The Aromatic Ring by Doris Kolb
Illinois Central College East Peoria. Illinois 61635
"Fragrant" is its literal meaning, but aromatic has come to we a special meaning for chemists. Early in the history of ganic chemistry it became clear that there were certain whon compounds that were somehow different from the rest. heir chemical formulas suggested that their molecules were waturated, containing several double bonds, yet their ronerties indicated otherwise. Often thev had distinctive Iors, and 90 they hecame known a "nrnmatic" compounds. Hter it turned out that thme c o m ~ o u n d shad something.in bmmon. They all seemed to be refated to benzene. re Benzene Problem
In 1825 Michael Faraday isolated a colorless liquid with a easant odor from a condensed portion of "illuminating gas," ~tainedby distilling certain fats and oils and used as fuel for mps. The liquid boiled a t 80°C and solidified a t about 5% ater Eilhardt Mitscherlich (1834) prepared the same liquid om henzoic acid by distilling it with lime. He called the ~hstancebenzin, which became benzene in English. Chemical analysis showed henzene to he 92% carbon and 6 hydrogen by weight, with a molecular weight of 78. Its olecular formula therefore was CnHn. " " which was a t once verv m p k and f:xtremcly mnfusing. The exwptinnally low H to ratio in CnIlr. imsrd quite a Droblem for organic chemists. - -. lnce carbon was tetravhent &d hydrogen m&ovalent, most ' the structures Dro~osedfor benzene contained double mds. An early stLuc a carbon-carbon double bond is 1.33 A, and a single bond measures 1.54 A. X-ray analysis shows that in benzene all the C-C bond distances are identical, and equal to 1.40 A). Perhaps the benzene ring might best be represented as ~
~
The simple circle-inscribed hexagon a t the right has become a popular alternative to the classical Kekul6 formula and is probably the henzene formula most widely used today. How aromatic a particular ring system is can be assessed by determining its "resonance energy." One way to do this is to measure its heat of hydrogenation. When hydrogen is added to a double bond, the heat of reaction is about 120 kilojoules per mole.
(3+- - 0 R,
According to Thiele's hypothesis, any ring system with a completely conjugated set of double bonds (resulting in partial valences all around the ring) should be aromatic. Thus he exoected both cvclobutadiene and cvclooctatetraene (with 2 ani1.l ronj11~m.4 double bonds, reipe>tivclv) to have ar;,miltic ch:~r;wter.\\'hen Willstarter was unahle to svntheiize cvclohutadiene, and when upon preparing cycl&ctatetraene he showed that it was totally lacking in aromatic properties,
not aromatic
aromatic
not aromatic
support for Thiele's theory largely disappeared. During the early 1920's Armit and Robinson noted that it was only those conjugated cyclic systems with six multiple bonding electrons that seemed to have special stability. Their
AH = -119.6 kJ
If benzene were really cyclohexatriene, its heat of hydrogenation should be about three times that of cvclohexane. or
The actual heat of hydrogenation for benzene, is only 204.8 kJ/mol. This is 150 kJ/mol less than we would expect i t to he if the molecule really contained 3 double bonds. The 150 kJ/mol is a measure of the extra stability that benzene has because its r electrons are delocalized and is referred to as the stabilization energy of benzene. Hiickel's Rule Rewnnnce was useful for describing the aromatic ring, but it could not explain why benzene was anmatic while s~milar conjugated rings with 201 4 double bonds were not. The moVolume 56, Number 5, May 1979 / 335
~
~
lecular-orbital method provided a key for solving that prohlem. Following the lead of Edward Condon and others who had used a molecular-orbital approach to describe the bonding in the hydrogen molecule (19271, Erich Huckel applied the molecular orhital theory to conjugated ring systems. By calculating a-orbital energies for various monocyclic conjugated systems, he was able to develop an explanation for the stability of henzene and certain other "aromatic" compounds. Huckel's rule can he expressed as 4n 2, which represents thenumber of ?r electrons that impart aromatic stahility to an unsaturated planar ring. Since n may equal 0,1,2,3,. . .etc., a monocyclic aromatic system might have 2, 6, 10, 14,. . . etc. a electrons according to Huckel's rule. For henzene, and its simple derivatives and analogs, n = 1 and 4n + 2 = 6, the aromatic sextet.
