Cycloheptatrienyl cation molecular orbitals

Molecular orbitals allyl cation, 397 [10]-annulene, 425 benzene, 407, 424 bonding and antibonding, 34—35 1,3-butadiene, 397—398 cyclobutadiene, 424 cycloheptatrienyl cation, 427—428 cis, tram -1,3-cyclooctadiene, 524 cyclooctatetraene, 424 cyclopentadienide anion, 428 ethylene, 386—397 frontier, 386... 

Huckel realized that his molecular orbital analysis of conjugated systems could be extended beyond neutral hydrocarbons He pointed out that cycloheptatrienyl cation also called tropyhum ion contained a completely conjugated closed shell six tt electron sys tern analogous to that of benzene... 

Figure 11.14 shows a molecular orbital diagrfflTt for cycloheptatrienyl cation. There are seven tt MOs, three of which are bonding and contain the six tt electrons of the cation. Cycloheptatrienyl cation is a Hiickel (4n + 2) system and is an aromatic ion. 

Problem 15.10 I Show the relative energy levels of the seven 77 molecular orbitals of the cvclohepta-trienyl system. Tel) which of the seven orbitals are filled in the cation, radical, and anion, and account for the aromaticity of the cycloheptatrienyl cation. 

FIGURE 11.14 Then molecular orbitals of cycloheptatrienyl cation. 

Huckel realized that his molecular orbital analysis of conjugated systems could be extended beyond the realm of neutral hydrocarbons. He pointed out that cycloheptatrienyl cation contained a tt system with a closed-shell electron configuration similar to that of benzene (Figure 11.13). Cycloheptatrienyl cation has a set of seven tt molecular orbitals. Three of these are bonding and contain the six tt electrons of the cation. These six tt electrons are delocalized over seven carbon atoms, each of which contributes one 2p orbital to a planar, monocyclic, completely conjugated tt system. Therefore, cycloheptatrienyl cation should be aromatic. It should be appreciably more stable than expected on the basis of any Lewis structure written for it. 

FIGURE 11.13 The tt molecular orbitals of cycloheptatrienyl (tropylium) cation. 

Following the instructions for drawing the tt molecular orbital energy levels of the compounds shown in Figure 15.2, draw the tt molecular orbital energy levels for the cyclohep-tatrienyl 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 andaromatic ... 

To be classified as aromatic, a compound must have an uninterrupted cyclic cloud of rr electrons that contains an odd number of pairs of tt electrons. An antiaromatic compound has an uninterrupted cyclic cloud of tt electrons with an even number of pairs of tt electrons. Molecular orbital theory shows that aromatic compounds are stable because their bonding orbitals are completely filled, with no electrons in either nonbonding or antibonding orbitals in contrast, antiaromatic compounds are unstable because they either are unable to fill their bonding orbitals or they have a pair of TT electrons left over after the bonding orbitals are filled. As a result of their aromaticity, the cyclopentadienyl anion and the cycloheptatrienyl cation are unusually stable. 

Refer to the Frost circle constructed in the answer to Example 21.1. In the ground-state electron configuration of the cycloheptatrienyl cation, the six tt electrons occupy the 77, 772, 3 molecular orbitals, all of which are bonding. 

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. 

Figure 12.4 Energy Levels and Occupancies of the Molecular Orbitals of Cycloheptatrienyl Cation... 

Hiickel pointed out that, on the basis of molecular orbital theory, monocyclic conjugated polymethines have filled shells of tt-electrons when the number of TT-electrons is An + 2, where n is an integer. These systems may be expected to be stable. The rule may be illustrated by reference to Fig. 2.1. If = 0, then a system with 27r-electrons should be stable. Such a situation is found in the cyclopropenyl positive ion, which has been isolated as the hexachloroanti-monate. For n = the prediction is that the cyclopentadienyl anion, benzene and the cycloheptatrienyl (tropylium) cation are stable. This is certainly in accord with experience. The stability of benzene is well known, the cydo-pentadienyl anion is readily formed by the action of potassium metal on cyclopentadiene, and the cycloheptatrienyl cation is one of the most stable carbonium ions known. Huckel s rule also predicts that some of the larger cyclic conjugated systems are stable, e.g. those with 10,14 and 18 rr-electrons. However, the situation is complicated by steric problems (see for example Garratt, 1971) and need not be considered further here. 

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