The Diverse Nature of the c6H6 Molecule J. H. Potgieter Physical Metallurgy Division, Council for Mineral Technology, Private Bag X3015, Randburg 2125. Republic of South Africa The chemical formula CsHs is usually associated with henzene, the simplest of the aromatic hydrocarbons. Benzene is widely used in industry as a solvent as well as a precursor for other chemicals, e.g., styrene, phenol, and nylon. The chemistry of benzene and its derivatives is covered extensively in undergraduate chemistry courses and is described in a large number of standard textbooks. The purpose of this discussion is to show that the chemical formula CsHs actually describes more than just one kind of benzene. Benzene was discovered in 1825 by Faraday in cylinders that had held acetylene gas. I t is therefore not surprising that one of the reactions by which benzene is currently produced involves the heating of acetylene in the presence of an iron catalyst:
The structural formula used for henzene today was proposed in 1865 by August Kekul6 and consisted of a ring of carbon atoms that are bonded to each other by alternating single and double bonds, with one hydrogen atom attached to each carbon:
has been suggested as early as 1867 by James Dewar. The existence of a compound with this structure was proved in 1963 when Tamelen and Pappas (4) succeeded in synthesizing it. The method involved UV irradiation of cis-1,2-dihydrophthalic anhydride (3) in ether to form bicyclo[2.2.0]hexa-5-ene-2,3-dicarboxylicacid anhydride (4), according to the reaction:
The photoanhydride 4 was converted to the hicyclohexadiene 2 by direct oxidative decarboxylation with lead tetraacetate in a pyridine solution.
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Kekul6 claimed that the structure he proposed for benzene came to him in a dream. However, this claim has been disputed hy modern historians ( I ) . Recently Wiswesser (2) reported that Johann Loschmidt, a little-known Viennese chemist, proposed a ring of six carbon atoms for the structure of henzene four years earlier than Kekul6. He puhlished his idea in a private pamphlet. Accordng to Wiswesser, Kekul6 certainly knew of Loschmidt's proposal through a mutual friend, even though he never acknowledged the pamohlet. The problem with KekulB's structure was that it portrays benzene as a cvclic coniueated triene. Yet benzene does not . undergo easily any of the addition reactions normally associated with coniueated dienes or ordinarv alkenes. Instead. it undergoes substitution reactions. ~ h u i the , Kekul6 stricture clearly had difficulties in explaining the chemical behavior of henzene. However, the ring structure of henzene as proposed by Kekul6 has been proved beyond doubt by a number of modern-day analytical techniques such as NMR, CMR, and X-ray diffraction. A further very striking conformation of this ring structure was obtained in 1988, when scientists a t IBM "photographed" benzene with a scanning tunnelling microscope (3). The inadequacy of Kekul6's model to explain the chemical behavior of benzene as well as the general ingenuity of scientists led to the proposal of several other structures for the CsHs molecule. In total seven valence isomers for henzene have been proposed. The second structure for henzene, socalled Dewar benzene:
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The Dewar henzene is reasonablv stable in the ovridine ., solution at room temperature, but rearranges quantitatively LO benzene on standing (half-lifeof 2 -2 davs~. One of the rarest isokers of benzene is h&alene (5):
Benzvalene was isolated in low yield from sensitized photolysis of henzene by Wilzbach et al. (5) in 1967. An improved synthesis was developed hy Katz et al. (6) that entailed the reaction between lithium cyclopentadienide (6), methylene chloride (CHzC12) and methyl lithium in dimethyl ether according to the following scheme:
6
(29% yield)
Although large quantities of benzvalene can he safely prepared in solution, henzvalene is extremely explosive when it is isolated. Another characteristic of henzvalene is its extremely foul odor. Another early structure, prismane, was proposed for benzene by Ladenburg in 1869 (7).Also called Ladenburg beu-
zene, its structure consisted of six carbon-hydrogen units that are disposed a t the corners of a triangular prism:
The synthesis of 7 was accomplished in 1973 by Katz and Acton ( 8 ) from benzvalene according to the following reaction scheme:
The structure of 8 was confirmed by NMR spectroscopy. Bicyclopropenyl (2,2) was found to be stable only below 10 OC; above this temperature it slowly decomposes to a yellow solid, as yet unidentified. Thus it differs from the other benzene isomers, which on decomposition all revert back to benzene. The bicyclopropenyl (1,2) compound has been made by a similar reaction:
I t was found that prismane is an explosive odorless liquid with properties quite different from those of benzene. Although it is stable at room temperature, it decomposes to benzene a t 90 OC in toluene (half-life-11 h). The only remaining isomers of CsHs that are possible are the double-ring systems known as bicyclopropenyls that have been calculated (9) to be the CeHs species of highest energy. There are three isomers, namely 1,1, 1,2, and 2,2, which differ only in the positions of their double bonds.
Although Billups and Haley also daim to have synthesized the bicyclopropenyl (1,l) compound, no details about it or its behavior are currently available. I t is clear, therefore, that the CsHs molecule is indeed a chemical chameleon that can take on a variety of forms that exhibit different chemical behaviors and characteristics. Acknowledgment
The author expresses his appreciation to T. Modro of the Organic Chemistry Division in the Department of Chemistry at the University of Pretoria for his valuable suggestions in the preparation of this paper. Literature Clted
In a recent breakthrough the 2,2 and 1,2 isomers of bicyclopropenyl, the last of the remaining benzene isomers, were synthesized by Billups and Haley (10).For the synthesis of the bicyclopropenyl(2,2) compound 1,4-bis(trimethylsi1yl)buta-1,3-diene (11) was reacted with methyl lithium in dichloromethane. The product (12) was treated with tetra-nbutylammonium fluoride, resulting in a 45% yield of the bicyclopropenyl(2,2) derivative (8).
I. i a i Kohn. M J Chem. Edue. 1945.381-384. 1bJAnahulr. R. Be,. 1912. G . 5 3 9 2. Wiswelsor, W.Aldiirhimico A r l a 1988, 11.17-18. 3. Ems1ey.J. New Sciontist 1990, IFebr. 171.32. 4. Tarn~lon.E. E.; Psppas. S. P. J . Am. Cham. Soc. 1963.85.3297-3298. 5. (sJ Wilrbach. K. E.: Ritscher, J. S.: K s p l s n , L J.Am. Chem. Sor. l967,89, LOB1 (b1 Kepian, L.: Wilrbach. K. E. J. Am. Chsm. Sor. 1968, 90,3291. (cl Ward, H. R., Wirhnok, J. S . J . A m . Chem. Sor. 1968,90. 1035. 6 . KaU.T. J.: Wane.E. J.:Acton.N. J. Am. Chem S o e 1971.93.3782-3783. 7. A. ~ ; e r n . ~ 1869,2.140. w 8. Kstr.T.J.;Aefon. N. J.Am. Cham. Soc. 1973,95,273&27?9. 9. Greenburg,A.; Liebman. J. F.J. Am. C h m . S a r . 1981.109.U. 10. Billups, W.E.:Hsley. M. M. Angew. Cham. Int. Ed. Enpi. 1989,28 (121.171L-1712.
Volume 88
Number 4
April 1991
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