+
Polycyclic Aromatic Compounds Huckel's calculations were made for monocyclic conjugated systems, so his rule is not generally applicable to polycyclic systems. Even though many polycyclic aromatic compounds do follow Huckel's rule
naphthalene (n = 2)
4n+2=10
phenanthrene (n = 3) 4n+2=14
lJ1,>
-
fl* +
OorO
This was the first instance of aromatic character being attributed to such an unusual kind of ring. Among the fascinating derivatives that have been made from this very stahle carbanion are the organometallic "sandwich" molecules such as ferrocene (discoveredin 1951) and other metallocenes.
It was not until 1945 that M. J. S. Dewar, in working with some natural products with 7-membered rings, suggested that a similar type of aromaticity should exist for the tropylium ion (the cation derived from cycloheptatriene),
chrylene (n = 4)
4n+2=18
others do not. Pyrene and coronene, for example, have 16 and 24 a electrons, respectively, and therefore do not ohey Huckel's rule, hut both are aromatic.
pyrene
In polycyclic aromatic systems it often happens that one ring has more aromatic character than another. In phenanthrene. for examole. the center rinz has less electron delocalizathn, and t h i s less aromaticity,ihen the two rings on the ends. This is easiest to understand by considering the 5 contributing resonance structures for phenanthrene.
& B h & Each of the end rings has a 3-double-bond "Kekul6" arrangement in 4 out of 5 of the structures, hut only 2 structures show that condition for the middle ring. I n 4 of the 5 structures a double hond appears in the 9,10 position (top middle), suggesting that this is a largely localized hond. This is confirmed experimentally by the fact that addition readily occurs across this 9,10 double hond, whereas the end rings undergo typical aromatic suhstitution. Polycyclic aromatic compounds need not he henzenoid. Purine is one of the multi-ring heterocyclic compounds that exhibit aromatic properties.
ty; 11
punne
Another polycyclic compound worthy of special mention is azulene.
a nzulene
Named for its deep blue color, azulene is a completely conjugated hicyclic molecule with one 5- and one 7-memhered ring fused together. Although not as stable as its isomer naph336 / Journal of Chemical Education
thalene, azulene is still decidedly aromatic. I t is one of a zrowing numher of what might he called non-classical aromatic compounds. Non-Classical Aromatic Compounds Cyclopentadiene as early as 1901 had been recognized as an acidic hydrocarbon. In 1928 Ingold suggested that this curious acidity was due to the stahility of the cyclopentadienyl anion because of its aromatic sextet of electrons.
a fact confirmed in 1954 hy Doering and Knox. Since then the di-anion of cyclohutadiene and the di-cation of cyclooctatetraene have also been shown to exhibit aromaticity.
All four of these aromatic ions are. like benzene., . ~ l a n acvclic r systems with 6 ff electrons. According to Huckel's rule the cation of cvcloorooene should also he aromatic (n = 0: 4n
+
Breslow and others have established that cvclooronenium derivatives do indeed have aromatic properties. ~ h ~ ~ l a n a r anion of cvclononatetraene and the di-anion of cvclooctatetraene wo&d also he expected to have aromatic itability (n =2;4n+2=10)
and both of them do. In addition, Huckel's rule predicted aromatic stahility for some macrocvclic called annulenes. Sondheimer and " oolvenes . others have prepared and studied a numher of these conjugated rings and have found that (181-annulene (a fully conjugated ring with 18 carbon atoms) does have aromatic properties, especially a t lower temperatures. [14]-Annulene is less stahle, hut still aromatic
m
w
'-=v-'
1141-annulene and so is [22]-annulene. These rings contain 14,18, and 22 a electrons, which represent values of 4n 2 where n = 3,4, and 5. The [12]-, [16]-, [20]-,and [24]-annulenes, on the other hand, do not ohey Huckel's rule and are not aromatic. All of the aromatic rings mentioned thus far have been planar, fully conjugated systems. But since 1962 Pettit and others have nreoared some homoaromatic comoounds, in which the r i n g are not cvmpletely planar and the& is definite inwrruptiun oft he conjugatum pattern. (This kind of aromatic [ I 81-annulcne
+
system had, in fact, been predicted by Winstein). When cyclooctatetraene is treated with acid, for example, the "homotropylium" ion is formed CH.
