Neutral 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 of Page
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.
Top
of
page
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. Top of page
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 of page
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 of page
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. Top of page
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 of page
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. Top of page
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 of page
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 of page
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.