Structure of carbocations

Clearly there was no lack of devoted adversaries (perhaps a more proper term than enemies) on both sides of the norbornyl ion controversy. It is to their credit that we today probably know more about the structure of carbocations, such as the norbornyl cation, than about most other chemical species. Their efforts also resulted not only in rigorous studies but also in the development or improvement of many techniques. Although many believe that too much effort was expended... 

General agreement on what to call these ions has not yet been reached. The term s complex is a holdover from the time when much less was known about the structure of carbocations and it was thought they might be complexes of the type discussed in Chapter 3. Other names have also been used. We will call them arenium ions, following the suggestion of Olah, G.A. J. Am. Chem. Soc., 1971, 94, 808. 

It should be kept in mind that quantum chemical calculations of structures and magnetic properties generally are done for the isolated carbocation without taking into account its environment and media effects such as solvent, site-specific solvation or counterion effects. This is a critical question since NMR spectra of carbocations with a few exceptions are studied in superacid solutions and properties calculated for the gas-phase species are of little relevance if the electronic structure of carbocations is strongly perturbed by solvent effects. Provided that appropriate methods are used,... 

The structure of carbocation has been measured by 13C nmr shift of the carbon atom carrying the positive charge. The shift agrees with the electron density on this carbon. 

X-ray diffractometry is the most powerful method to determine atomic coordinates of molecules in the solid state. X-ray crystal structure analysis was, however, rarely applied in the early years of development of persistent, long-lived alkyl carbocations and studies were only performed to investigate structures of carbocations of aryl derivatives and aromatic systems.65 This is due to the low thermal stability of alkyl carbocations and to the difficulties in obtaining single crystals of carbocations suitable for analysis. Since then, however, methods and instrumentation have improved significantly and X-ray crystal structure analysis has become a powerful tool to solve structural problems of carbocations.65,66... 

Recent developments in computational chemistry have established the exact structure of carbocations by combining computational and experimental results.78,79 Furthermore, accurate 1H and 13C NMR chemical shifts of carbocations and other organic molecules can be calculated with the application of recent coupled cluster methods, such as GIAO-CCSD(T).80... 

Having established the three-dimentional structure of carbocations as planar, we can now study the stereochemical progression of Sk 1 reactions as compared to Sk2 reactions. As shown in Scheme 5.6, the stereochemical course of an Sk2 reaction is well defined because nucleophilic displacement of a leaving group proceeds with inversion of stereochemistry. Thus, the stereochemical outcome is defined by the stereochemistry of the starting material. As for SnI reactions, since the step required for initiation of these reactions involves formation of a planar species, incoming nucleophiles have equal access to both sides of the reactive carbocation. As shown in Scheme 5.7, this results in complete elimination of... 

As alluded to at the beginning of this section, carbocations generated during SNI mechanisms are subject to side reactions that include eliminations and rearrangements. Considering the possibility of these side reactions, one must question the stability of carbo-cationic species. To clarify, if carbocations were inherently stable, they would not be readily subject to additional transformations. Having already addressed the structure of carbocations, attention can now be focused on the factors influencing stability. 

Q Draw and describe the structures of carbocations, carbanions, free radicals, and carbenes and the structural features that stabilize them. Explain which are electrophilic and which are nucleophilic. 

Structure-reactivity relationship in polyarylcarbocation systems 334 Conformations of carbocations 334 Reactivity-conformation relationship 337 Stabilities of carbocations in the gas phase 343 Structural effects 343 Tlie resonance demand parameter 355 Theoretically optimized structures of carbocations 362 Reaction mechanisms and transition-state shifts 365 Extended selectivity-stability relationships 365 Ground-state electrophilic reactivity of carbocations 366 Sn2 reactions of 1-arylethyl and benzyl precursors 372 Concluding remarks 378 Acknowledgements 379 References 379... 

Theoretical reasons and the analogy with boron suggest the trigonal planar structure of carbocations which has recently been confirmed by spectroscopy in superac-... 

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