Chapter 16
Aromatic Compounds

16-00-03UN

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Title
Resonance in Benzene
Caption
Benzene is actually a resonance hybrid between the two Kekule structures. This representation implies that the pi electrons are delocalized, with a bond order of 1.5 between adjacent carbon atoms. The carbon-carbon bond lengths in benzene are shorter than typical single-bond lengths, yet longer than typical double-bond lengths.
Notes
Benzene's resonance can be represented by drawing a circle inside the six memebred ring as a combined representation.
Keywords
Kekule, benzene, delocalization
16-01

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Title
Orbital Representation of Benzene
Caption
Figure 16-1 Benzene is a flat ring of sp2 hybrid carbon atoms with their unhybridized p orbitals all aligned and overlapping. The carbon-carbon bond lengths are all 1.397 A, and all the bond angles are exactly 120o.
Notes
The conjugation and delocalization of the electrons in benzene gives this compound greater stability than non-conjugated cycles. The term aromatic compound is used to describe a cyclic compound with conjugated double bonds and that is stabilized by resonance.
Keywords
benzene, overlap, aromatic
16-02

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Title
Molar Heats of Hydrogenation
Caption
Figure 16-2 The molar heats of hydrogenation and the relative energies of cyclohexene, 1,4-cyclohexadiene, 1,3-cyclohexadiene, and benzene.
Notes
Benzene does not have the predicted heat of hydrogenation of -85.8 kcal/mol. The observed heat of hydrogenation is -49.8 kcal/mol, a difference of 36 kcal. This difference between the predicted and the observed value is called the resonance energy.
Keywords
heat of hydrogenation, resonance energy
16-03-01UN

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Title
Annulenes
Caption
Hydrocarbons with alternating double and single bonds are called annulenes. For example, benzene is a six-membered annulene, so it can be named [6]-annulene. Cylobutadiene is [4]-annulene, cyclooctatetraene is [8]-annulene.
Notes
For a compound to be aromatic it must be a cycle with conjugated double bonds, it must be planar to allow the p orbitals to overlap and it must have (4N+2) number of pi electrons. Cyclobutadiene and cyclooctatetraene are not aromatic.
Keywords
annulenes, aromatic
16-04

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Title
Molecular Orbitals of Benzene
Caption
Figure 16-4 The six p molecular orbitals of benzene viewed from above. The number of nodal planes increases with energy, and there are two degenerate MOs at each intermediate energy level.
Notes
The first MO is the lowest in energy. The electrons in the orbitals are delocalized over all six carbon atoms.
Keywords
molecular orbitals, nodal planes, degenerate MOs
16-05

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Title
Energy Diagram for the MOs of Benzene
Caption
Figure 16-5 Energy diagram of the molecular orbitals of benzene. Benzene's six pi electrons fill the three bonding orbitals, leaving the antibonding orbitals vacant.
Notes
The configuration with all the bonding MOs filled is energetically very stable.
Keywords
bonding orbitals, antibonding orbitals
16-08

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Title
Energy Diagram for the MOs of Cyclobutadiene, Benzene , and Cyclooctatetraene
Caption
Figure 16-8 The polygon rule predicts that the MO energy diagram for these annulenes will resemble the polygonal shapes of the annulenes.
Notes
Only benzene has all the bonding MOs filled.
Keywords
polygon rule
16-09

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Title
Derivation of Huckel's Rule
Caption
Figure 16-9 In a cyclic conjugated system, the lowest-lying MO is filled with two electrons. Each of the additional shells consists of two degenerate MOs, with space for four electrons. If a molecule has (4N + 2) pi electrons, it will have a filled shell. If it has 4N electrons, there will be two unpaired electrons in two degenerate orbitals.
Notes
To be aromatic the compound has to be cyclic, with conjugated double bonds, planar, and all its shells much be filled. Aromatic compounds have (4N + 2) electrons, while antiaromatic compounds have only 4N electrons.
Keywords
shell, Huckel's rule
16-09-02UN

