Hydrocarbon structures carbocation reactivity

Experimental studies on the structures, physical and chemical properties, and thermodynamic stabilities of carbocations are especially difficult because of the inherent instability of these reactive intermediates. Of particular fundamental interest are experimental methods for the determination of the structures and stabilities of carbocations in the gas pha.se. These methods can be used to gather data for direct comparison with the results of ah initio theory, without the need for consideration of solvation effects. In this article, we will show comparisons between theory and experiment for hydrocarbon and carbocation stabilities in order to test the performance of... 

When we say cycloheptatriene is not aromatic but cycloheptatrienyl cation is, we are not comparing the stability of the two to each other. Cycloheptatriene is a stable hydrocarbon but does not possess the special stability required to be called aromatic. Cycloheptatrienyl cation, although aromatic, is still a carbocation and reasonably reactive toward nucleophiles. Its special stability does not imply a rock-like passivity, but rather a much greater ease of formation than expected on the basis of the Lewis structure drawn for it. A number of observations indicate that cycloheptatrienyl cation is far-more stable than most other car bocations. To emphasize its aromatic nature, chemists often write the structure of cycloheptatrienyl cation in the Robinson circle-in-a-ring style. 

This concludes the discussion of the stabilities of carbocations with hydrocarbon-based structures and also of different methods for deriving equilibrium constants to express these stabilities. The remainder of the chapter will be concerned mainly with measurements of stabilities for oxygen-substituted and metal ion-coordinated carbocations. Consideration of carbocations as conjugate acids of carbenes and derivations of stabilities based on equilibria for the ionization of alkyl halides and azides will conclude the major part of the chapter and introduce a discussion of recent studies of reactivities. 

The very existence of a remarkably simple and extensive relationship between thermodynamic and kinetic reactivity in solution and intrinsic stability is intriguing, on account of the variety of stabilities, structures and exposures to solvent involved. This raises important questions about, inter alia, the role of C-H- solvent hydrogen bonding interactions of carbocations in solution as well as charge delocalization effects within hydrocarbon frameworks of the carbocations. 

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