Carbocations cycloheptatrienyl, stability

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 reac tive 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 struc ture drawn for it A number of observations indicate that cycloheptatrienyl cation is far more stable than most other carbocations To emphasize its aromatic nature chemists often write the structure of cycloheptatrienyl cation m the Robinson circle m a ring style... 

A special case of carbocation stability arises where the cation complies with the Hiiekel (4n+2) rule governing aromatic structures. Of these, the best known and most useful is the cycloheptatrienyl cation, more frequently referred to as the tropylium ion. For an informative and wide ranging account of structures, stabilities, properties and reactions of almost every type of carbocation, reference should be made to the series of monographs edited by Olah and Schleyer (18). 

Figure 4.26 shows the FfMOs for cyclopropenyl carbocation, square planar cyclobutadiene, cyclopentadienyl anion, benzene, cycloheptatrienyl carbocation, and planar (Ds/,) cyclooctatetraene. If we place electrons into the molecular orbitals of each species according to the aufbau principle, we notice an important relationship between the stability of the systems and the... 

The cycloheptatrienyl carbocation (20) is also a six n electron system, and all six electrons can go into bonding orbitals. Its delocalization energy is calculated to be 2.99)8 (Figure 4.26), which is close to the value of 50 kcal/mol of delocalization stabilization calculated by other methods. Therefore, the cycloheptatrienyl cation is an especially stable carbocation, although it is still a cation and is certainly not as stable as benzene. On the other hand, the cycloheptatrienyl anion is a 4n Ji system and thus is predicted to be antiaromatic by HMO theory. More advanced calculations suggest that any energy consequences of electron delocalization in the anion must be very small. 

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... 

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