Chemistry 350 Organic Chemistry I

Study Guide :: Unit 14

Conjugated Compounds and Ultraviolet Spectroscopy

Unit Preview

You have already studied the chemistry of compounds that contain one carbon-carbon double bond. In this unit, you will focus your attention on compounds that contain two or more such bonds. In particular you will study the properties of those compounds that contain two carbon-carbon double bonds which are separated by one carbon-carbon single bond. These compounds are called “conjugated dienes.”

To understand the properties exhibited by conjugated dienes, you must first examine their bonding in terms of the molecular orbital theory introduced in Section 1.5. Then, you must learn how the products of a reaction are dependent on both thermodynamic and kinetic considerations. Which of these two factors is the most important can sometimes determine which one of two possible products will predominate when a reaction is carried out under specific conditions. Although we shall not make extensive use of ultraviolet spectroscopy in this course, this technique can often provide important information when conjugated compounds are being investigated. In general, ultraviolet spectroscopy is less useful than the other spectroscopic techniques introduced in earlier units.

Unit Objectives

After you have completed Unit 14, you should be able to

  1. fulfil all of the detailed objectives listed under each individual section.
  2. use the reactions discussed in this unit, along with those from previous units, when designing multi-step syntheses.
  3. use the reactions and concepts discussed in this unit to solve road-map problems.
  4. use ultraviolet-spectral data, in conjunction with other spectral data, to elucidate the structure of an unknown compound.
  5. define, and use in context, the key terms introduced.

14.0  Introduction

Objective

After completing this section, you should be able to determine whether or not a molecule contains a conjugated system, given its Kekulé, condensed or shorthand formula.

Learning Activities

Read 14.0 Introduction and do any associated exercises.

14.1  Stability of Conjugated Dienes: Molecular Orbital Theory

Objectives

After completing this section, you should be able to

  1. write a reaction sequence to show a convenient method for preparing a given conjugated diene from an alkene, allyl halide, alkyl dihalide or alcohol (diol).
  2. identify the reagents needed to prepare a given diene from one of the starting materials listed in Objective 1, above.
  3. compare the stabilities of conjugated and nonconjugated dienes, using evidence obtained from hydrogenation experiments.
  4. discuss the bonding in a conjugated diene, such as 1,3-butadiene, in terms of the hybridization of the carbon atoms involved.
  5. discuss the bonding in 1,3-butadiene in terms of the molecular orbital theory, and draw a molecular orbital for this and similar compounds.

Learning Activities

Read 14.1 Stability of Conjugated Dienes: Molecular Orbital Theory and do any associated exercises.

14.2  Electrophilic Additions to Conjugated Dienes: Allylic Carbocations

Objectives

After completing this section, you should be able to

  1. write an equation for the addition of one or two mole equivalents of a halogen or a hydrogen halide to a nonconjugated diene.
  2. write an equation for the addition of one or two mole equivalents of a halogen or a hydrogen halide to a conjugated diene.
  3. write the mechanism for the addition of one mole equivalent of hydrogen halide to a conjugated diene, and hence account for the formation of 1,2- and 1,4-addition products.
  4. explain the stability of allylic carbocations in terms of resonance.
  5. draw the resonance contributors for a given allylic carbocation.
  6. predict the products formed from the reaction of a given conjugated diene with one mole equivalent of halogen or hydrogen halide.
  7. predict which of the possible 1,2- and 1,4-addition products is likely to predominate when one mole equivalent of a hydrogen halide is reacted with a given conjugated diene.
  8. use the concept of carbocation stability to explain the ratio of the products obtained when a given conjugated diene is reacted with one mole equivalent of hydrogen halide.

Learning Activities

Read 14.2 Electrophilic Additions to Conjugated Dienes: Allylic Carbocations and do any associated exercises.

