Cycloheptatrienyl cation from 1,3,5-trienes

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. 

Similar arguments can be used to predict the relative stabilities of the cyclo-heptatrienyl cation, radical, and anion. Removal of a hydrogen from cyclohepta-triene can generate the six-77-electron cation, the seven-77-electron radical, 01 the eight-77-elec iron anion (Figure 15.6). All three species again have numerous resonance forms, but HiickeTs rule predicts that only the six-7r-electron cyclohep-tatrienyl cation should be aromatic. The seven-77-electron cycloheptatrienyl radical and the eight-77-electron anion are antiaromatic. 

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