Cycloheptatrienyl carbocation

Energies of the pi molecular orbitals of cyclooctatetraene and the cycloheptatrienyl carbocation. Note that the electrons are not yet shown in the MOs in these diagrams. 

The cycloheptatrienyl carbocation, also known as the tropylium cation, has six pi electrons. It is also aromatic and is quite stable. In fact, 7-bromo-l,3,5-cycloheptatriene actually exists as an ionic compound. 

The benzylic cation rearranges to form cycloheptatrienyl cation, which is even more stable. Note that the cycloheptatrienyl carbocation is aromatic 6 7T electrons) and has the positive charge delocalized around the entire ring. 

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 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). 

The seven resonance forms for tropylium cation (cycloheptatrienyl cation) may be generated by moving tt electrons in pairs toward the positive charge. The resonance forms are simply a succession of allylic carbocations. 

The cycloheptatrienyl cation is called the tropylium ion. This aromatic ion is much less reactive than most carbocations. Some tropylium salts can be isolated and stored for months without decomposing. Nevertheless, the tropylium ion is not necessarily as stable as benzene. Its aromaticity simply implies that the cyclic ion is more stable than the corresponding open-chain ion. 

Triphenylmethyl halides and tropylium halides ionize to form trityl, (f>3C, and cycloheptatrienyl (tropylium), CyRt", carbocations [Eqs. (P8.20.1) and (P8.20.2)], which are too stable to efficiently polymerize less reactive monomers such as isobutylene and styrene, but polymerization of p-methoxystyrene, vinyl ethers and N-vinyl carbazole, which are more reactive, proceeds rapidly. 

Similarly if Kd is less than 0.1, the system consists of more than 90% free ions and less than 10% less reactive ion pairs see Problem 8.28) and hence one can safely equate fc with k. Such conditions may be obtained in polymerizations initiated by stable carbocation salts such as hexachloroantimonate (SbCl ) salts of triphenyl methyl [(C6H5)3C" "] and cycloheptatrienyl (CtH ) carbocations, which are thus useful for evaluating the free-ion propagation rate constant [24]. However, since these cations are stable, their use is limited to the initiation of the more reactive monomers like N-vinylcarbazole and aUcyl vinyl ethers. The dissociation constant Kd)... 

In addition to neutral molecules, certain cation and anion intermediates meet the criteria for aromaticity. If the cyclopropenyl cation (115) and the cycloheptatrienyl cation (116) are examined, both have a continuous array of p-orbitals confined to a ring and a number of n-electrons that fit the 4n -i- 2 series (two for 115 and six for 117). Both of these carbocations are aromatic, which means that they are very stable, easy to form, and relatively long-lived intermediates. Compare these carbocations with the cyclopentadienyl cation (117), which meets the criterion of having a continuous array of p-orbitals confined to a ring, but has 4n ji-electrons (not a number in the 4n -i- 2 series) and is not aromatic. Indeed, it is considered to be antiaromatic, is very unstable, and is very difficult to form. 

It is important to recognize the difference between the hydrocarbon cycloheptatriene and cycloheptatrienyl cation. The carbocation is aromatic the hydrocarbon is not. Although cycloheptatriene has six tt electrons in a conjugated system, the ends of the triene system are separated by an yp -hybridized carbon, which prevents continuous cyclic tt electron delocalization. 

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... 

When 1,3,5-cycloheptatriene is heated with bromine, a stable salt is formed, cycloheptatrienyl bromide. In this molecule, the organic cation contains six delocalized tt electrons, and the positive charge is equally distributed over seven carbons (as shown in the electrostatic potential map in the margin). Even though it is a carbocation, the system is remarkably unreactive, as is expected for an aromatic system. In contrast, the cycloheptatrienyl anion is antiaromatic, as indicated by the much lower acidity of cycloheptatriene (pA"a = 39) compared with that of cyclopentadiene. 

Among the odd-membered rings, aromatic ions are readily prepared. Cyclopentadiene is deprotonated by alkoxide bases while cycloheptatriene is not, even with stronger bases. On the other hand, bromocycloheptatriene is ionic while 5-bromocyclopentadiene is not. Tripropylcyclopropenyl perchlorate exists largely as the carbocation in aqueous acetonitrile at pH 7 [4]. Electron configurations for the cyclopentadienyl anion, benzene, and the cycloheptatrienyl cation are shown in Figure 5.9. For simplicity, the molecular orbitals are represented by horizontal lines. 

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