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22: Arenes, Electrophilic Aromatic Substitution

  • Page ID
    21986
  • Benzene and other aromatic hydrocarbons usually have such strikingly different properties from typical open-chain conjugated polyenes, such as 1,3,5-hexatriene, that it is convenient to consider them as a separate class of compounds called arenes. In this chapter we shall outline the essential features of the chemistry of arenes, particularly their reactions with electrophilic reagents which result in the substitution of a ring hydrogen with other functional groups. Some of the important properties of benzene were discussed in Chapter 21 in connection with the valence-bond and molecular-orbital theories, which rationalize the bonding in benzene and account for the remarkable stability and low reactivity of benzene (Section 21-3A). This chapter is especially concerned with chemical properties of benzene and its derivatives as well as related ring systems.

    • 22.1: Nomenclature of Arenes
      The naming of benzene derivatives is relatively straightforward. However, many benzene derivatives have acquired trivial names, and we draw your attention to a few of these below. The accepted name for the -C6H5 group as a substituent is phenyl
    • 22.2: Physical Properties of Arenes
      The pleasant odors of the derivatives of many arenes is the origin of the name aromatic hydrocarbons. The arenes themselves generally are quite toxic; some are carcinogenic and inhalation of their vapors should be avoided. The volatile arenes are highly flammable and burn with a luminous sooty flame, in contrast to alkanes and alkenes, which usually burn with a bluish flame leaving little carbon residue.
    • 22.3: Spectral Properties of Arenes
      The presence of a phenyl group in a compound can be ascertained with a fair degree of certainty from its infrared spectrum.
    • 22.4: Electrophilic Aromatic Substitution
      In this section we shall be mainly interested in the reactions of arenes that involve attack on the carbon atoms of the aromatic ring. We shall not elaborate now on the reactions of substituent groups around the ring.
    • 22.5: Effect of Substituents on Reactivity and Orientation in Electrophilic Aromatic Substitution
      In planning syntheses based on substitution reactions of substituted benzenes, it is imperative to be able to predict in advance which of the available positions of the ring are likely to be most reactive. This is now possible with a rather high degree of certainty, thanks to the work of many chemists during the past 100 years. Few, if any, other problems in organic chemistry have received so much attention over so many years.
    • 22.6: Orientation in Disubstituted Benzenes
      The orientation and reactivity effects of substituents discussed for the substitution of monosubstituted benzenes also hold for disubstituted benzenes, except that the directing influences now come from two groups. Qualitatively, the effects of the two substituents are additive on the reactivity. We therefore would expect 4-nitromethylbenzene to be less reactive than methylbenzene because of the deactivating effect of a nitro group.
    • 22.7: IPSO Substitution
      For all practical purposes, electrophilic aromatic substitution is confined to the substitution of a ring hydrogen. Attack at the substituted carbon evidently does occur, but it does not lead directly to substitution products because demethylation, unlike deprotonation, does not occur. Instead, the nitro group changes positions to the neighboring ring carbon, which then can eliminate a proton to form a substitution product.
    • 22.8: Substitution Reactions of Polynuclear Aromatic Hydrocarbons
      Although naphthalene, phenanthrene, and anthracene resemble benzene in many respects, they are more reactive than benzene in both substitution and addition reactions. This increased reactivity is expected on theoretical grounds because quantum-mechanical calculations show that the net loss in stabilization energy for the first step in electrophilic substitution or addition decreases progressively from benzene to anthracene.
    • 22.9: Addition Reactions of Arenes
      Benzenoid compounds are not readily converted to cyclohexane derivatives. Nevertheless, several addition reactions are carried out on an industrial scale. Mention was made previously of the hydrogenation of benzene to cyclohexane in the presence of a nickel catalyst.
    • 22.E: Arenes, Electrophilic Aromatic Substitution (Exercises)
      These are the homework exercises to accompany Chapter 22 of the Textmap for Basic Principles of Organic Chemistry (Roberts and Caserio).
    • 22.10: Oxidation Reactions
      The reagents usually employed for the oxidation of alkenes normally do not attack benzene. Although at high temperatures, benzene can be oxidized to cis-butenedioic (maleic) anhydride by air with a vanadium pentoxide catalyst. Ozonization of aromatic hydrocarbons is possible .e.g, ozonation of benzene gives ethanedial (glyoxal).
    • 22.11: Sources and Uses of Aromatic Hydrocarbons
      Benzene and many of its derivatives are manufactured on a large scale for use in high-octane gasolines and in the production of polymers, insecticides, detergents, dyes, and many miscellaneous chemicals. Most of the benzene and almost all of the methylbenzene and the dimethylbenzenes produced in the United States are derived from petroleum.
    • 22.12: Some Conjugated Cyclic Polyenes
      There are several compounds that possess some measure of aromatic character typical of benzene, but do not possess a benzenoid ring. Appropriately, they have (4n+2)(4n+2) ππ electrons and are classified as nonbenzenoid aromatic compounds
    • 22.13: Fluxional Compounds
      A number of compounds are known to rearrange from one structure to an entirely equivalent structure, sometimes with extraordinary facility. Such compounds are said to be fluxional to distinguish from tautomers (which usually involve rearrangements between nonequivalent structures).

    Contributors and Attributions

    John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."