It has six a electrons spread around a 7-membered planar ring that is interrupted a t one point by a CH2 group lying in a perpendicular plane. I n spite of the discontinuity, the homotropylium ion is aromatic. What is Aromaticity?
I t may be impossible to state a concise definition for aromaticity that would he completely acceptable to everyone. At one time the "aromatic" label applied only to benzene and its derivatives. Then its meaning was broadened to include any compounds that were benzene-like. Chemical activity was the criterion for decidine whether a substance was aromatic, benzene being the chemical standard. Many chemists still tend to think of aromatic molecules as very stable unsaturated rings that react by substitution rather than addition. Followina the formulation of the electronic theory of valence arouna 1920, aromaticity began to be viewed as a special kind of molecular bonding. Aromatic compounds have been defined as cyclic molecules in which all the atoms in the ring are involved in a single conjugated bonding system. Aromatic rings may also be described as planar cyclic systems with circular clouds of delocalized a electrons. For some the aromatic rine is best defined as a cvclic unsaturated svstem that is stabilized by resonance. An aromatic ring might even he described as a fully conjugated monocyclic system that has 4n 2 a electrons, in keeping with Hiickel's rule. Perhaps the most practical way to define aromaticity is according to whatever parameter is being used to measure it. Thus. an unsaturated rina is aromatic if its bond distances show "niformity, as determined hy X-ray analysis, microwave spectroscopy, or electron diffraction. Or, an unsaturated ring compound is aromatic if its heat of combustion or heat of hydrogenation (as measured calorimetrically) is considerably less than would be expected if the ring contained true double bonds. If a cyclic compound is aromatic, there are certain characteristic features that show up in its electronic spectra, ~
~~~~
~
+
-~~
such as longer wavelength absorption bands in the ultraviolet. An aromatic compound also has diamagnetic anisotropy; in other words, its magnetic susceptibility is much greater along one axis (of a single crystal) than along the other two axes. Today i t is probably most convenient to define aromaticity in terms of how well a rine of a electrons can sustain an induced "ring current," as measured by nuclear magnetic resonance. If the protons attached to an unsaturated ring are shifted downfield in the nmr spectrum (from where they would be if the ring contained double bonds), the system is said to be diatropic. A ring exhibits this low field chemical shift (down to around 7-8 ppm) because its a electrons are delocalized. A diatropic ring is therefore aromatic. Mystified by its unsaturation, Chemists long had pursued information On the benzenoid classification. Benzene has been quite problematic. After much structure argumentation, Kekuli: made his recommendationThe hexagonal representation, However. its form was not static. Were there two farms in swift oscillation? Rapid shifting of ring conjugation? Instantaneous eauilihration? . . . The subject remained enigmatic. Now at last there is sound explanation Far this special ring stabilization: Pi electron delocalization Is what makes a ring aromatic. Some General References Rsdger. G. M., "Ammatie Character and Aromstieity."Cambridge Univenity Press. London.
1969.
Garratt, P. J., "Aromaticity."McGrau-Hill, London, 1971. i n & C. K., ''Structure and Mechanism in Organic Chemistry." 2nd 4.Cornell . Univenity Press. Ithaca, New Ynrk. 1969. "KekuI6Centennial." ACS Symposium, Advances in Chemistry Series No. 61. ACS Puhlications, Washington. D.C. 1966. March, J., "Advanced Organic Chemistry."2nd ed.. Chapter 2, McGrau-Hill. New Yurk.
."... ow7
I,.,"TheNatureofthechemieslBond,"3rd ed.. Cornell U n i w r i t y Prers, Ithaca. NPWYork, 1960. Snyder. J. P. (Edit",) "Nonhenzenoid Aromatics." organic Chemistry Monograph No. 16. 2 unlumer, Academic P r e . New York. 1969. Sulomona, T. W. G.,"Organic Chemistcy." Chapter 12. John Wiley & Suns. Near York. Pauling.
1976. 1965.
Whelsnd. G. W.. -RPsonance in
organic Chemirtry."
Jnhn Wiley & sons,
New Yurk,
Solar Energy Potential The following solar energy diagram was prepared hy J. C. White. former Head ol'the Electrc,chemistry Branch ol'the Chemistry Division oI The Naval Research Laboratory. It was provided hy .Jeanne Burhank, Southern Arizona Sect.ion, ACS.
Vo'olume56. Number 5. May 1979 1 337
