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CHAPTER 17 609 
 
17.7. 
(a) According to Figure 17.1, the conversion from 
cyclohexane to cyclohexene is uphill in energy (ΔH has a 
positive value). 
(b) According to Figure 17.1, the conversion from 
cyclohexene to benzene is uphill in energy (ΔH has a 
positive value). 
(c) According to Figure 17.1, the conversion from 
cyclohexadiene to benzene is downhill in energy (ΔH 
has a negative value). 
 
17.8. 
(a) This compound has six  bonds, for a total of twelve 
 electrons. Twelve is not a Hückel number, so the 
compound is not expected to be aromatic. 
(b) This compound has seven  bonds, for a total of 
fourteen  electrons. Fourteen is a Hückel number. In 
Section 17.5, we saw that [14]annulene is somewhat 
destabilized by a steric interaction between the hydrogen 
atoms positioned inside the ring. Although [14]annulene 
is nonplanar, it does indeed exhibit aromatic 
stabilization, because the deviation from planarity is not 
too great. 
(c) This compound has eight  bonds, for a total of 
sixteen  electrons. Sixteen is not a Hückel number, so 
the compound is not expected to be aromatic. 
 
17.9. We draw a circle and inscribe a triangle inside the 
circle, with one of the points of the triangle at the bottom 
of the circle. Each location where the triangle touches 
the circle represents an energy level. The energy level 
on the bottom of the circle is a bonding MO, and the two 
energy levels on top are antibonding MOs. The structure 
has two  electrons, which both occupy the bonding MO. 
This Frost circle indicates that there is only one bonding 
MO and it is filled, while the antibonding MOs are 
empty. This structure is expected to exhibit aromatic 
stabilization. 
 
 
17.10. This structure exhibits a ring of continuously 
overlapping p orbitals, and there are 22 π electrons (a 
Hückel number). Therefore, the compound is expected 
to exhibit aromatic stabilization. 
 
17.11. 
(a) This structure exhibits a ring of continuously 
overlapping p orbitals, but there are four  electrons, 
rendering the structure antiaromatic. This anion is 
expected to be very high in energy (very unstable). 
(b) This structure exhibits a ring of continuously 
overlapping p orbitals (C+ represents a carbon atom with 
an empty p orbital), and there are two  electrons, 
rendering the structure aromatic. 
(c) This structure exhibits a ring of continuously 
overlapping p orbitals, but there are four  electrons, 
rendering the structure antiaromatic. 
(d) This structure exhibits a ring of continuously 
overlapping p orbitals (the lone pair occupies a p orbital 
because it is resonance stabilized). There are ten  
electrons, rendering the structure aromatic. 
 
17.12. In order to predict whether this dianion is 
aromatic, we must determine if the following two criteria 
have been met: 
1. Does the compound contain a ring comprised of 
continuously overlapping p orbitals? 
2. Is there a Hückel number of  electrons in the ring? 
 
The lone pairs can occupy p orbitals providing for 
continuous overlap of p orbitals around the ring, so the 
first criterion has been met. To determine if the second 
criterion has been met, we must count the number of  
electrons. Each  bond counts as two electrons, and each 
lone pair counts as two electrons, for a total of 10  
electrons. Ten is a Hückel number, so the dianion is 
aromatic. All ten  electrons are thus completely 
delocalized around the eight-membered ring. 
 
17.13. 
(a) One of the lone pairs occupies a p orbital, thereby 
rendering the compound aromatic (six  electrons). The 
other lone pair will occupy an sp2 hybridized orbital, in 
the plane of the ring, extending away from the ring. 
(b) One of the lone pairs occupies a p orbital, thereby 
rendering the compound aromatic (six  electrons). The 
other lone pair will occupy an sp2 hybridized orbital, in 
the plane of the ring, extending away from the ring. 
(c) If the lone pair were to occupy a p orbital, there 
would be a continuous system of overlapping p orbitals 
with eight  electrons (antiaromatic). To avoid the 
instability associated with antiaromaticity, the lone pair 
is expected to occupy an sp3 hybridized orbital. 
(d) One of the lone pairs on the sulfur atom occupies a p 
orbital, thereby rendering the compound aromatic (six  
electrons). The lone pair on the nitrogen atom occupies 
an sp2 hybridized orbital that is in the plane of the ring 
(because the nitrogen atom is already using a p orbital to 
establish aromaticity). 
(e) There is only one lone pair (on oxygen) and it is not 
participating in aromaticity. That oxygen atom is already 
using a p orbital to establish aromaticity. 
(f) Each nitrogen atom has one lone pair, and neither is 
participating in aromaticity. In each case, the nitrogen 
atom is already using a p orbital to establish aromaticity. 
(g) The compound is not aromatic. In order to achieve a 
continuous system of overlapping p orbitals, each 
oxygen atom would need to contribute a lone pair in a p 
orbital, and that would give 8 π electrons (not a Hückel 
number). 
(h) One of the lone pairs on the oxygen atom occupies a 
p orbital, thereby rendering the compound aromatic (six 
 electrons). The lone pair on the nitrogen atom 
occupies an sp2 hybridized orbital that is in the plane of 
the ring (because the nitrogen atom is already using a p 
orbital to establish aromaticity). 
 
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