OpenStax Organic Chemistry: A Tenth Edition Student Solutions Manual
Chapter 15 – Benzene and Aromaticity
Solutions to Problems
15.1 An ortho disubstituted benzene has two substituents in a 1,2 relationship. A meta
disubstituted benzene has two substituents in a 1,3 relationship. A para disubstituted
benzene has two substituents in a 1,4 relationship.
(a) (b) (c)
15.2 Remember to give the lowest possible numbers to substituents on trisubstituted rings.
(a) (b) (c)
(d) (e) (f)
15.3
(a) (b) (c)
(d)
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15.4
The electronic descriptions of pyridine and benzene are very similar. The pyridine ring is
formed by the σ overlap of carbon and nitrogen sp2 orbitals. In addition, six p orbitals,
perpendicular to the plane of the ring, hold six electrons. These six p orbitals form six π
molecular orbitals that allow electrons to be delocalized over the π system of the
pyridine ring. The lone pair of nitrogen electrons occupies an sp2 orbital that lies in the
plane of the ring.
15.5
Cyclodecapentaene has 4n + 2 π electrons (n = 2), but it is not flat. If cyclodecapentaene
were flat, the starred hydrogen atoms would crowd each other across the ring. To avoid
this interaction, the ring system is distorted from planarity.
15.6
A compound that can be described by several resonance forms has a structure that can be
represented by no single form. The structure of the cyclopentadienyl anion is a hybrid of
all of the above structures and contains only one kind of carbon atom and one kind of
hydrogen atom. All carbon–carbon bond lengths are equivalent, as are all carbon–
hydrogen bonds lengths. Both the 1H NMR and 13C NMR spectra show only one
absorption.
15.7 When cyclooctatetraene accepts two electrons, it becomes a (4n + 2) π electron aromatic
ion. Cyclooctatetraenyl dianion is planar with a carbon–carbon bond angle of 135°, that
of a regular octagon.
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15.8 This diagram resembles Figure 15.6 but has one less antibonding orbital.
15.9 Furan is the oxygen analog of pyrrole. Furan is aromatic because it has 6 π electrons in
a cyclic, conjugated system. Oxygen contributes two lone-pair electrons from a p orbital
perpendicular to the plane of the ring.
15.10
The heterocyclic thiazolium ring contains six π electrons. Each carbon contributes one
electron, nitrogen contributes one electron, and sulfur contributes two electrons to the
ring π system. The thiazolium ion is aromatic because it has 6 π electrons in a cyclic,
planar, conjugated system.
15.11
Azulene is aromatic because it has a conjugated cyclic π electron system containing ten
π electrons (a Hückel number).
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Chapter 15 – Benzene and Aromaticity
Solutions to Problems
15.1 An ortho disubstituted benzene has two substituents in a 1,2 relationship. A meta
disubstituted benzene has two substituents in a 1,3 relationship. A para disubstituted
benzene has two substituents in a 1,4 relationship.
(a) (b) (c)
15.2 Remember to give the lowest possible numbers to substituents on trisubstituted rings.
(a) (b) (c)
(d) (e) (f)
15.3
(a) (b) (c)
(d)
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15.4
The electronic descriptions of pyridine and benzene are very similar. The pyridine ring is
formed by the σ overlap of carbon and nitrogen sp2 orbitals. In addition, six p orbitals,
perpendicular to the plane of the ring, hold six electrons. These six p orbitals form six π
molecular orbitals that allow electrons to be delocalized over the π system of the
pyridine ring. The lone pair of nitrogen electrons occupies an sp2 orbital that lies in the
plane of the ring.
15.5
Cyclodecapentaene has 4n + 2 π electrons (n = 2), but it is not flat. If cyclodecapentaene
were flat, the starred hydrogen atoms would crowd each other across the ring. To avoid
this interaction, the ring system is distorted from planarity.
15.6
A compound that can be described by several resonance forms has a structure that can be
represented by no single form. The structure of the cyclopentadienyl anion is a hybrid of
all of the above structures and contains only one kind of carbon atom and one kind of
hydrogen atom. All carbon–carbon bond lengths are equivalent, as are all carbon–
hydrogen bonds lengths. Both the 1H NMR and 13C NMR spectra show only one
absorption.
15.7 When cyclooctatetraene accepts two electrons, it becomes a (4n + 2) π electron aromatic
ion. Cyclooctatetraenyl dianion is planar with a carbon–carbon bond angle of 135°, that
of a regular octagon.
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, OpenStax Organic Chemistry: A Tenth Edition Student Solutions Manual
15.8 This diagram resembles Figure 15.6 but has one less antibonding orbital.
15.9 Furan is the oxygen analog of pyrrole. Furan is aromatic because it has 6 π electrons in
a cyclic, conjugated system. Oxygen contributes two lone-pair electrons from a p orbital
perpendicular to the plane of the ring.
15.10
The heterocyclic thiazolium ring contains six π electrons. Each carbon contributes one
electron, nitrogen contributes one electron, and sulfur contributes two electrons to the
ring π system. The thiazolium ion is aromatic because it has 6 π electrons in a cyclic,
planar, conjugated system.
15.11
Azulene is aromatic because it has a conjugated cyclic π electron system containing ten
π electrons (a Hückel number).
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