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15: Benzene and Aromaticity

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    448698
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    Learning Objectives

    After you have completed Chapter 15, you should be able to

    1. fulfillall of the detailed objectives listed under each individual section.
    2. use the information presented in this chapter, along with material from earlier chapters, to solve problems, particularly road-map problems and those requiring an understanding of spectroscopy.
    3. explain the concept of aromaticity and the stability of aromatic compounds.
    4. define, and use in context, the key terms introduced.

    In Chapter 3, we identified an aromatic compound as being a compound which contains a benzene ring (or phenyl group). It is now time to define aromaticity in a more sophisticated manner. In this chapter, we discuss the stability of benzene and other aromatic compounds, explaining it in terms of resonance and molecular orbital theory. You will study the nomenclature of aromatic compounds and the Hückel (4n + 2) rule for predicting aromaticity. The chapter concludes with a brief summary of the spectroscopic properties of arenes.

    • 15.0: Why This Chapter?
      This chapter introduces the significance of benzene and aromatic compounds in organic chemistry. It highlights their unique stability due to resonance and the delocalization of π electrons, differentiating them from non-aromatic compounds. Understanding these properties is crucial for grasping reactions and applications involving aromaticity, which plays a vital role in the structure and function of many biological molecules and synthetic materials.
    • 15.1: Naming Aromatic Compounds
      Aromatic substances, more than any other class of organic compounds, have acquired a large number of nonsystematic names. IUPAC rules discourage the use of most such names but do allow some of the more widely used ones to be retained. Thus, methylbenzene is known commonly as toluene; hydroxybenzene as phenol; aminobenzene as aniline; and so on.
    • 15.2: Structure and Stability of Benzene
      Benzene (C6H6) has six fewer hydrogens than the six-carbon cycloalkane cyclohexane (C6H12) and is clearly unsaturated, usually being represented as a six-membered ring with alternating double and single bonds. Yet it has been known since the mid-1800s that benzene is much less reactive than typical alkenes and fails to undergo typical alkene addition reactions.
    • 15.3: Aromaticity and the Hückel 4n + 2 Rule
      In 1931, German chemist and physicist Erich Hückel proposed a theory to help determine if a planar ring molecule would have aromatic properties. His rule states that if a cyclic, planar molecule has 4n+2 π electrons, it is considered aromatic. This rule would come to be known as Hückel's Rule.
    • 15.4: Aromatic Ions
      Aromatic ions, such as cyclopentadienyl and phenyl cations, maintain aromatic stability due to their cyclic, planar structures and delocalized π electrons. These ions can participate in electrophilic aromatic substitution reactions. Their unique stability is attributed to resonance, allowing for multiple contributing structures. This stability influences their reactivity and properties in various organic reactions.
    • 15.5: Aromatic Heterocycles - Pyridine and Pyrrole
      The definition of aromaticity discussed previously was a cyclic, conjugated molecule containing 4n + 2 π electrons. Nothing in this definition says that the atoms in the ring must be carbon. In fact, heterocyclic compounds can also be aromatic. A heterocycle is a cyclic compound that contains atoms of more than one element in its ring, usually carbon along with nitrogen, oxygen, or sulfur.
    • 15.6: Polycyclic Aromatic Compounds
      The Hückel rule is strictly applicable only to monocyclic compounds, but the general concept of aromaticity can be extended to include polycyclic aromatic compounds. Naphthalene, with two benzene-like rings fused together; anthracene, with three rings; benzo[a]pyrene, with five rings; and coronene, with six rings, are all well-known aromatic hydrocarbons.
    • 15.7: Spectroscopy of Aromatic Compounds
      The spectroscopy of aromatic compounds focuses on techniques like UV-Vis, IR, and NMR spectroscopy. UV-Vis helps identify conjugated systems and their π→π* transitions, while IR spectroscopy reveals characteristic C=C stretching vibrations. NMR provides insights into the hydrogen environment in aromatic systems, distinguishing between protons on different carbon atoms. These techniques are essential for understanding the structure and behavior of aromatic compounds.
    • 15.8: Chemistry Matters—Aspirin, NSAIDs, and COX-2 Inhibitors
      Whatever the cause—whether tennis elbow, a sprained ankle, or a wrenched knee—pain and inflammation seem to go together. They are, however, different in their origin, and powerful drugs are available for treating each separately. For minor pains and inflammation, both problems are often treated together by using a common over-the-counter medication called an NSAID, or nonsteroidal anti-inflammatory drug.
    • 15.9: Key Terms
    • 15.10: Summary
      Aromatic rings are a common part of many biological structures and are particularly important in nucleic acid chemistry and in the chemistry of several amino acids. In this chapter, we’ve seen how and why aromatic compounds are different from such apparently related compounds as cycloalkenes.


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