Structure and stability of carbocations

The critical step in the ionization mechanism for nucleophilic substitution is the generation of the carbocation intermediate. For this mechanism to operate, it is essential 

The relative stability of the carbocation can be expressed in terms of its p/fR+, which is defined as 

Several very stable carbocations are included in the Other Carbocations part of Table 4.15. The tricyclopropylmethyl cation, for example, is more stable than the 

As discussed in Section 3.4.1, carbocation stability can also be expressed in terms of hydride affinity. Hydride affinity values based on solution measurements can be 

It is possible to obtain thermodynamic data for the ionization of alkyl chlorides by reaction with SbFj, a strong Lewis acid, in the nonnucleophilic solvent S02C1F. The solvation energies of the carbocations in this medium are small and do not differ much from one another, which makes comparison of nonisomeric systems reasonable. As long as subsequent reactions of the carbocation can be avoided, the thermodynamic characteristics of the ionization reactions provide a measure of the relative ease of carbocation formation in solution. There is good correlation between these data and the 

The heterolytic fission of a C—X bond in an organic molecule, in which X is more electronegative than carbon, generates the negatively charged anion (X ) and positive charged species known as carbocations (called carbonium ions in the older literature). 

The carbon atom in a typical carbocation is sp hybridized. The pz orbital is empty and is perpendicular to the plane of the other three bonds. Thus, carbocation adopts a trigonal planar shape. 

The electron-donating groups attached to positively charged carbons in carbocations increase the stability of the carbocations by inductive effect and/or hyperconjugation (no bond resonance). Thus, a tertiary carbocation is more stable than a secondary carbocation, which in turn is more stable than a primary carbocation. 

However, the presence of electron-withdrawing groups adjacent to the carbon atom bearing positive charge makes the carbocation less stable (by —I and/or —M effects). 

Resonance effect further stabilizes the carbocations when present. By resonance the positive charge on the central carbon atom gets dispersed over other carbon atoms and this renders stability to the carbocation. The more the canonical (resonating) structures for a carbocation, the more stable it will be. For example, benzyl and allyl carbocations are very stable because of resonance. 

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To understand why Markovnikov s rule works, we need to learn more about the structure and stability of carbocations and about the general nature of reactions and transition states. The first point to explore involves structure. 

Having addressed the structure and stability of carbocations, discussions will now be directed to the specific side reactions to which carbocations are subject. Specifically, this section focuses on rearrangements of carbocations known as hydride shifts and alkyl shifts. 

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... 

The major carbon centered reaction intermediates in multistep reactions are carbocations (carbenium ions), carbanions, free radicals, and carbenes. Formation of most of these from common reactants is an endothermic process and is often rate determining. By the Hammond principle, the transition state for such a process should resemble the reactive intermediate. Thus, although it is usually difficult to assess the bonding in transition states, factors which affect the structure and stability of reactive intermediates will also be operative to a parallel extent in transition states. We examine the effect of substituents of the three kinds discussed above on the four different reactive intermediates, taking as our reference the parent systems [CH3], [CHi]", [CHi] , and [ CH2]. 

Hyperconjugation has a profound effect on structure and stability of cyclohexyl cations. An elegant study combined theoretical results with experimental data to confirm that different hyperconjugative stabilization patterns lead to the formation of two equilibrating conformers of the 1-methyl-1-cyclohexyl cation where the carbocation p-orbital is oriented either pseudoaxiaUy or pseudoequatoriaUy. 

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