Determining Aromaticity
Overview:
Aromaticity is special stability provided to a molecule upon possessing four specific qualities
mentioned below. To determine if a molecule is aromatic, investigate its structure for the qualifiers.
If all are present within the molecule, then it is aromatic.
Criteria for Aromaticity:
  bonds must lie within cyclic structure
 Each atom in the cycle must have p orbital, forming p orbital loop
 All p orbitals in the loop must overlap (planarity)
 Hückelʼs Rule: orbital arrangement must result in a lowering of energy.
4n + 2electrons (n is an integer: 0, 1, 2, 3, etc…) in the loop
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The stability of Benzene
 Benzene is much more stable than would be expected based on calculations for
“cyclohexatriene”.
 The prediction for the heat of hydrogenation of hypothetical cyclohexatriene is -360 kJ
mol-1. (3 times that of cyclohexene, -120 kJ mol-1)
 The experimentally heat of hydrogenation for benzene is -208 kJ mol-1, 152 kJ mol-1 more
stable than hypothetical cyclohexatriene. This difference is called the resonance energy.
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Experimental
data
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Model
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NMR Spectroscopy
Evidence for Electron Delocalization in Aromatic Compounds
When benzene is placed in a strong magnetic field a -electron ring current is
induced which reinforces the applied magnetic field at the location of the protons.
The net effect is that the protons of benzene are highly
deshielded (their signal is a singlet at 7.27) and this is
evidence for aromaticity.
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Non-benzenoid Aromatic Compounds
The Annulenes
 Annulenes are monocyclic compounds with alternating double and single bonds.
 Annulenes are named using a number in brackets that indicates the ring size
Ex. Benzene is [6]annulene and cyclooctatetraene.
 An annulene is aromatic if it has 4n+2 electrons and a planar carbon skeleton.
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Non-benzenoid Aromatic Compounds
The Annulenes
 The [10]annulenes below should be aromatic but none of them can be planar.
 4 is not planar because of steric interaction of the indicated hydrogens.
 5 and 6 are not be planar because of large angle strain in the flat molecules.
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Non-benzenoid Aromatic Compounds
Aromatic Ions
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Cyclopentadiene
• Cyclopentadiene is unusually acidic (pKa = 16) because it becomes the aromatic
cyclopentadienyl anion when a proton is removed.
• Cyclopentadienyl anion has 6 electrons in a cyclic, continuous electron system, and
hence follows the 4n + 2 rule for aromaticity.
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Cyclopentadiene
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Electron Delocalization of Cyclopentadienyl Anion
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Cycloheptatriene
• Cycloheptatriene is not aromatic because its  electrons are not delocalized around
the ring (the sp3-hybridized CH2 group is an “insulator”).
• Lose of hydride produces the aromatic cycloheptatrienyl cation (tropylium cation).
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Cyclooctatetraene
• Cyclooctatetraene is not aromatic because it is not planar and behave as a simple
cyclic polyene.
• Cyclotetraenyl dianion follows the 4n + 2 rule for aromaticity.
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Aromatic, Antiaromatic, and Nonaromatic Compound
A comparison of cyclic annulenes with their acyclic counterparts provides
a measure of the stability conferred by aromaticity.
• If the ring has lower -electron energy than the open chain, then it is aromatic.
• If the ring has the same -electron energy as the open chain, then it is nonaromatic.
• If the ring has higher -electron energy than the open chain, then it is antiaromatic.
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Aromatic, Antiaromatic, and Nonaromatic Compound
A comparison of cyclic annulenes with their acyclic counterparts provides
a measure of the stability conferred by aromaticity.
• If the ring has lower -electron energy than the open chain, then it is aromatic.
(4n+2)
• If the ring has the same -electron energy as the open chain, then it is nonaromatic.
• If the ring has higher -electron energy than the open chain, then it is antiaromatic.
4n 
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Other Aromatic Compounds
Benzenoid Aromatic Compounds
Polycyclic benzenoid aromatic compounds have two or more benzene rings fused together.
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Other Aromatic Compounds
Pyrene has 16  electrons, a non-Hückel number, yet is known to be aromatic.
Ignoring the central double bond, the periphery of pyrene has 14  electrons, a Hückel
number, and on this basis it resembles the aromatic [14]annulene.
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Other Aromatic Compounds
Fullerenes
Buckminsterfullerene is a C60 compound shaped like a soccer ball with interconnecting
pentagons and hexagons, and each carbons is sp2 hybridized and bonds to 3 other
carbons. It is aromatic.
C60
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C70
Heterocyclic Aromatic Compounds
Pyridine has an sp2 hybridized nitrogen
•
•
•
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The p orbital on nitrogen is part of the aromatic  system of the ring.
The nitrogen lone pair is in an sp2 orbital orthogonal to the p orbitals of the ring; these
electrons are not part of the aromatic system.
The lone pair on nitrogen is available to react with protons and so pyridine is basic.
Heterocyclic Aromatic Compounds
Pyridine has an sp2 hybridized nitrogen
•
•
•
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The p orbital on nitrogen is part of the aromatic  system of the ring.
The nitrogen lone pair is in an sp2 orbital orthogonal to the p orbitals of the ring; these
electrons are not part of the aromatic system.
The lone pair on nitrogen is available to react with protons and so pyridine is basic.
Heterocyclic Aromatic Compounds
The nitrogen in pyrrole is sp2 hybridized and the lone pair resides in the p orbital.
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•
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This p orbital contains two electrons and participates in the aromatic system.
The lone pair of pyrrole is part of the aromatic system and not available for protonation;
pyrrole is therefore not basic.
Pyrrole
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Heterocyclic Aromatic Compounds
In furan and thiophene an electron pair on the heteroatom is also in a p orbital
which is part of the aromatic system.
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Furan
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Frostʼs Circle
A graphic method for determining the relative order of  MOs in planar,
fully conjugated monocyclic compounds.
1. Draw a circle of radius 2 and centered at .
2. Inside the circle, inscribe the regular polygon corresponding to the molecule of interest
One vertex must be pointed downward.
All vertices must just touch the circle.
3. The energy levels of the system are now given by the points at which the line drawing
touches the circle.
4. Orbitals with negative energies are bonding; those with positive energies are antibonding.
5. Orbitals of zero (or ) energy are non-bonding.
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Frostʼs Circle
The diagram on the left represents benzene. The six -electrons then may be placed in the
orbitals with spins paired, beginning with the lowest. The six electrons just fill all the bonding
orbitals.
On the right is cyclobutadiene, with four -electrons. Since Hund's rule won't let us pair
electrons in an orbital until there is no other option, Frost's circle predicts cyclobutadiene to
be a diradical (or biradical).
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Frostʼs Circle
• Below the horizontal line through the center of the ring are bonding MOs.
• on the horizontal line are nonbonding MOs.
• above the horizontal line are antibonding MOs.
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Aromatic, Antiaromatic, and Nonaromatic Compound
Any cyclic, flat, fully conjugated system may be aromatic or anti-aromatic. The level
of stability really depends on the number of -electrons in the cyclic conjugated system,
but it will always follow the rules stated above, and may always be proven using the
Frost Circle to derive the orbital energies and determine where the electrons are located.
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