Cycloheptatrienyl cation, aromaticity hydrogenation

Similar arguments can be used to predict the relative stabilities of the cycloheptatrienyl cation, radical, and anion. Removal of a hydrogen from cycloheptatriene can generate the six-vr-electron cation, the seven-ir-electron radical, or the eight-ir-electron anion (Figure 15.7, p. 572). Once again, all three species have numerous resonance forms, but Huckel s rule predicts that only the six-vr-electron cycloheptatrienyl cation should be aromatic. The seven-or-electron cycloheptatrienyl radical and the eight-or-electron anion are antiaromatic. 

As a hydride ion is ranoved from the —CH2— group of cycloheptatriene, a vacant p orbital is created, and the carbon atom becomes hybridized. The cation that results has seven overlapping p orbitals containing six delocalized tt electrons. The cycloheptatrienyl cation is, therefore, an aromatic cation, and all of its hydrogen atoms should be equivalent again, this is exactly what we find experimentally. 

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). 

FIGURE 9.3 The aromatic six-iT-electron cyclopentadienyl anion can be formed by removing a hydrogen ion (H" ) from the CH2 group of cyclopenta-i,3-diene. Similarly, the aromatic six-iT-electron cycloheptatrienyl cation can be generated by removing a hydride ion (H ) from the CH2 group of cyclohepta-i,3,5-triene. 

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. 

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. 

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