14.3  Kinetic versus Thermodynamic Control of Reactions

Objectives

After completing this section, you should be able to

  1. explain the difference between thermodynamic and kinetic control of a chemical reaction; for example, the reaction of a conjugated diene with one equivalent of hydrogen halide.
  2. draw a reaction energy diagram for a reaction which can result in both a thermodynamically controlled product and a kinetically controlled product.
  3. explain how reaction conditions can determine the product ratio in a reaction in which there is competition between thermodynamic and kinetic control.

Learning Activities

Read 14.3 Kinetic versus Thermodynamic Control of Reactions and do any associated exercises.

14.4  The Diels-Alder Cycloaddition Reaction

Objectives

After completing this section, you should be able to

  1. write an equation to represent a typical Diels-Alder reaction.
  2. draw the structure of the product formed when a given conjugated diene reacts with a given dienophile in a Diels-Alder reaction.
  3. identify the diene and dienophile that must be used to prepare a given compound by a Diels-Alder reaction.
  4. explain the general mechanism of the Diels-Alder reaction, without necessarily being able to describe it in detail.

Learning Activities

Read 14.4 The Diels-Alder Cycloaddition Reactions and do any associated exercises.

14.5  Characteristics of the Diels-Alder Reaction

Objectives

After completing this section, you should be able to

  1. determine whether or not a given compound would behave as a reactive dienophile in a Diels-Alder reaction.
  2. predict the stereochemistry of the product obtained from the reaction of a given diene with a given dienophile.
  3. recognize that in order to undergo a Diels-Alder reaction, a diene must be able to assume an s-cis geometry, and determine whether or not a given diene can assume this geometry.

Learning Activities

Read 14.5 Characteristics of the Diels-Alder Reaction and do any associated exercises.

14.6  Diene Polymers: Natural and Synthetic Rubbers

Objectives

After completing this section, you should be able to

  1. show that the polymerization of a diene, such as 1,3-butadiene or isoprene (2‑methyl-1,3-butadiene), can result in the formation of either a cis or trans polymer.
  2. draw the structure of natural rubber.
  3. explain, briefly, the process of vulcanization.

Learning Activities

Read 14.6 Diene Polymers: Natural and Synthetic Rubbers and do any associated exercises.

14.7  Structure Determination of Conjugated Systems: Ultraviolet Spectroscopy

Objectives

After completing this section, you should be able to

  1. identify the ultraviolet region of the electromagnetic spectrum which is of most use to organic chemists.
  2. interpret the ultraviolet spectrum of 1,3-butadiene in terms of the molecular orbitals involved.
  3. describe in general terms how the ultraviolet spectrum of a compound differs from its infrared and NMR spectra.

Learning Activities

Read 14.7 Structure Determination of Conjugated Systems: Ultraviolet Spectroscopy and do any associated exercises.

14.8  Interpreting Ultraviolet Spectra: The Effect of Conjugation

Objective

After completing this section, you should be able to use data from ultraviolet spectra to assist in the elucidation of unknown molecular structures.

Learning Activities

Read 14.8 Interpreting Ultraviolet Spectra: The Effect of Conjugation and do any associated exercises.

14.9  Conjugation, Colour and the Chemistry of Vision

Objectives

After completing this section, you should be able to

  1. explain why some organic compounds have different colours based on compound structure and our perception of light.
  2. state the relationship between frequency of light absorbed and the extent of conjugation in an extended pi electron system.

Learning Activities

Read 14.9 Conjugation, Colour and the Chemistry of Vision and do any associated exercises.

Summary

In this unit, you learned about the differences between conjugated dienes and nonconjugated dienes. The unusual stability of the former was explained in terms of the molecular orbital theory. The addition reactions of conjugated dienes were discussed, and you saw how the products obtained in these reactions depend on both thermodynamic and kinetic considerations. The Diels-Alder reaction was described, and ultraviolet spectroscopy was introduced.

When you are confident that you understand the material presented in Units 12 to 14, please complete Self Test 5.