Chemistry 350 Organic Chemistry I

Study Guide :: Unit 6

An Overview of Organic Reactions

Unit Preview

This unit is designed to provide a gentle introduction to the subject of reaction mechanisms. Two types of reactions are introduced—polar reactions and radical reactions. It also gives a short review of a number of topics with which you should be familiar, including rates and equilibria, elementary thermodynamics and bond dissociation energies. You must have a working knowledge of these topics to obtain a thorough understanding of organic reaction mechanisms. Reaction energy diagrams are used to illustrate the energy changes that take place during chemical reactions, and to emphasize the difference between a reaction intermediate and a transition state.

Unit Objectives

After you have completed Unit 6, you should be able to

  1. fulfill the detailed objectives listed under each individual section.
  2. identify the polarity pattern in the common functional groups, and explain the importance of being able to do so.
  3. describe the essential differences between polar and radical reactions, and assign a given reaction to one of these two categories.
  4. discuss how kinetic and thermodynamic factors determine the rate and extent of a chemical reaction.
  5. use bond dissociation energies to calculate the ΔH° of simple reactions, and vice versa.
  6. draw and interpret reaction energy diagrams.
  7. define, and use in context, the new key terms.

6.1  Kinds of Organic Reactions

Objective

After completing this section, you should be able to list and describe the four important “kinds” of reactions that occur in organic chemistry.

Learning Activities

Read 6.1 Kinds of Organic Reactions and do any associated exercises.

6.2  How Organic Reactions Occur: Mechanisms

Objectives

After completing this section, you should be able to

  1. explain the difference between heterolytic and homolytic bond breakage, and between heterogenic and homogenic bond formation.
  2. state the two reaction types involved in symmetrical and unsymmetrical processes.

Learning Activities

Read 6.2 How Organic Reactions Occur: Mechanisms and do any associated exercises.

6.3  Radical Reactions

Objectives

After completing this section, you should be able to

  1. give an example of a radical substitution reaction.
  2. identify the three steps (initiation, propagation and termination) that occur in a typical radical substitution reaction.
  3. write out the steps involved in a simple radical substitution reaction, such as the chlorination of methane.
  4. explain why the halogenation of an alkane is not a particularly useful method of preparing specific alkyl halides.

Learning Activities

Read 6.3 Radical Reactions and do any associated exercises.

6.4  Polar Reactions

Objectives

After completing this section, you should be able to

  1. identify the positive and negative ends of the bonds present in the common functional groups.
  2. explain how bond polarity can be enhanced by the interaction of a functional group with a solvent, metal cation or acid.
  3. explain how the polarizability of an atom can be an important factor in determining the reactivity of a bond.
  4. describe the heterolytic bond-breaking process.
  5. use curved (curly) arrows to indicate the movement of electron pairs during bond breakage and bond formation.
  6. predict whether a given species (compound or ion) is likely to behave as a nucleophile or as an electrophile.

Learning Activities

Read 6.4 Polar Reactions and do any associated exercises.

6.5  An Example of a Polar Reaction: Addition of HBr to Ethylene

Objectives

After completing this section, you should be able to

  1. give an example of a simple polar reaction (e.g., a electrophilic addition).
  2. identify the electrophile and nucleophile in a simple polar reaction.

Learning Activities

Read 6.5 An Example of a Polar Reaction: Addition of HBr to Ethylene and do any associated exercises.

6.6  Using Curved Arrows in Polar Reaction Mechanisms

Objective

After completing this section, you should be able to use curved (curly) arrows, in conjunction with a chemical equation, to show the movement of electron pairs in a simple polar reaction, such as electrophilic addition.

Learning Activities

Read 6.6 Using Curved Arrows in Polar Reaction Mechanisms and do any associated exercises.

6.7  Describing a Reaction: Equilibria, Rates and Energy Changes

Objectives

After completing this section, you should be able to

  1. write the equilibrium constant expression for a given reaction.
  2. assess, qualitatively, how far a reaction will proceed in a given direction, given the value of Keq.
  3. explain the difference between rate and equilibrium.
  4. state the relationship between ΔG° and Keq, and use this relationship to determine the value of either of the two variables, given the other.
  5. state the relationship between Gibbs free-energy, enthalpy and entropy, and use the relationship to calculate any one of ΔG°, ΔH° and ΔS°, given the other two.
  6. make a qualitative assessment of whether ΔS° for a given process is expected to be positive or negative.

Learning Activities

  1. Read 6.7 Describing a Reaction: Equilibria, Rates and Energy Changes and do any associated exercises.
  2. Examine the brief reviews of thermodynamics (Appendix A) and kinetics (Appendix B) at the end of this Study Guide. Note that we have also provided selected physical constants and data in Appendix C.

6.8  Describing a Reaction: Bond Dissociation Energies

Objectives

After completing this section, you should be able to

  1. predict the value of ΔH° for a gas-phase reaction, given the necessary bond dissociation energy data.
  2. predict the dissociation energy of a particular bond, given ΔH° for a reaction involving the bond and any other necessary bond dissociation energy data.
  3. outline the limitations of using bond dissociation energies to predict whether or not a given reaction will occur.

Learning Activities

Read 6.8 Describing a Reaction: Bond Dissociation Energies and do any associated exercises.

6.9  Describing a Reaction: Energy Diagrams and Transition States

Objectives

After completing this section, you should be able to

  1. sketch the reaction energy diagram for a single-step reaction, given some indication of whether the reaction is fast or slow, exothermic or endothermic.
  2. interpret the reaction energy diagram for a single-step process (e.g., use the diagram to decide whether the reaction is exothermic or endothermic).
  3. suggest possible transition-state structures for simple one-step processes.
  4. assess the likelihood of a reaction occurring at room temperature, given the value of the activation energy ΔG‡.

Learning Activities

Read 6.9 Describing a Reaction: Energy Diagrams and Transition States and do any associated exercises.

6.10  Describing a Reaction: Intermediates

Objectives

After completing this section, you should be able to

  1. explain the difference between a transition state and an intermediate.
  2. draw a reaction energy diagram for a given multistep process.
  3. interpret the reaction energy diagram of a multistep process (e.g., determine which of the steps is rate-determining).

Learning Activities

Read 6.10 Describing a Reaction: Intermediates and do any associated exercises.

6.11  A Comparison between Biological Reactions and Laboratory Reactions

Objectives

No objectives have been identified for this section.

Learning Activities

Read 6.11 A Comparison between Biological Reactions and Laboratory Reactions and do any associated exercises.

Summary

This unit introduced the subject of reaction mechanisms. Two types of mechanisms were described, with emphasis on the ways in which covalent bonds may be broken and formed. The unit also explored various aspects of physical chemistry that are important when considering organic reaction mechanisms. Topics discussed included reaction kinetics, equilibria and bond dissociation energies. Much of this material should be familiar from your general first-year chemistry course.