The Cyclopentadienyl Anion and Cation

Other kinds of substances besides benzene-like compounds can also be aromatic. For example, the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic ions. Pyridine, a six-membered, nitrogen-containing heterocycle, is aromatic and resembles benzene electronically. Pyrrole, a hve-membered heterocycle, resembles the cyclopentadienyl anion. 

By considering the n MOs of the cyclopentadienyl system (C5H5) to result from an interaction between cri-butadiene n MOs and an sp1 hybridized C atom, explain the stability of the cyclopentadienyl anion and the instability of the cyclopentadienyl cation. 

Fig. 4.23 Hiickel s rule says that cyclic n systems with An + 2 n electrons ( = 0, 1, 2,. .. An + 2 = 2, 6, 10,. ..) should be especially stable, since they have all bonding levels full and all antibonding levels empty. The special stability is usually equated with aromaticity. Shown here are the cyclopropenyl cation, the cyclobutadiene dication, the cyclopentadienyl anion, and benzene formal structures are given for these species - the actual molecules do not have single and double bonds, but rather electron delocalization makes all C/C bonds the same...

The cyclopentadienyl anion and the tropylium cation both illustrate an important principle The number of n electrons determines aromaticity, not the number of atoms in a ring or the number of p orbitals that overlap. The cyclopentadienyl anion and tropylium cation are aromatic because they each have six it electrons. 

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). 

According to the Htickel criteria for aromaticity described in the preceding section, a molecule must be cyclic, conjugated (that is, be nearly planar and have a p orbital on each carbon), and have 4n + 2 tt electrons. Nothing in this definition says that the numhera of p orbitals and tt electrons must be the same. In fact, they can be different. The 4n + 2 rule is broadly applicable to many kinds of molecules, not just to neutral hydrocarbons. For example, both the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic. 

One of the most famous results of the simple MO approach is the prediction 24> of stability for the cyclopentadienyl anion and cyclohept-atrienylium (tropylium) cation, which has been fully confirmed by the experimental findings 161>. The Ji-electron configuration of these ions is fairly similar to that of benzene, and this explains the results obtained in the first approximation. 

A natural extension of the above studies is to charged aromatic molecules containing silicon. The potential for aromaticity in the silacyclopentadienyl anion 101 and the silacyclopropenium cation 102, both formally possessing an aromatic (4n + 2) -electron network, was studied by Gordon and coworkers2463. 101 is isoelectronic with the cyclopentadienyl anion and 102 with the cyclopropenium cation, both well established aromatic systems with high delocalization energies. 

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. 

In addition to the neutral molecules that we have discussed so far, there are a number of monocyclic species that bear either a positive or a negative charge. Some of these ions show unexpected stabilities that surest that they are aromatic ions. Hiickel s rule is helpful in accounting for the properties of these ions as well. We shall consider two examples the cyclopentadienyl anion and the cycloheptatrienyl cation. 

PROBLEM 13.9 How many signals do the NMR spectra of the cyclopentadienyl anion and the tropylium cation show ... 

Look back again at the definition of aromaticity in the previous section "... a cyclic, conjugated molecule containing 4n + 2 tt electrons. Nothing in this definition says that the number of tt electrons mnst he the same as the number of atoms in the ring or that all the atoms in the ring mnst he carbon. In fact, both ions and heterocyclic compounds, which contain atoms of different elements in their ring, can also he aromatic. The cyclopentadienyl anion and the cycloheptatrienyl cation are perhaps the best known aromatic ions, while pyridine and pyrrole are common aromatic heterocycles. 

Other kinds of molecules besides benzene-like compounds can also be aromatic. The cyclopentadienyl anion and cycloheptatrienyl cation, for instance, are aromatic ions. Pyridine and pyrimidine are srx-memhered, nitrogen-containing, aromatic heterocycles. Pyrrole and imidazole are five-membered, nitrogen-containing heterocycles. Naphthalene, quinoline, indole, and many others are polycyclic aromatic compounds. 

To see why the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic, imagine starting from the related neutral hydrocarbons, 1,3-cyclo-pentadiene and 1,3,5-cycloheptatriene, and removing one hydrogen from the saturated CH2 carbon in each. If that carbon then rehybridizes from sp to sp, the resultant products would be fully conjugated, with a p orbital on every carbon. There are three ways in which the hydrogen might be removed. 

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