CHM 2211C
6th edition Notes
Chapter 15
Benzene and Aromaticity
By
Dr. Andrea Wallace
Coastal Georgia Community College
Edited by
John T. Taylor
Florida Community College at Jacksonville
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Chapter 15 Benzene and Aromaticity
____________________ - word first used to describe fragrant substances such as
benzaldehyde (from cherries, almonds, and peaches.)
____________________ are actually benzene and its derivatives and are grouped
together due to their chemical behavior. (The association of aromaticity and fragrance is
now lost.)
See p. 499 for more examples of aromatics.
15.1 Sources of Aromatic Hydrocarbons, p. 499
Simple aromatic hydrocarbons have two main sources - _______________ and
_____________________.
1) Coal – when coal is heated at 1000 oC without air, a mixture of volatile products
called coal tar is released. Fractional distillation of coal tar yields a variety of
aromatic compounds.
2) Petroleum – few aromatics found in petroleum. However, alkanes which are plentiful
in petroleum can be transformed into aromatics via dehydrogenation and cyclization
reactions.
See examples on p. 499
15.2 Naming Aromatic Compounds, p. 500
Large # of nonsystematic names. This is usually discouraged by IUPAC, but in this case
some are retained.
See p. 500
Monosubstituted - some common names –toluene, phenol, etc. and then others are more
obvious – bromobenzene, ethylbenzene, etc.
Alkyl substituted benzenes are sometimes called arenes. The name is based on the length
of the alkyl group.
Use phenyl group when the alklyl portion of the molecule has > 6 carbons.
Phenyl
Benzyl
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Disubstituted Benzenes use prefixes - o (ortho), m (meta), and p (para)
1,2
ortho
1,3
meta
1,4
para
o-dichlorobenzene
m-chlorphenol
Examples:
p-xylene
(Useful since reactions may prefer one position vs. another)
Benzenes with more than two substituents – use lowest #’s and then alphabetize. May
end with benzene or parent name.
1,2-dichloro-4-nitrobenzene
2-bromo-3-chlorophenol
Problem 15.1. p. 502
Tell whether the following compounds are ortho, meta, or para disubstituted.
Problem 15.2, p. 502
Give IUPAC names for the following compounds.
a.
c.
d.
e.
f.
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Problem 15.3, p. 502
Draw structures corresponding to the following IUPAC names:
a) p-bromochlorobenzene
b) p-bromotoluene
c) m-chloroaniline
d) 1-chloro-3,5-dimethylbenzene
15.3 Structure and Stability of Benzene, p. 502
Aromatics do not undergo alkene addition reactions.
Aromatics undergo substitution reactions.
Example:
--Fe
catalyst
Benzene
Bromobenzene
(substitution pdt)
Substitution reactions allow for retention of the conjugation of the benzene ring.
The benzene ring is very stable, more stable than what would be expected for the
hypothetical “cyclohexatriene”.
See Figure 15.2 on p. 503.
Evidence for this unusual stability is seen in the Hhydrogenation data. (Energy released
upon addition of H2.) The less energy released, the more stable the compound.
Cyclohexene  Cyclohexane
Hhydrogenation = - 118 kJ/mol
Benzene  Cyclohexane
Predicted Hhydrogenation = - 118 x 3(approx) =
- 356 kJ/mol
Actual Hhydrogenation = - 206 kJ/mol
Difference is nearly 150 kJ/mol
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Actual value is about 150 kJ/mol less than expected.
More evidence for stability – all bond lengths are equal in benzene. They are 139 pm.
(C-C is 154 pm and C=C is 134 pm)
Bond order for C-C bonds and benzene is 1.5
Benzene is a resonance hybrid of two equivalent resonance forms.
Figure:
Pi electron delocalization brings about great stability.
15.4 Molecular Orbital Description of Benzene, p. 504
See Figure 15.3, p. 505
Molecular orbital description emphasizes cyclic conjugation and 6 C-C bonds in benzene.
All C-C-C bonds are 120o, sp2, with p orbitals perpendicular to the plane of the six
membered ring.
Electrons are delocalized over the entire ring. Benzene’s pi electron system could be
described as 2 doughnut shaped clouds – one above and one below.
15.5 Aromaticity and the Huckel 4n+2 Rule, p. 505
Review Benzene’s aromaticity and then extend to other molecules.
1)
2)
3)
4)
5)
cyclic conjugated molecule
Hhydrogenation is 150 kJ/mol less than expected
planar, hexagon, bond angle of 120o, C-C bond lengths of 139 pm
Undergoes substitution reactions that retain cyclic conjugation
Forms a resonance hybrid
More information is necessary to determine the aromaticity of other molecules.
