Cycloheptatrienyl cation electronic configuration

Figure 29.10. Aromatic compounds with 6 w electrons. Configuration of IT electrons in cyclopentadienyl anion, benzene, and 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 11.15 shows the Huckel MOs of cyclopentadienyl anion. Like benzene and cycloheptatrienyl cation, cyclopentadienyl anion has six tt electrons and a closed-shell electron configuration. 

Construct an MO energy diagram for the cycloheptatrienyl cation and describe its ground-state electron configuration. 

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. 

HiickeFs rule also pertains to charged cyclic conjugated systems. The cyclo-propenyl (2 tt electrons), cyclopentadienyl anion (6 tt electrons), and cycloheptatrienyl (tropylium) cation (6 tt electrons) are examples of stabilized systems. We say much more about the relationship between MO configuration and aromaticity in Chapter 9. 

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