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:
(1)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:
(2)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:
(3)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.
(4)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.