Chapter 16 – Aromaticity
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Structure and properties of benzene
o Formula C6H6
 Learn to recognize that C6H5 is a phenyl ring
 Very common to see it this way on the ACS
o Empirical formula CH
o Very, very stable
 The reactions of alkenes will not react with benzene unless there are
extremely reactive conditions (high pressure, temperature)
Heats of hydrogenation
"cyclohexatriene"
"expected" -85.5 kcal
-28.6 kcal
-57.4 kcal
actual - 49.8 kcal
difference - about 35 kcal of resonance energy
With “cyclohexatriene” you’re pretending that the “double bonds” are not even conjugated.
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Requirements of aromaticity
o It must be cyclic.
o All atoms in the ring must be sp2-hybridized
 This means that there is an unhybridized p orbital.
 This also means that the ring is completely conjugated.
o The ring must be planar.
o There must be 4n+2 pi electrons – Hückel’s Rule
 n has nothing to do with the structure, so don’t try to count something in
the structure to determine how many electrons you want involved.
 It just tells you that if 2, 6, 10, 14, etc. pi electrons are present in
the ring, you might have an aromatic system.
 When do I count a lone pair in this?
 Baker’s Rule – Count it when it helps, don’t count it when it
doesn’t
 If you just remember Baker’s Rule, you will miss out on the deeper
understanding that is often required on this test. You need to be
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looking at the hybridization of the given atom and understand
where the lone pair is.
For instance, if a nitrogen is already involved in a double bond,
then you know that the p orbital is already occupied. This means
that the lone pair is in an sp2 orbital and cannot be involved in the
pi system
Aromatic ions
o Carbocations have an empty p orbital
not aromatic
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aromatic
Why is 7-bromocyclohepta-1,3,5-triene water-soluble?
aromatic
not aromatic
o Carbanions can become sp2-hybridized if that makes it aromatic.
 This means the lone pair is in the p-orbital, so you count it.
 This is why cyclopentadiene is a particularly acidic hydrocarbon.
 The pKa is around 15!
NaOH
not aromatic
aromatic
o Boron with three bonds has an empty p-orbital.
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If any part of a molecule is aromatic, it is aromatic!!!
o This means that if you are circling the aromatic structures from a group of
molecules, a huge molecule with one phenyl group is indeed aromatic.
 Circle it!
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Aromatic compounds you need to know
(I’m not sure what flash cards are used for, but I think this might be one of the
times when they are useful)
o Pyrrole
 The lone pair is in a p orbital, so it is included in the pi system.
o Pyridine
 The lone pair is in an sp2 orbital, thus not involved in the pi system
o Pyrimidine
 Both lone pairs are in sp2 orbitals, thus not involved in the pi system
o Purine
 One lone pair is involved in the aromaticity.
o Furan
 One of the lone pairs of oxygen is sp2 and the other is in a p-orbital.
o Naphthalene
o Anthracene
o Benzene Derivatives
Phenol
Toluene
Aniline
NH2
Anisole
Styrene
Benzoic acid
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Benzaldehyde
Benzene sulfonic acid
Nomenclature of Benzene Derivatives
o If a substituted benzene has no functional group that gives it one of the common
names above, then name it as a substituted benzene.
bromobenzene
isopropyl benzene
ethoxybenzene
o Ortho, meta, and para positions
 When you have two and only two substituents, you can use the terms
ortho, meta, and para
 These terms tell you where two pieces are relative to one another
ortho
meta
para
o With three or more substituents, you need to go back to numbering just like you
would with other naming.
o Naming benzene derivatives where part of it has a common name
 If one of the substituents gives you a common name, then that is the
“parent.”
NH2
meta-chloroaniline
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When there are other substituents, count from the carbon whose
substituent gives the parent its name.
If you have two competing parents, then most likely you have a new
common name, for which you are not responsible.
 Ex. You wouldn’t have to name this.
OH
H2N
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Polarity of aniline (more fun with resonance)
There are two more resonance forms with the negative charge at the two ortho
positions as well.
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Review of acidity of phenols
o Phenol by itself has a pKa of 10.
o Every nitro group you add to an ortho or para position decreases the pKa by
about 3 log units.
Allotropes of carbon
o Diamond – all carbons sp3-hybridized
 Each diamond is essentially one huge molecule.
o Graphite – lattice of fused aromatic rings
 The layers are not covalently bonded, so they slide off each other easily.
o Bucky balls – balls of alternating six- and five-membered rings
 Named after Buckminster Fuller, the architect who was a big proponent
of geodesic domes in housing.
o Carbon nanotubes – very similar in structure to the Bucky ball
 Oh, the awesomeness of carbon nanotubes. Mmmmm.
 Possible solution to the space elevator?