Top
of PageNeutral Annulenes [4]-Annulene [6]-Annulene [8]-Annulene [10]-Annulene
Charged Annulenes Cyclopropenium cation Cyclopentadienyl anion Cycloheptatrienyl cation
Heteroannulenes Replacement heteroatom in benzene: Nitrogen Oxygen Sulphur
Other Ring Sizes Replacement in cyclopentadienyl anion: Nitrogen Oxygen Sulphur
Polycyclic
Aromatics Top
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Cyclobutadiene,
[4]-annulene, is very unstable and highly reactive, much less stable
than
its acyclic counterpart, buta-1,3-diene,
and consequently considered to be antiaromatic. The ring system
may also be stabilised as the tetra
t-butyl derivative. The very bulky alkyl groups shield the
pi-system
from attack. Physical measurements with this system have shown that the
4-membered ring shows alternation of bond lengths, consistent with it
being
non-aromatic.
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Benzene, [6]-annulene, is the prototype aromatic system to which all others are referenced. Its chemistry has already been dealt with in detail in module CS204.
Cyclooctatetraene, [8]-annulene, is well-known and
stable.
It behaves as a typical alkene. The tub-shaped
conformer has alternating long and short bonds and is more stable than
the planar molecule.
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The all
cis-isomer
of cyclodecapentaene, [10]-annulene, would have bond angles of 144o
and therefore would be highly strained. This strain has to be relieved
by twisting within the molecule and the planarity required for
aromaticity
is lost. The mono-trans
isomer
is also twisted out of planarity and, while the di-trans
isomer would have 120o bond angles if planar, two of its
hydrogens would be strongly sterically opposed and so the ring is
distorted.
[10]-Annulenes of this type have been shown not to be aromatic,
consistent
with the molecule being twisted out of planarity. However when a
methylene
bridge is introduced the resulting bridged
annulene can achieve sufficient planarity for the pi-system to support
a ring current (NMR evidence) and to undergo electrophilic aromatic
substitution
under mild conditions. In the bridged system the methylene hydrogens
lie
"within" the ring in the shielded region and resonate at -0.5ppm.
The corresponding oxa-bridged
annulene is also aromatic. Top
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Cyclodecahexaene, [12]-annulene, would not be expected to be aromatic since it is not a 4n+2 system. The inner hydrogens interfere with one another sterically and the ring is puckered.
Cyclotetradecaheptaene, [14]-annulene, has 4n+2
pi-electrons
and has been shown to be aromatic. The larger ring size permits planarity
of the ring. The outer hydrogens absorb in the NMR spectrum at 7.6ppm
(PhH
absorbs at 7.3ppm), and the inner ones at 0.0ppm, being strongly
shielded
by the ring current. Top
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Cyclohexadecaoctaene, [16]-annulene, would not be expected to be aromatic since it is not a 4n+2 system. The inner hydrogens interfere with one another sterically and the ring is puckered.
Cyclooctadecanonaene, [18]-annulene, has 4n+2 pi-electrons and has been shown to be aromatic. The outer hydrogens absorb in the NMR spectrum at 9.3ppm, and the inner ones at -3.0ppm. The molecule can readily achieve the planar conformation required for aromaticity.
C3:
Cyclopropenium cation (n = 0):
The three-membered ring with a p-orbital at each corner achieves special aromatic stability as the cyclically delocalised, resonance stabilised cyclopropenium cation, two of the carbons contributing one p-electron each to the p-system, and the third contributing none. Hydrogens absorb at 11.1ppm in the NMR spectrum - more shielded than in benzene (7.3 ppm) due to the additive deshielding effects of both the ring current and positive charge.
C5:
Cyclopentadienyl anion (n = 1):
Each corner carbon again provides a p-orbital to
the cyclically
delocalised odd-numbered ring size p-MO system. Four of the five
carbons
donate one electron each to the pi-system (analogous to benzene), with
the fifth contributing two electrons. Thus, the fifth carbon is
carbanionic,
and this 5-membered system is the cyclopentadienyl
anion, formed by treating cyclopentadiene
with a base. All carbons and hydrogens in the anion are identical.
