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Chapter 15
Aromaticity

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15-00CO
Title
Molecules of Some Simple Aromatic Compounds
Caption
Potential maps of molecules of benzene, pyrrole, and pyridine.
Notes
Aromatic compounds need to have 4n+2 pi electrons in the ring system. Benzene has 6 pi electrons (n = 1) so it is aromatic. In order for pyrrole to have 6 pi electrons it needs to contribute the lone pair of electrons on its nitrogen to the aromatic ring system. In order for pyridine to have only 6 pi electrons it needs to not contribute the lone pair electrons on its nitrogen to the aromatic ring system. This results in pyrrole's nitrogen lone pair being more available for bonding to electrophiles (i.e., protons) than pyridine's lone pair. Pyrrole is therefore more basic than pyridine.
Keywords
potential, maps, benzene, pyrrole, pyridine
15-01
Title
Figure 15.1
Caption
Formation of pi molecular orbitals from carbon's p atomic orbitals in benzene, and an electrostatic potential map of benzene.
Notes
Every (carbon) atom in the ring path in benzene contributes a p atomic orbital to develop the pi molecular orbital system which resides above and below the plane of the atoms in the ring. An electrostatic potential map of benzene shows that the electrons in the pi system are evenly distributed around the ring.
Keywords
figure, 15.1, p, pi, orbitals, benzene, potential, map
15-01-06
Title
Buckminsterfullerene
Caption
Ball-and-stick model of the C60 form of buckminsterfullerene.
Notes
Buckminsterfullerenes are made of interlocking five- and six-membered rings containing only carbon atoms. The six-membered rings have alternating single and double bonds connecting the carbons in each six-membered ring. Since these rings are bent out of planarity by the shape of the molecule, the double bonds in the six-membered rings do not mix their pi electrons to form an aromatic system, and buckminsterfullerene shows chemical reactivity characteristic of alkenes rather than aromatic compounds.
Keywords
buckminsterfullerene, buckyball, C60
15-01-13.1UN
Title
Benzene, Pyridine, and Pyrrole
Caption
Potential maps of molecules of benzene, pyridine, and pyrrole.
Notes
Aromatic compounds need to have 4n+2 pi electrons in the ring system. Benzene has 6 pi electrons (n = 1) so it is aromatic. In order for pyrrole to have 6 pi electrons it needs to contribute the lone pair of electrons on its nitrogen to the aromatic ring system. In order for pyridine to have only 6 pi electrons it needs to not contribute the lone pair electrons on its nitrogen to the aromatic ring system. This results in pyrrole's nitrogen lone pair being more available for bonding to electrophiles (i.e., protons) than pyridine's lone pair. Pyrrole is therefore more basic than pyridine.
Keywords
potential, maps, benzene, pyrrole, pyridine
15-01-13.2UN
Title
Orbital Structure of Pyridine
Caption
Schematic of a pyridine molecule showing p atomic orbitals on the nitrogen and carbon atoms before the p orbitals merge to form a pi system.
Notes
The lone-pair electrons in pyridine are held in an sp2 hybrid orbital perpendicular to the orbitals in the pi system, so they are not part of the pi system.
Keywords
orbital, pyridine, structure
15-01-14.1UN
Title
Orbital Structures of Pyrrole and Furan
Caption
Schematic of pyrrole and furan molecules showing p atomic orbitals on the ring atoms before these p orbitals merge to form a pi system.
Notes
Pyrrole orients its nitrogen lone pair so that it becomes part of the developing pi system. Furan has two lone pairs on its oxygen atom. One of these becomes part of the ring pi system and the other is oriented perpendicular to the pi system in an sp2 hybrid orbital.
Keywords
orbital, structures, pyrrole, furan
15-01-23UN
Title
Antiaromaticity
Caption
Relative stabilities of aromatic, nonaromatic, and antiaromatic compounds.
Notes
Antiaromatic compounds have even numbers of electron pairs in planar ring systems. Since they are less stable than nonaromatic compounds, ring systems with more or less than 4n+2 pi electrons attempt to achieve nonplanar conformations so that the pi orbitals containing these electrons do not overlap. In cases where the architecture of a molecule forces a ring system to exist in a flat conformation with an antiaromatic number of pi electrons, an extremely unstable pi system results.
Keywords
antiaromaticity, antiaromatic, aromatic, nonaromatic, stability
15-04
Title
Figure 15.4
Caption
Reaction coordinate diagrams for electrophilic aromatic addition and electrophilic aromatic substitution.
Notes
Electrophilic aromatic addition yields products which are less stable than reactants because aromaticity is lost, whereas electrophilic aromatic substitution preserves aromaticity and yields products having stabilities comparable to those of reactants. For this reason aromatic compounds generally undergo substitution reactions instead of addition reactions.
Keywords
figure, 15.4, reaction coordinate, diagram, electrophilic, substitution, addition, benzene
15-05
Title
Figure 15.5
Caption
Reaction coordinate diagram for the sulfonation of benzene.
Notes
The first step is rate-determining in the forward reaction, whereas the second step is rate-determining in the reverse reaction. Since reactant and product have similar stabilities, this class of reaction can be run in either direction. Dry reaction conditions where HSO3+ concentrations are much higher than H3O+ concentrations favor the forward reaction, and wet reaction conditions favor the reverse reaction. The forward reaction is often run to introduce sulfonate as a blocking group, which is later removed by running the reverse reaction.
Keywords
figure, 15.5, reaction coordinate, diagram, sulfonation, benzene
15-05-40P30
Title
End-of-Chapter Problem 30
Caption
Proton NMR spectrum associated with end-of-chapter Problem 30.
Notes
The proton NMR spectrum shown results from obtaining the spectrum of the product of the reaction of benzene with a Friedel-Crafts acylation reagent followed by Clemmensen reduction of the acylation product. Determine the identity of the acyl chloride used in the Friedel-Crafts acylation reaction.
Keywords
end-of-chapter, problem, 30, NMR

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