Chemistry 350 Organic Chemistry I

Study Guide :: Unit 15

Benzene and Aromaticity

Unit Preview

In Unit 3, we identified an aromatic compound as a compound which contains a benzene ring (or phenyl group). It is now time for us to define aromaticity in a more sophisticated manner. In this unit, 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 unit concludes with a brief summary of the spectroscopic properties of arenes.

Unit Objectives

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

  1. fulfil all of the detailed objectives listed under each individual section.
  2. use the information presented in this unit, along with material from earlier units, to solve problems, particularly road-map problems and problems 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.

15.0  Introduction

Objectives

After completing this section, you should be able to

  1. explain what is meant by the term “aromatic compound.”
  2. identify the aromatic portions present in naturally occurring compounds, given the necessary structures.

Learning Activities

Read 15.0 Introduction and do any associated exercises.

15.1  Sources and Names of Aromatic Compounds

Objectives

After completing this section, you should be able to

  1. draw the structure of each of the common aromatic compounds in Figure 16 of the reading, given their IUPAC-accepted trivial names.
  2. write the IUPAC-accepted trivial name for each of the compounds in Figure 16, given the appropriate Kekulé, condensed or shorthand structure.
  3. identify the ortho, meta and para positions in a monosubstituted benzene ring.
  4. use the ortho/meta/para system to name simple disubstituted aromatic compounds.
  5. draw the structure of a simple disubstituted aromatic compound, given its name according to the ortho/meta/para system.
  6. provide the IUPAC name of a given aromatic compound containing any number of the following substituents: alkyl, alkenyl or alkynyl groups; halogens; nitro groups; carboxyl groups; amino groups; hydroxyl groups.
  7. draw the structure of an aromatic compound containing any number of the substituents listed in Objective 6, above, given the IUPAC name.
  8. provide the IUPAC name of a given aromatic compound in which the phenyl group is regarded as being a substituent.
  9. draw the Kekulé,condensed or shorthand structure of an aromatic compound in which the phenyl group is regarded as a substituent, given its IUPAC name.

Learning Activities

Read 15.1 Sources and Names of Aromatic Compounds and do any associated exercises.

15.2  Structure and Stability of Benzene

Objectives

After completing this section, you should be able to

  1. compare the reactivity of a typical alkene with that of benzene.
  2. use the heat of hydrogenation data to show that benzene is more stable than might be expected for “cyclohexatriene.”
  3. state the length of the carbon-carbon bonds in benzene, and compare this length with those of bonds found in other hydrocarbons.
  4. describe the geometry of the benzene molecule.
  5. describe the structure of benzene in terms of resonance.
  6. describe the structure of benzene in terms of molecular orbital theory.
  7. draw a molecular orbital diagram for benzene.

Learning Activities

Read 15.2 Structure and Stability of Benzene and do any associated exercises.

15.3  Aromaticity and the Hückel 4n + 2 Rule

Objectives

After completing this section, you should be able to

  1. define aromaticity in terms of the Hückel 4n + 2 rule.
  2. use the Hückel 4n + 2 rule to determine whether or not a given polyunsaturated cyclic hydrocarbon should exhibit aromatic properties.
  3. describe the difference in properties between an aromatic hydrocarbon, such as benzene, and a non-aromatic polyunsaturated cyclic hydrocarbon, such as cyclobutadiene or cyclooctatetraene.
  4. draw molecular orbital diagrams for aromatic species, such as benzene, the cyclopentadienyl anion and pyridine,and compare these diagrams with those obtained for non-aromatic species, such as cyclobutadiene and the cyclopentadienyl cation.

Learning Activities

Read 15.3 Aromaticity and the Hückel 4n + 2 Rule and do any associated exercises.

15.4  Aromatic Ions

Objectives

After completing this section, you should be able to

  1. use the Hückel 4n + 2 rule to explain the stability of the cyclopentadienyl anion,the cycloheptatrienyl cation and similar species.
  2. use the Hückel 4n + 2 rule to determine whether or not a given unsaturated cyclic hydrocarbon anion or cation is aromatic.
  3. draw the resonance contributors for the cyclopentadienyl anion, cation, and radical,and similar species.

Learning Activities

Read 15.4 Aromatic Ions and do any associated exercises.

15.5  Aromatic Heterocycles: Pyridine and Pyrrole

Objectives

After completing this section, you should be able to

  1. draw the structure of the common aromatic heterocycles pyridine and pyrrole.
  2. use the Hückel 4n + 2 rule to explain the aromaticity of each of pyridine and pyrrole.
  3. draw a diagram to show the orbitals involved in forming the conjugated six-pi-electron systems present in aromatic heterocycles such as pyridine, pyrrole, etc.

Learning Activities

Read 15.5 Aromatic Heterocycles: Pyridine and Pyrrole and do any associated exercises.

15.6  Polycyclic Aromatic Compounds

Objective

After completing this section, you should be able to draw the resonance contributors for polycyclic aromatic compounds, such as naphthalene, anthracene, etc.

Learning Activities

Read 15.6 Polycyclic Aromatic Compounds and do any associated exercises.

15.7  Spectroscopy of Aromatic Compounds

Objectives

After completing this section, you should be able to

  1. determine whether an unknown compound contains an aromatic ring by inspection of its infrared spectrum, given a table of characteristic infrared absorptions.
  2. state the approximate chemical shift of aryl protons in a proton NMR spectrum.
  3. explain why signals resulting from the presence of aryl protons are found downfield from those caused by vinylic protons in a proton NMR spectrum.
  4. propose possible structures for an unknown aromatic compound, given its proton NMR spectrum,other spectroscopic data (such as a 13C NMR or infrared spectrum), or both.

Learning Activities

Read 15.7 Spectroscopy of Aromatic Compounds and do any associated exercises.

Summary

In this unit, you learned how to name aromatic compounds; saw how the structure of benzene can be explained in terms of both valence bond theory and molecular orbital theory; became familiar with the Hückel 4n + 2 rule, and how it can be applied; and were presented with an overview of the application of a number of spectroscopic techniques to the elucidation of aromatic structures. In the next unit, you will study the reactions of aromatic compounds, particularly electrophilic aromatic substitution reactions.