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C7: Cycloheptatrienyl
cation (n = 1):
In the cycloheptatrienyl
cation (tropylium cation), six of the seven carbons donate one
electron each to the pi-system with the seventh electron contributing
no
electrons, i.e. the seventh carbon is carbocationic. It is formed by
treating
cycloheptatriene with
phosphorus
pentachloride. Top
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Just as a carbon atom in a hydrocarbon chain can be replaced by heteroatoms to give, for example, ethers or amines, so can the carbons of aromatic rings such as those described above be replaced by heteroatoms to give HETEROAROMATIC systems.
Nitrogen
as replacement heteroatom in benzene:
To formally replace a C atom of benzene, nitrogen must fulfill the same orbital and electronic criteria as the carbon it is to replace, i.e. it must be sp2-hybridised and have the same number of electrons (the principle of isoelectronic replacement).
Replacing one carbon with a nitrogen yields pyridine.
Replacing two carbons in different positions can yield a variety of componds like pyridazine (1,2-diazine), pyrimidine (1,3-diazine), or pyrazine (1,4-diazine).
Replacing three carbons with three nitrogens generates 1,3,5-triazine and replacing four carbons by four nitrogens produces 1,2,4,5-tetrazine.
Oxygen
as replacement heteroatom in benzene:
For oxygen to achieve the same orbital and electronic requirements as a carbon in benzene it has to lose an electron becoming O+. By replacing one carbon in benzene with an O+, a pyrilium cation is formed.
Sulphur
as replacement heteroatom in benzene:
For sulphur to achieve the same orbital and
electronic
requirements as carbon it also has to lose an electron becoming S+.
By replacing one carbon in benzene with an S+,
a thiopyrilium cation is
formed.
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Just as one or more carbons in a benzene can be replaced by heteroatoms, so can the carbons of non-benzenoid systems, such as the cyclopentadiene anion and the tropylium cation be similarly replaced to give heteroaromatic systems of ring size other than six.
Nitrogen
as replacement in cyclopentadienyl anion:
Pyrrole may be visualised as being formed by replacement of the "carbanionic carbon" of the cyclopentadienyl anion with nitrogen. The nitrogen effectively contributes its "lone pair" electrons to the pi-system.
Imidazole and pyrazole, in contrast, may be considered to be formed by replacement of "neutral" corner carbons (analogous to the carbons in benzene) by nitrogen.
Multiple replacements of carbon by nitrogen lead
to 1,2,3-triazole
and 1,3,4-triazole. Top
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Oxygen
as replacement in cyclopentadienyl anion:
The furan structure is obtained by replacing the "carbanionic carbon" of the cyclopentadienyl anion with a neutral oxygen, which contributes a lone pair of electrons to the aromatic pi-system.
Sulphur
as replacement in cyclopentadienyl anion:
Thiophene
is
the sulphur analogue of furan, involving replacement of the
cyclopentadienyl
carbon by a neutral sulphur. Top
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Nitrogen/oxygen
and Nitrogen/sulphur systems:
Replacement of a carbon of furan with nitrogen generates oxazole or isoxazole depending at which position the nitrogen is inserted.
Similarly, replacement of a thiophene carbon atom by nitrogen generates thiazole or isothiazole.
Further nitrogens may be introduced leading to the 1,2,3-oxadiazole, the 1,3,4-oxadiazole, the 1,2,3-thiadiazole and the 1,3,4-thiadiazole systems.
These contain more than one aromatic ring, with adjacent rings having atoms in common, i.e. two (or more) rings having a common side. They are said to be condensed or fused. They include polycyclic aromatic hydrocarbons (PAHs) such as the 10-carbon naphthalene, the 14-carbon anthracene and phenanthrene, and the 24-carbon coronene, as well as heterocyclic systems such as indole, benzofuran, and benzothiophene.
Systems containing a number of heteroatoms are also common. These include benzimidazole, benzoxazole and benzthiazole, and their positional isomers benzpyrazole, benzisoxazole and benzisothiazole.