chemistry — Tags: , — sankirnam @ 10:22 am

For those who are familiar with carbocation chemistry, the title will seem a huge shock. Yet, it has been obtained! The norbornyl cation was the center of the “nonclassical ion controversy”, which basically centered around arguments as to whether carbon could undergo σ-delocalization in order to mitigate charge-charge repulsive effects. The big players in this debate were Saul Winstein (UCLA) and Nobel Laureate H. C. Brown; after Winstein’s untimely demise in 1969, his position was informally handed over to Olah. In his early years at Case Western Reserve University, Olah had proposed the concept of “σ-basicity” of alkanes, proposing that C-C and C-H bonds could act as Bronsted bases when reacting with the strongest superacids known, Magic Acid or fluoroantimonic acid. Nonclassical structures (keep in mind the term was first coined by Prof. J. D. Roberts (Caltech), now 95 years old!) were simply an extension of this concept. H. C. Brown had proposed that boron could bond with more than 4 atoms at once (e.g. diborane) but for whatever reason, was unwilling to accept that carbon could do the same. It is now known that boranes and carbocations, being isoelectronic analogues, undergo similar types of bonding.

The isolation of stable crystals of norbornyl ion salts is in itself no trivial task; I can say this from my own experience of trying to crystallize carbocation salts! One breakthrough in this case is the use of bromoaluminate counterions; I have not seen these used as frequently as other counterions (such as hexafluoroantimonate, hexafluorophosphate, tetrafluoroborate, or carboranes). This effort was led by Karsten Meyer, at Erlangen-Nuremberg. Off the top of my head, he is more famous as an organometallic chemist; I remember him giving an interesting talk at our department a few years ago on organouranium complexes. It is therefore interesting that he got involved in this area of research. As he mentions, not only were the crystals air- and moisture-sensitive, but they were prone to undergoing phase transitions leading to disorder if heated or cooled too rapidly.

This paper thus leads to the end of the famous “norbornyl ion controversy”; although this debate has been claimed to be closed unsuccessfully in the past, I think that this time it will be for good.


1 Comment »

  1. […] mentioned last year that the structure of the long-controversial 2-norbornyl cation had at last been solved […]

    Pingback by Prof. Paul Schleyer passes away | musings on music and life — November 21, 2014 @ 9:39 pm

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