Cyclopentadienyl anion resonance structures

Write resonance structures for cyclopentadienyl anion suffi cient to show the delocalization of the negative charge over all five carbons J... 

Problem 15.6 Draw the five resonance structures of the cyclopentadienyl anion. Are all carbon-carbon bonds equivalent How many absorption lines would you expect to see in the lH NMR and, 3C NMR spectra of the anion ... 

Fio. 6. Molecular orbital and resonance structure representation of cyclopentadienyl anion. 

The peak potentials from the cyclic voltammetry of 2,1-cyclopentathiazine 46 were registered at 100 mV s in a 5x 10 M solution in DCM <2005JOC9314>. This material displayed a reversible reduction wave at —0.95 V, which is attributed to the stability of the delocalized cyclopentadienyl radical anion, as depicted in resonance structures 66 and 67 (Scheme 8). 

Azulene can be written as fused cyclopentadiene and cycloheptatriene rings, neither of which alone is aromatic. However, some of its resonance structures have a fused cyclopentadienyl anion and cycloheptatrienyl cation, which accounts for its aromaticity and its dipole moment of 1.0 D. 

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. 

A compound that can be described by several resonance forms has a structure that can be represented by no one form. The structure of the cyclopentadienyl anion is a hybrid of all of the above structures and contains only one kind of carbon atom and one kind of hydrogen atom. All carbon-carbon bond lengths are equivalent, as are all caibon-hydrogen bonds lengths. Both the H NMR and 13C NMR spectra show only one absorption. 

Deprotonation of 8 is readily achieved by treatment of 8 with Na[Et3BH] in hexane. The sodium 2,4-dicarba-n/db-hexaborate(r) 9 has been characterised in solution by H, nB and 13C NMR, including various 1H 1 B double and 13C H,llB] triple resonance experiments and also in the solid state by X-ray structural analysis.13 In the solid state, 9 crystallises from toluene without solvent as a dimer in which the two sodium cations are imbedded into a cave formed by the ten basal ethyl groups of the two anions. There are numerous close contacts between the sodium cations and the ethyl groups the B-H-B hydrogen atoms are not involved in those contacts. The electron balance of the anion 9 is comparable to that of cyclopentadienyl anions. 

Although live equivalent resonance structures can be drawn for all three species, Huckel s rule predicts that only the six-ir-electron anion. shouldbe aromatic. The four-TT-electron cyclopentadienyl carbocation and tlic- five-7r-electron cyclopentadienyl radical are predicted to be unstable and antiaromatic. 

We can draw five equivalent resonance structures for the cyclopentadienyl anion, delocalizing the negative charge over every carbon atom of the ring. 

Although five resonance structures can also be drawn for both the cyclopentadienyl cation and radical, only the cyclopentadienyl anion has six ji electrons, a number that satisfies Huckel s rule. The cyclopentadienyl cation has four ji electrons, making it antiaromatic and especially unstable. The cyclopentadienyl radical has five ji electrons, so it is neither aromatic nor antiaromatic. Having the right number of electrons is necessary for a species to be unusually stable by virtue of aromaticity. 

In some cases, a resonance structure is required to see an aromatic system. The increased stability associated with an aromatic system is found for the structure, although the compounds do not appear aromatic unless the resonance structure is considered. Azulene, which can be drawn as a cyclopentadienyl anion fused to a cycloheptatriene cation, and cyclopropenone, which can be written as possessing a cyclopropenyl cation, are two examples (see margin). 

All the potential products formed by removing a hydrogen from cyclo-penta-1,3-diene and from cyclohepta-l,3,5-triene can be drawn with numerous resonance structures, hut only the six-Tr-elecfron cyclopentadienyl anion and cycloheptatrienyl cation are predicted by the 4n -1- 2 rule to be aromatic (Figure 9.3). 

Write the resonance structures of the cyclopentadienyl anion to show how the negative charge can be delocalized over five carbon atoms. 

Examination of the resonance structures that can be drawn for the SNAr intermediate in the first case reveals that the five-membered ring has cyclopentadienyl anion character. This is a stabilizing feature due to aromaticity. 

It is very common for begiiming students to say that resonance structures that look like mirror images of each other (or rotated versions of one another) are the same, redundant, and therefore it is not necessary to draw them. This is incorrect. For example, in the last two CTQ s of this activity, the five resonance structures of cyclopentadienyl anion look like the same resonance structure, just rotated 1/5 of a turn each time, BUT all five resonance structures are unique and necessary for a complete picture of the ion. This seeming contradiction comes about because the drawing of resonance structures is a thought experiment in which you claim to be able to pin down the molecule in space and identify individual atoms even if they are otherwise identical. 

In the cartoon below the resonance structure on the right sacrifices one "cyclopentadienyl anion" to generate the fluorenyl carbanion hence, there should be little net loss in aromatidty. This "no net loss of aromaticity" has recently been exploited by Marder and coworkers in designing NLO-phores with large h3q>erpolarizabilities. ... 

---