Cyclopentadienyl cation, antiaromaticity

Two other systems that have been studied as possible aromatic or antiaromatic four-electron systems are the cyclopropenyl anion (59) and the cyclopentadienyl cation (60). In these cases also the evidence supports, antiaromaticity, not aromaticity. With respect to 59, HMO theory predicts that an unconjugated 61 (i.e., a single canonical form) is more stable than a conjugated 59, so that 61 would... 

Among the isomers of C4BH5 (157)-(160), borole is the most stable (Table XV), which may be attributed to its smaller antiaromaticity relative to the cyclopentadienyl cation. Similar to borabenzene, borole is apt to act as a strong Lewis acid complexing even with nitrogen thus structure... 

Cations structurally related to benzhydryl are anthracenyl (7)75 87 and fluorenyl (8).69 There has been some dispute as to whether or not the flve-membered ring of the fluorenyl cation is antiaromatic. Clearly the antiaromatic character is less than for the indenyl or cyclopentadienyl cations, but current opinion favors antiaromaticity also for the fluorenyl cation.90 92 This is supported by the large difference in pAR from the anthracenyl cation, (although an additional reason for this difference will be noted later in the chapter, p. 61). Again, the brackets indicate correction of a measurement from TFE H20 to water. 

Hiickel s rule predicts that the cyclopentadienyl cation, with four pi electrons, is antiaromatic. In agreement with this prediction, the cyclopentadienyl cation is not easily formed. Protonated cyclopenta-2,4-dien-l-ol does not lose water (to give the cyclopentadienyl cation), even in concentrated sulfuric acid. The antiaromatic cation is simply too unstable. 

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. 

Systems containing 4 n Jt-electrons (such as cyclobutadiene and the cyclopentadienyl cation) are antiaromatic. 

The El mass spectra of methyl-substituted cyclopentadienes were studied by Harrison and coworkers" and their fragmentation behaviour was found to be very similar to that of the isomeric cyclohexadienes. Major fragmentation paths were suggested to lead to protonated alkylbenzenes such as benzenium (CeKv+j and toluenium (C7H9+) ions. Obviously, formation of antiaromatic cyclopentadienyl cations is circumvented however, other isomers may also be formed along with ihe (most stable) arenium-type fragment ions (see below). Open-chain 1,3,5-hexatriene isomers were also found to give similar El mass spectra. 

Consult the literature to find discussions of the aromaticity or antiaromaticity of the following structures tropylium ion, cyclopentadienyl cation, COT dianion. Are the results consistent with familiar genercdizations about aromatic character ... 

PRACTICE PROBLEM 14.8 The cyclopentadienyl cation is apparently antiaromatic. Explain what this means in... 

It suggests that it is not the size of the ring but the number of electrons present in it determines whether a molecule would be aromatic or antiaromatic. In fact the molecules with An+ 2) n electrons are aromatic whereas with (An, 0) n electrons are antiaromatic. Thus, benzene, cyclopropenyl cation, cyclobutadiene dication (or dianion), cyclopentadie-nyl anion, tropylium ion, cyclooctatetraene dication (or dianion), etc. possess (4 + 2) ti electrons and hence aromatic whereas cyclobutadiene, cyclopentadienyl cation, cycloheptatrienyl anion, cyclooctatetraene (non-planar) etc. have An n electrons which make them antiaromatic . Systems like [10] annulene are forced to adopt a nonplanar conformation due to transannular interaction between two hydrogen atoms and hence their aromaticity gets reduced even if they have (An + 2)n electrons. On the other hand the steric constraints in systems like cyclooctatetraene force it to adopt a tube-like non-planar conformation which in turn reduces its antiaromaticity. Various derivatives of benzene like phenol, toluene, aniline, nitrobenzene etc. are also aromatic where the benzene ring and the n sextet are preserved. In homoaromatic " systems, like cyclooctatrienyl cation, delocalization does not extend over the whole molecule. 

In addition to neutral molecules, certain cation and anion intermediates meet the criteria for aromaticity. If the cyclopropenyl cation (115) and the cycloheptatrienyl cation (116) are examined, both have a continuous array of p-orbitals confined to a ring and a number of n-electrons that fit the 4n -i- 2 series (two for 115 and six for 117). Both of these carbocations are aromatic, which means that they are very stable, easy to form, and relatively long-lived intermediates. Compare these carbocations with the cyclopentadienyl cation (117), which meets the criterion of having a continuous array of p-orbitals confined to a ring, but has 4n ji-electrons (not a number in the 4n -i- 2 series) and is not aromatic. Indeed, it is considered to be antiaromatic, is very unstable, and is very difficult to form. 

Describe the ground-state electron configuration of the cyclopentadienyl cation and radical. Assuming each species is planar, would you expect it to be aromatic or antiaromatic ... 

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