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Title
Cyclopentadienyl Ions
Caption
We can draw a five-membered ring of sp2 hybrid carbon atoms with all the unhybridized p orbitals lined up to form a continuous ring. With five pi electrons, this system would be neutral, but it would be a radical because of an odd number of electrons cannot all be paired. With four pi electrons Huckel's rule predicts the system to be antiaromatic. With six pi electrons, Huckel's rule predicts aromaticity.
Notes
The cyclopentadienyl anion (six pi electrons) can be easily formed by abstracting a proton from cyclopentadiene with a base.
Keywords
cyclopentadienyl cation, cyclopentadienyl anion, Huckel's rule
16-09-03UN

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Title
Deprotonation of Cyclopentadiene
Caption
Cyclopentadiene is unusually acidic because loss of a proton converts the nonaromatic diene to the aromatic cyclopentadienyl anion.
Notes
By deprotonating the sp3 carbon of the -CH2-, the electrons in the p orbitals can be delocalized over all five carbon atoms.
Keywords
cyclopentadienyl anion, deprotonation
16-09-06UN

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Title
Cyclopentadienyl Cation
Caption
Huckel's rule predicts that the cyclopentadienyl cation, with four pi electrons, is antiaromatic. In agreement with this prediction, the cyclopentadienyl cation is not easily formed.
Notes
When 2,4-cyclopentadienol is treated with sulfuric acid, it does not dissociate into water and the cyclopentadienyl cation. The cation is not stable so it does not form.
Keywords
cyclopentadienyl cation
16-09-07UN

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Title
Resonance Forms of Cyclopentadienyl Ions
Caption
With conjugated systems such as cyclopendadienyl cation and anion, the resonance approach is a poor predictor of stability. The Huckel rule, based on molecular orbital theory, is a much better predictor of stability for these aromatic and antiaromatic systems.
Notes
The resonance pictures suggests that both ions are stable. Only cyclopentadienyl anion is aromatic.
Keywords
cyclopentadienyl ions
16-09-10UN

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Title
Cycloheptatrienyl Cation
Caption
The cycloheptatrienyl cation is easily formed by treating the corresponding alcohol with dilute (0.01N) aqueous sulfuric acid.
Notes
The cycloheptatrienyl cation is commonly known as the tropylium ion.
Keywords
cycloheptatrienyl cation
16-09-13UN

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Title
Cyclooctatetraene Dianion
Caption
Dianions of hydrocarbons are rare and are usually much more difficult to form. Cyclooctatetraene reacts with potassium metal, however, to form an aromatic dianion.
Notes
Cyclooctatetraene is antiaromatic but cyclooctatetraene, with 10 electrons, is aromatic (4N + 2, N = 2).
Keywords
cyclooctatetraene
16-10

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Title
Pyridine Pi System
Caption
Figure 16-10 Pyridine has six delocalized electrons in its pi system. The two non-bonding electrons on nitrogen are in an sp2 orbital, and they do not interact with the pi electrons of the ring.
Notes
The lone pair of electrons will not have any effect on the aromaticity of pyridine.
Keywords
pyridine
16-11

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Title
Protonation of Pyridine
Caption
Figure 16-11 Pyridine is basic, with non-bonding electrons available to abstract a proton. The protonated pyridine (a pyridinium ion) is still aromatic.
Notes
The nonbonding electrons are in an sp2 orbital perpendicular to the plane of the ring and do not interact with the electrons of the aromatic system.
Keywords
pyridinium ion, pyridine
16-12

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Title
Pyrrole Pi System
Caption
Figure 16-12 The pyrrole nitrogen atom is sp2 hybridized with a lone pair of electrons in the p orbital. This p orbital overlaps with the p orbitals of the carbon atoms to from a continuous ring.
Notes
Pyrrol is aromatic because it has 6 pi electrons (N = 1).
Keywords
pyrrol
16-13

