Benzene and Aromatic Compounds

A cyclic, planar, completely conjugated compound that contains 4 + 2 jr electrons ( = 0, 1,2, 3, and so forth). 

An aromatic compound is more stable than a similar acyclic compound having the same number of it electrons. 

A compound that lacks one (or more) of the requirements to be aromatic or antiaromatic. 

We begin with benzene and then examine other cycUc, planar, and conjugated ring systems to learn the modern definition of what it means to be aromatic. Then, in Chapter 18, we will learn about the reactions of aromatic compounds, highly unsaturated hydrocarbons that do not undei o addition reactions like other unsaturated compounds. An explanation of this behavior relies on an understanding of the structure of aromatic compounds presented in Chapter 17. 

For 6 C s, the maximum number of H s = 2n + 2 = 2(6) + 2 = 14. Because benzene contains oniy 6 H s, it has 14-6 = 8 H s fewer than the maximum number. This corresponds to 8 H s/2 H s for each degree of unsaturation = four degrees of unsaturation in benzene. 

Benzene (CeH ) is the simplest aromatic hydrocarbon (or arene). Since its isolation by Michael Faraday from the oily residue remaining in the illuminating gas lines in London in 1825, it has been recognized as an unusual compound. Based on the calculation introduced in Section 10.2, benzene has four degrees of unsaturation, making it a highly unsaturated hydrocarbon. But, whereas unsaturated hydrocarbons such as alkenes, alkynes, and dienes readily undergo addition reactions, benzene does not. For example, bromine adds to ethylene to form a dibromide, but benzene is inert under similar conditions. 

Benzene does react with bromine, but only in the presence of FeBr3 (a Lewis acid), and the reaction is a substitution, not an addition. 

any stmcture proposed for benzene must account for its high degree of unsaturation and its lack of reactivity towards electrophilic addition. 

Benzene occupies a special place in the field of organic chemistry. It is an especially stable compound, and because of this stability, substituted benzenes are widely distributed among natural products and industrial chemicals. Efforts by organic chemists to understand this stability have contributed significantly to our current models for the electronic structure of organic compounds and have led to the development of theories that not only explain the special properties of benzene but also help to explain and predict which other compounds have this special stability that has come to be called aromaticity. 

This chapter begins with a discussion of some experimental observations that support the conclusion that benzene is especially stable. Then a model based on molecular orbital theory is presented to explain this stability. This model is generalized so that it can be applied to other compounds that are especially stable and also to some that are especially unstable. Several different classes of such compounds are discussed, along with examples of a variety of experimental observations that can be rationalized based on this theory. 

Benzene was discovered in 1825 by Michael Faraday, who isolated it from the liquid that condensed from the gas that was burned in the street lamps of London. Although Faraday was able to deduce that the formula of benzene is C6Hb, it was not until 1866 that the correct structure was proposed by Kekule (see the Focus On box in Chapter 12 on page 469). 

From the beginning, it has been apparent that benzene does not behave like an alkene. For example, the addition 

Look for this logo in the chapter and go to OrganicChemistryNow at http //now.brookscole.com/hornback2 for tutorials, simulations, problems, and molecular models. 

Capsaicin is responsible for the characteristic spicy flavor of jalapeho and habanero peppers. Although it first produces a burning sensation on contact with the mouth or skin, repeated application desensitizes the area to pain. This property has made it the active ingredient in several topical creams for the treatment of chronic pain. Capsaicin has also been used as an animal deterrent in pepper sprays, and as an additive to make birdseed squirrel-proof. Capsaicin is an aromatic compound because it contains a benzene ring. In this chapter, we learn about the characteristics of aromatic compounds like capsaicin. 

The hydrocarbons we have examined thus far—including the alkanes, alkenes, and alkynes, as well as the conjugated dienes and polyenes of Chapter 16— have been aliphatic hydrocarbons. In Chapter 17, we continue our study of conjugated systems with aromatic hydrocarbons. 

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Draw the Product of a Diels-Alder Reaction 588 Benzene and Aromatic Compounds... 

Reactions of lithium alkyls are generally considered to be carbanionic in nature, but in reactions with alkyl halides free radicals have been detected by electron spin resonance.32 Lithium alkyls are widely employed as stereospecific catalysts for the polymerization of alkenes, notably isoprene, which gives up to 90% of 1,4-cA-polyisoprene numerous other reactions with alkenes have been studied.33 The TMED complexes again are especially active not only will they polymerize ethylene but they will even metallate benzene and aromatic compounds, as well as reacting with hydrogen at 1 atm to give LiH and alkane. 

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