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Unformatted text preview: 1 6-3 continued (b) 16-5 Figure 1 6-8 shows that the fi rst 3 pairs of electrons are in three bonding molecular orbitals of cyclooctatetraene. Electrons 7 and 8, however, are located in two different nonbonding orbitals . As in cyclobutadiene, a planar c yc looctatetraene is predicted to be a diradical , a particular l y unstable electron configuration.
1l: Models show that the angles between p orbitals on adjacent b onds approach 90°. 1 6-7 (a) nonaromatic: internal hydrogens prevent planarity (b) nonaromatic: not al l atoms in the ri ng have a p orbital (c) aromatic: [l4]annulene (d) aromatic: also a [ 1 4]annulene in the outer ring: the internal alkene is not part of the aromatic system 16-8 Azulene satisfies all the criteria for aromaticity, and it has a Huckel number of 1l: e lectrons: 10. Both heptalene ( 1 2 1[ electrons) and pentalene (8 1l: e lectrons) are anti aromatic . 1 6-9 (a)
1l:6* 1l: 4 1l:2 1[3 1[7* 1[5 1l:s* (b ) n onbonding
�!1�!g.x 1l:6* 1l:4 1[2 1 HHH
1l:1 1 1l:7* 1l:s 1l:3 This electronic c onfiguration is antiaromatic. 3 42 ...
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