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Title
Protonation of Pyrrole
Caption
Figure 16-13 The pyrrole nitrogen atom must become sp3 hybridized to abstract a proton. This eliminates the unhybridized p orbital needed for aromaticity.
Notes
Protonation yields a nonaromatic compounds so the protonation will not occur on the nitrogen. Pyrrole can be protonated by strong acids on C2 rather than at the nitrogen.
Keywords
pyrrole
16-14

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Title
Pyrrole, Furan, and Thiophene
Caption
Figure 16-14 Pyrrole, furan, and thiophene are isoelectronic. In furan and thiophene, the pyrrole N-H bond is replaced by a nonbonding pair of electrons in the sp2 hybrid orbital.
Notes
All these species are aromatic. Furan and thiophene have two pairs of nonbonding electrons but only one pair is in the unhybridized p orbital and is able to overlap with the carbons of the ring.
Keywords
furan, thiophene
16-14-08UN

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Title
Naphthalene
Caption
Naphthalene is the simplest fused aromatic hydrocarbon, consisting of two fused benzene rings.
Notes
There are a total of 10 pi electrons in naphthalene giving it a resonance energy of 60 kcal/mol.
Keywords
naphthalene, fused rings
16-14-09UN

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Title
Anthracene and Phenanthrene
Caption
As the number of aromatic ring increases, the resonance energy per ring continues to decrease and the compounds become more reactive.
Notes
Anthracene and phenanthrene can undergo reactions that are more characteristic of nonaromatic polyene units.
Keywords
anthracene, phenanthrene
16-14-13UN

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Title
Carcinogenic Effect of Benzo[a]pyrene
Caption
Benzo[a]pyrene, one of the most thoroughly studied carcinogens, is formed whenever organic compounds undergo incomplete combustion.
Notes
Its carcinogenic effect appears to result from its epoxidation to arene oxides, which can be attacked by nucleophilic sites of DNA. The resulting DNA derivatives cannot be properly transcribed; on replication they produce mutations.
Keywords
Benzo[a]pyrene, DNA, replication, mutation, carcinogen, combustion
16-16

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Title
Diamond and Graphite
Caption
Figure 16-15 Structures of diamond and graphite. Diamond is a lattice of tetrahedral carbon atoms linked in a rigid three-dimensional array. Graphite consists of planar layers of fused aromatic rings.
Notes
Diamond is the hardest naturally occuring substance known. Graphite is slightly more stable than diamond.
Keywords
diamond, graphite
16-17

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Title
Fullerenes
Caption
Figure 16-16 Structure of C60 amd a carbon nanotube. Each carbon in C60 is a bridgehead carbon for a five-membered ring and two six-membered rings.
Notes
C60 is commonly know as buckyball, a short form of its full name buckminsterfullerene.
Keywords
buckminsterfullerene, buckyball, fullerenes
16-17-03UN1-3

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Title
Ortho, Para, and Meta Derivatives of Benzene
Caption
Disubstituted benzenes are named using the prefixes ortho-, meta-, and para- to specify the substitution patterns. These terms are abbreviated o-, m-, and p-.
Notes
Numbers can also be used to identify the relationship between the groups; ortho- is 1,2-disubstituted, meta- is 1,3, and para- is 1,4.
Keywords
ortho-, meta-, para-, substitution pattern
16-18

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Title
Mass Spectrometry of Alkylbenzene Derivatives
Caption
Figure 16-18 The mass spectrum of n-butylbenzene has its base peak at m/z = 91, corresponding to cleavage of a benzylic bond. The fragments are a benzyl cation and a propyl radical. The cation rearranges to the tropylium ion, detected at m/z = 91.
Notes
Alkylbenzenes frequently give ions corresponding to the tropylim ion.
Keywords
tropylium ion, alkylbenzene, benzyl cation, rearrangement
16-19

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Title
Ultraviolet Spectra of Benzene and Styrene
Caption
Figure 16-19 Ultraviolet spectra of benzene and styrene
Notes
The most characteristic part of the spectrum is the band centered at 254 nm, called the benzenoid band. About three to six small, sharp peaks (called fine structure) usually appear in this band.
Keywords
benzenoid band, ultraviolet spectroscopy, fine structure

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