Benzene is a colorless liquid and boils at 80°C. It was discovered in 1825 by Michael Faraday in the condensate from compressed illuminating gas. Benzene is a major industrial chemical today, and has attracted public concern over the safety hazard which it presents to workers, since it is toxic if inhaled or ingested, occasionally even causing leukemia. Its formula, C6H6, suggests unsaturation - a saturated alkane would have the formula C6H14 and a cycloalkane, C6H12; however, much of the behavior of benzene more closely resembles that of the saturated rather than of the unsaturated hydrocarbons. For example, benzene will not react with either bromine or chlorine under conditions where alkenes and dienes such as hexene and hexadiene rapidly add both. Benzene is also inert to hydrogen bromide, hydrogen chloride, and other reagents that readily add to double and triple bonds. When it does react, it usually does not undergo addition, but rather substitution, as in its reaction with bromine in the presence of ferric bromide (Fig. 4-1).
Figure 4-1. As compared to most unsaturated compounds, benzene is highly unreactive. Under vigorous reaction conditions it is substituted by bromine but adds hydrogen.
Benzene shows that it is actually unsaturated because it adds hydrogen or chlorine, although only when allowed to react under very vigorous conditions (higher temperature or pressure) compared to those required for alkenes and alkynes. When reduced with hydrogen, it forms cyclohexane, and 1,2,3,4,5,6-hexachlorocyclohexane (the insecticide Lindane) can be prepared from benzene by free-radical addition of chlorine. These reactions suggest that benzene has its six carbon atoms arranged in a ring, and it must have the equivalent of three double bonds, since three molecules of hydrogen or chlorine react if forced to do so.
Figure 4-2. The cyclohexatriene structure accounts for the addition reactions of benzene, but not for its substitution reactions or its failure to react with the usual alkene reagents such as bromine and hydrogen bromide.
These addition reactions could be accounted for if benzene were 1,3,5-cyclohexatriene (Fig. 4-2), but such a molecule should be highly reactive in marked contrast to benzene. Although the peculiar nature of benzene has occupied the attention of organic chemists for a century, they have only acquired a general understanding of the compound's structure and its unusual chemical properties since the 1930s. The explanation now accepted has required the best modern physical evidence and mathematical theories.
Some of the best evidence involves physical measurement of bond lengths by electron diffraction or X-ray diffraction. If the structure in Fig. 4-2 were correct for benzene, there would be three double bonds of length 1.34 Å and three single bonds of length about 1.54 Å. However, all measurements show that the carbons are arranged in a regular hexagon, meaning that all carbon-carbon bonds have the same length, which turns out to be 1.39Å.
Figure 4-3. Models of benzene. All the carbon-carbon bonds in benzene are the same length, 1.39 Å. This corresponds to a bond order of 1.5, a one-and-one-half bond.
We discussed the variation of bond length with bond order in "Unsaturated Compounds" and presented this variation in graphic form in Fig. 2-6. That graph is reproduced in Fig. 4-3. Note that a bond length of 1.39 Å, as in benzene, corresponds almost exactly to a bond order of 1.5, namely, to one and one-half bonds. How can we account for half bonds?
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