Huckel – a German physicist developed a 4n+2 Rule
To be aromatic, a compound must be a planar, cyclic system of conjugation with a p
orbital on each atom and the p orbital system must contain 4n+2  electrons.
n = 0, 1, 2, etc.
#  electrons = 2, 6, 10, 14, 18, etc.
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Planar, conjugated, 4n  electron systems are said to be __________________ delocalization of pi electrons would lead to an increase in energy. (Destabilized by
delocalization of pi electrons.)
1) Cyclobutadiene (____ pi electrons)
Extremely reactive – no aromaticity. Synthesized in 1965 but could not be isolated at
– 78 oC it will dimerize by a __________________________ Reaction.
2) Benzene
4n+2 =?
3) Cyclooctatetrene (See p. 507)
4n + 2 = ?
Not Aromatic, first prepared in 1911. Reacts with HCl, Br2, KMnO4 – just like other
alkenes. Has no cyclic conjugation. Why? It is tub-shaped rather than planar. It
lacks proper alignment for p orbitals to overlap. Pi electrons are localized into 4
discrete double bonds.
X-ray studies show C-C at 147 pm and C=C at 134 pm.
Problem 15.5, p. 507
To be aromatic, a molecule must have 4n+2 pi electrons and must have cyclic
conjugation. The cyclodecapentaene shown below in a stereo view fulfills one of these
criteria but not the other, and has resisted all attempts at synthesis. Explain.
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15.6 Aromatic Ions, p. 507
Broad applications – only constraints – 4n+2 pi electrons, cyclic, planar, conjugated p
orbital on each Carbon.
Cyclopentadienyl anion
Cyclopentadiene
Cation
___ pi electrons
Radical
___ pi electrons
Anion
___ pi electrons
_____________
______________
_______________
Most hydrocarbons have a pKa > 45 indicating that they are _________ acids.
However, cyclopentadiene has a pKa = 16 (similar to water)
Why?
Cycloheptatrienyl cation
Cycloheptatriene
Cation
___ pi electrons
Radical
___ pi electrons
Anion
___ pi electrons
_____________
______________
_______________
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Problem 15.6, p. 510
Draw the five resonance structures of the cyclopentadienyl anion. Are all carbon-carbon
bonds equivalent? How many absorption lines would you expect to see in the 1H NMR
and 13C NMR of the anion?
Problem 15.7, p. 510
Cyclootatetraene readily reacts with potassium metal to form cyclooctatetraene dianion,
C8H82-. Why do you suppose this reaction occurs so easily? What geometry do you
expect for the cyclooctatetraene dianion?
15.7
Aromatic Heterocycles: Pyridine and Pyrrole, p. 510
Heterocycles can also be aromatic. (It is not necessary for all atoms in conjugation to be
Carbon atoms.)
Heterocycle – ring with one or more atoms other than carbon like N, O, S, P, etc.
Pyridine is aromatic.
____ pi electrons
N’s one pairs are in a sp2 orbital perpendicular to the ring system. They are not involved
with the pi system since they do not have the correct alignment.
Figure of N’s hybridization:
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Pyrrole is aromatic
___ pi electrons
Figure of N’s hybridization:
Differences:
N in pyridine is double bonded and contributes only 1 pi electron (like a C in a benzene).
N in pyrole is not double bonded and contributes 2 pi electrons (like in the
cyclopentadienyl anion).
Problem 15.8, p. 512
Draw an orbital picture of furan to show how the molecule is aromatic.
Problem 15.9, p. 512
The aromatic five-membered hetereocycle imidazole is important in many biological
processes. One of its nitrogen atoms is pyridine like in that it contributes one pi electron
to the aromatic sextet, and the other nitrogen is pyrrole-like in that it contributes two pi
electrons. Draw an orbital picture of imidazole and account for its aromaticity. Which
nitrogen atom is pyridine like and which is pyrole like? Which nitrogen atom is more
electron rich, and why?
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15.8
Why 4n + 2?, p. 512
See Figure 15.10, p. 513 - Full Molecular orbital of benzene – no electrons in the
antibonding orbitals, bonding orbitals are full – very stable.
See Figure 15.11, p. 513 – Cyclopentadienyl Anion –(Full bonding, none in antibonding)
Problem 15.10, p. 513
Show the relative energy levels of the seven pi molecular orbitals of the cycloheptatrienyl
system. Tell which of the seven orbitals are filled in the cation, radical, and anion, and
account for the aromaticity of the cycloheptatrienyl cation.
