Cyclopentadienyl anion, aromaticity molecular orbitals

Active Figure 15.11 Energy levels of the five cyclopentadienyl molecular orbitals. Only the six-7r-electron cyclopentadienyl anion has a filled-shell configuration leading to aromaticity. Sign in at www.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

The operation of (d) is seen in cyclopentadiene (14) which is found to have a pKa value of 16 compared with 37 for a simple alkene. This is due to the resultant carbanion, the cyclopentadienyl anion (15), being a 6n electron delocalised system, i.e. a 4n + 2 Hiickel system where n = 1 (cf. p. 18). The 6 electrons can be accommodated in three stabilised n molecular orbitals, like benzene, and the anion thus shows quasi-aromatic stabilisation it is stabilised by aromatisation ... 

Using a simple resonance approach, we might incorrectly expect both of the cyclopentadienyl ions to be unusually stable. Shown next are resonance structures that spread the negative charge of the anion and the positive charge of the cation over all five carbon atoms of the ring. With conjugated cyclic systems such as these, the resonance approach is a poor predictor of stability. Hiickel s rule, based on molecular orbital theory, is a much better predictor of stability for these aromatic and antiaromatic systems. 

Energy levels of the five cyclopentadienyl molecular orbitals. Only the ix-ir-electron cyclopentadienyl anion has a filled-shell configuration leading to aromaticity. 

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. 

Cyclopentadiene is a muci stronger acid than propene. The difference in acidity is enormous. Look carefully at the structure of the cyclopentadienyl anion. Here too, we have a planar, cyclic, and fully conjugated system. The molecular orbitals can be derived from a Frost circle (Fig. 13.29).There are six electrons to put into the molecular orbitals, and, as in the tropylium ion or benzene, they fully occupy the lowest molecular orbital and the set of degenerate bonding molecular orbitals. The cyclopentadienyl anion can be described as aromatic, and for an anion, this species is remarkably stable. Do not fall into the trap of expecting this anion to be as stable as benzene. 

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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