Aromaticity

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Aromaticity

The literal meaning of "aromaticity" is "fragrance," but the word has a special meaning in chemistry. Aromaticity has to do with the unusual stability of the compound benzene and its derivatives, as well as certain other unsaturated ring compounds. The structures of these compounds are often shown to contain double bonds, but they do not actually behave like double bonds. For example, reagents such as bromine react with benzene by substitution rather than addition. Benzene and its derivatives had long been referred to as aromatic because of their distinctive odors.

The structure for benzene is often shown as a hybrid of the two Kekulé formulas:

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The double-headed "resonance arrow" does not signify an equilibrium in which the two structures are very rapidly shifting back and forth (as was once thought to be the case). Instead it means that the actual structure is not like either of the two Kekulé structures but is rather a resonance hybrid of the two. (The length of a carbon-carbon single bond is 1.54 Angstroms, and that of a double bond, 1.33 Angstroms. X-ray analysis shows that in benzene all the C-C distances are identical and equal to 1.40 Angstroms.) The C-C bonds in benzene are neither single nor double bonds, but something in between. Perhaps the benzene ring is best represented as follows:

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The simple circle-inscribed hexagon on the right has become a popular alternative to the classical Kekulé structure and is probably the benzene formula most widely used.

The aromaticity of the benzene ring can be assessed by measuring its "resonance energy." One way to do this is by measuring its heat of hydrogenation. When hydrogen is added to a double bond, the heat of reaction is about 120 kilojoules per mole. If benzene really had three double bonds, its heat of hydrogenation should be about 360 kilojoules per mole. In contrast, its actual heat of hydrogenation is only about 210 kilojoules per mole. This is 150 kilojoules per mole less than expected if benzene actually contained three double bonds. The 150 kilojoules per mole is a measure of the extra stability that benzene has because its π electrons are delocalized . (The π electrons are those involved in the second bonding pair of the double bond.)

Benzene is not the only compound that exhibits such unusual stability. The following heterocyclic unsaturated ring compounds also exhibit aromatic behavior:

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In 1890 Eugen Bamberger was the first to suggest that six was the magic number of "potential valences" that caused an unsaturated ring to be aromatic. In 1899 Johannes Thiele suggested that any ring that had a completely conjugated set of double bonds around the ring should be aromatic; when cyclobutadiene and cyclooctatetraene were prepared, however, neither turned out to be aromatic.

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In the 1920s Armit and Robinson pointed out that it was conjugated ring systems with six multiple bonding electrons that seemed to have special stability. They spoke of an "aromatic sextet" of electrons as being necessary for aromaticity.

Hückel's Rule

German physicist Erich Hückel used the molecular orbital theory to explain the stability of benzene and other aromatic compounds. Hückel's rule determines the number of π electrons that give stability to an unsaturated planar ring according to the formula 4n + 2. For benzene and its analogs, n = 1; therefore, 4n + 2 = 6 (the aromatic sextet). The rule was calculated for single ring molecules and does not generally apply to multiring systems. Although many polycyclic aromatic molecules do follow Hückel's rule (Figure 5), some do not (Figure 6). For example, pyrene has sixteen π electrons and coronene has twenty-four, but both are aromatic.

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The molecules in Figures 5 and 6 all have fused rings (in which some carbon atoms are part of two or more rings). In general, molecules with fused rings tend to be less stable than single aromatic rings.

Aromatic polycyclic compounds need not contain contain benzene rings. For example, purine, which contains two fused heterocyclic rings, is aromatic. Azulene, named for its deep blue color, is also aromatic, although one ring has five carbon atoms and the other has seven. It is not, however, as aromatic as its isomer naphthalene, which has two fused benzene rings.

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Aromatic Ions

Cyclopentadiene is an acidic hydrocarbon. In 1928 English chemist Christopher Ingold suggested that this was because the cyclopentadienyl anion had an aromatic sextet of electrons. This was the first case of aromatic character being attributed to an ion. An interesting derivative made from this very stable carbanion was ferrocene (discovered in 1951).

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In 1945 Michael J. S. Dewar suggested that the tropylium ion (the cation derived from cycloheptatriene) should also be aromatic (Figure 9). This was confirmed in 1954; since then, the dianion of butadiene and the dication of cyclooctatetraene have also been shown to be aromatic. Like benzene, all four of these ions are planar rings with six π electrons. According to Hückel's rule the cyclopropene cation should also exhibit aromaticity, and it does. (In this case n = 0, and 4n + 2 = 2.) The planar anion of cyclononatetraene and the dianion of cyclooctatetraene should also be aromatic (n = 2, and 4n + 2 = 10), and both of them are.

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Other Nonclassical Aromatic Compounds

Hückel's rule also predicted aromatic stability for certain large ring polyenes called annulenes. Fully conjugated [14]-annulene and [18]-annulene do have aromatic properties, especially at lower temperatures (Figure 12), as does [22]-annulene. They contain fourteen, eighteen, and twenty-two π electrons, respectively, corresponding to values of 4n + 2 where n = 3, 4, and 5, and all the molecules are planar. The [12]-, [16]-, [20]-, and [24]-annulenes, on the other hand, do not obey Hückel's rule, and they are not aromatic.

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Although aromatic rings are normally planar, with uninterrupted conjugation, that is not always the case. When cyclooctatetraene is treated with acid, the homotropylium ion is formed. It has six electrons in a seven-membered ring that has a CH₂ group lying in a perpendicular plane, yet is aromatic.

What Is Aromaticity?

At one time the term "aromatic" applied only to benzene. To some chemists it still simply means "like benzene." An aromatic molecule is a planar ring with a circular cloud of delocalized π electrons. It is an unsaturated cyclic molecule stabilized by resonance. It is a very stable unsaturated ring that reacts by substitution instead of addition. Or, according to Hückel's rule, it is a fully conjugated unsaturated ring that has 4n + 2 π electrons.

How do you know whether or not a molecule is aromatic? If an unsaturated ring compound is aromatic, its heat of hydrogenation and its heat of combustion will both be considerably less than they would be if double bonds were present. Its bond distances (as measured by x-ray or electron diffraction or by microwave spectroscopy ) will be uniform. It will have longer wavelength absorption bands in the ultraviolet region of the spectrum. An aromatic compound will have diamagnetic anisotropy (meaning that a crystal will have more magnetic susceptibility along one axis than the other two). It will also be diatropic ; in other words, because its π electrons are delocalized, protons attached to the ring will be shifted downfield in the nuclear magnetic resonance (NMR) spectrum from where they would be if double bonds were present. In fact, NMR can be used to measure the degree of aromaticity in a molecule by how well its ring of π electrons can sustain an induced "ring current."