15.9
Polycyclic Aromatic Compounds: Naphthalene, p. 514
PAH – polycyclic aromatic compounds
See p. 514
PAH can be represented by many resonance structures.
Resonance structures for Naphthalene
Naphthalene has 250 kJ/mol aromatic stabilization energy
Reaction:
(Substitution pdt – aromaticity is retained.)
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Naphthalene has ________ pi electrons – which is a Huckel # - high degree of electron
delocalization.
Problem 15.11, p. 515
Azulene, a beautiful blue hydrocarbon is an isomer of naphthalene. Is azulene aromatic?
Draw a second resonance form of azulene in addition to that shown.
Problem 15.12, p. 515
Naphthalene is sometimes represented with circles in each ring to represent aromaticity:
How many pi electrons are in each circle?
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15.10 Spectroscopy of Aromatic Compounds, p. 515
Infrared of Aromatic Compounds
C-H stretch at 3030 cm-1 (low intensity)
Characteristic series of peaks at 1450 – 1600 cm-1 (as many as four)
Usually two bands at 1500 and 1600 cm-1 that are most intense.
C-H out-of-plane bending at 690 – 900 cm-1 which is related to the substitution pattern
(mono or di – ortho, meta, or para)
See p. 515,
See Figure 15.13, IR spectrum of Toluene on p. 516
UV of Conjugated Pi System
Strong absorbance near 205 nm, less intense absorbance at 255 – 275 nm.
Their presence in an unknown is a sign of an aromatic ring.
NMR
Aromatic H’s are strongly deshielded by the ring and have a signal at 6.5 – 8.0 .
The nonequivalent aromatic protons on substituted rings often couple with each other
giving rise to spin-spin splitting patterns that give information about the substitution
pattern.
Difference in chemical shifts
Aromatic H+ (6.5 – 8.0 ) vs. Vinylic H+ (4.5 – 6.5 )
Aromatic protons are more deshielded and their signals occur further down field due to
ring current. Figure 15.14, p. 516 illustrates the electron delocalization that occurs.
Different effects are seen inside and outside of a ring.
Consider [18]Annulene on p. 517.
Inside H: -3.0  - shielded
Outside H: 9.3  - deshielded
Presence of the ring current s characteristic of all Huckel aromatic compounds. (Good
test for aromaticity)
Aromatic Benzene has a signal at 7.37  in the proton NMR.
Nonaromatic cyclooctatetraene has a signal at 5.78  in the proton NMR – like an alkene
signal.
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Benzylic protons (2.3- 3.0 ) absorb downfield to other alkanes (0.7 – 1.3 ).
See example on p. 519, Figure 15.15 (p-bromotoluene)
Match signals to structure.
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C NMR
See absorptions at 110 –140  . Same range as alkenes.
Examples on p. 518, Figure 15.16.
See Summary Table on p. 518.
See p. 519 and 520, Aspirin, NSAID’s, COX-2 Inhibitors
NSAID is an abbreviation for __________________________________________
They are used for minor pains and inflammation.
The most common NSAID is _____________________.
It’s precursor salicin can be isolated from the bark of a ______________ tree.
Salicin can be coverted to Salicyl Alcohol by hydrolysis and then oxidized into Salicylic
Acid. Salicylic Acid is a highly effective fever reducer (_________________), pain
reliever, (_________________), swelling reducer (___________________), and blood
thinner (________________). However, it is highly corrosive to the walls of the
stomach. Conversion of the phenol – OH into an acetate ester yields acetylsalicylic acid
which is just as potent but less corrosive to the stomach.
See p. 519
Aspirin can have some serious side effect including stomach ______________, allergic
reactions, and a condition called ________________________ can result when aspirin is
given to a child recovering from the flu.
Additional NSAID’s have been developed including ________________ (Advil, Motrin,
and Nuprin) and ________________ (Naprosyn and Aleve). Both have a similar activity
to aspirin, however, Ibuprofen is less corrosive to the stomach and Naproxen remains
active in the body 6 times longer.
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NSAID’s function by blocking the cyclooxygenase (COX) enzymes that synthesize
prostaglandins. There is a COX-1 (responsible for normal physiological production of
prostaglandins) and a COX-2 (responsible for the body’s response to arthritis and other
inflammatory conditions). The NSAID’s already mentioned block both COX-1 and
COX-2 thereby shutting down the response to inflammation and various protective
functions such as the control mechanism for the production of acids in the stomach.
_______________ (from Monsanto) and ________________ (from Merck) have recently
been developed and marketed as COX-2 inhibitors and are used for the treatment of
arthritis and other inflammatory conditions.
See p. 520.
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