Heptatrienyl cation

FIGURE 11 14 Their molecular orbitals of cycio heptatrienyl cation... 

Figure 11.15 shows the Fliickel MOs of cyclopentadienyl anion. Like benzene and cyclo-heptatrienyl cation, cyclopentadienyl anion has six tt electrons and a closed-shell electron configuration. 

Similar arguments can be used to predict the relative stabilities of the cyclo-heptatrienyl cation, radical, and anion. Removal of a hydrogen from cyclohepta-triene can generate the six-77-electron cation, the seven-77-electron radical, 01 the eight-77-elec iron anion (Figure 15.6). All three species again have numerous resonance forms, but HiickeTs rule predicts that only the six-7r-electron cyclohep-tatrienyl cation should be aromatic. The seven-77-electron cycloheptatrienyl radical and the eight-77-electron anion are antiaromatic. 

Using Equation 1.12, or its geometrical equivalent, calculate the coefficients for the frontier orbitals of the heptatrienyl cation. 

Each molecule is a hybrid of either five or seven equivalent structures, with the charge or odd electron on each carbon. Yet, of the six compounds, only two give evidence of unusually high stability the cyclopentadienyl anion and the cyclo-heptatrienyl cation (tropylium ion). 

Following the instructions for drawing the tt molecular orbital energy levels of the compounds shown in Figure 8.8, draw the tt molecular orbital energy levels for the cyclo-heptatrienyl cation, the cycloheptatrienyl anion, and the cyclopropenyl cation. For each compound, show the distribution of the tt electrons. Which of the compounds are aromatic Which are antiaromatic ... 

Fig. 4.2 illustrates the first few members of the series of equilibria of conjugated ions. In cations, they are the equilibria between the allyl 4.11 and the cyclopropyl cation 4.12, the pentadienyl 4.13 and the cyclopentenyl cation 4.14, and the heptatrienyl 4.15 and cycloheptadienyl cation 4.16, In anions, they are between the allyl 4.17 and the cyclopropyl anion 4.18, the pentadienyl 4.19 and the cyclopentenyl anion 4.20, and the heptatrienyl 4.21 and cycloheptadienyl anion 4.22. There are heteroatom-containing analogues, with nitrogen and oxygen lone pairs rather than a carbanion centre, and the systems can again have substituents and fused rings. 

Add the appropriate number of electrons to the MO energy level diagram for the cy-cloheptatrienyl cation in Figure 16.3. Is this ion aromatic or antiaromatic The cyclo-heptatrienyl anion has two more electrons than the cation. Do you expect this anion to be a stable species Explain. 

Problem 15.10 Show the relative energy levels of the seven n molecular orbitals of the cycle-heptatrienyl system. Indicate which of the seven orbitals are filled in the cation, radical, and anion, and account for the aromaticity of the cycloheptatrienyl cation. 

Problem 8.20 Give plausible explanation for the following facts. Primary and secondary alkyl halides are generally ineffective as initiators of cationic polymerization of monomers such as isobutene and styrene, but t-butyl and cumyl chlorides are effective. On the other hand, triphenylmethyl chloride and cyclo-heptatrienyl (tropylium) chloride are not very efBcient in polymerizing isobutylene and styrene but produces rapid polymerization of p-methoxystyrene, vinyl ethers and N-vinylcarbazole. 

In 1958 Dauben and Honnen prepared the fluoroborate salt of cyclo-heptatrienyl-molybdenum tricarbonyl through reaction of cycloheptatriene-molybdenum tricarbonyl with triphenylmethyl fluoroborate (88), Fischer and Fischer (89) have shown that an analogous reaction between cyclo-hexadiene-iron tricarbonyl (LXX) and the triphenylmethyl cation leads to the formation of a salt of the cyclohexadienyliron tricarbonyl cation (LXXI), Eq. 9. In the cation (LXXI), and in the analogous complexes described below, it is assumed that there are five ip -type carbon atoms simultaneously bonded to the iron atom. 

Cycloheptatriene metal complexes, where the cycloheptatriene is acting as a 6-electron ligand, may undergo hydride abstraction to give 7r-cyclo-heptatrienyl metal cations. 

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