Chemistry 350 Organic Chemistry I

Study Guide :: Unit 7

Alkenes: Structure and Reactivity

Unit Preview

This, the first of two units devoted to the chemistry of alkenes, begins by describing the natural occurrence of certain alkenes, and then shows the industrial importance of the two simplest members of this family: ethylene and propylene. The electronic structure of alkenes is reviewed, and their nomenclature discussed in detail. After dealing with the question of cis‑trans isomerism in alkenes, Unit 7 provides a general introduction to the reactivity of the carbon-carbon double bond. Finally, the unit focuses on one specific reaction, the addition of hydrogen halides to alkenes, using it to introduce a number of important concepts, including carbocation stability and the Hammond postulate.

Unit Objectives

After you have completed Unit 7, you should be able to

  1. fulfill all of the detailed objectives listed under each individual section.
  2. describe the importance of alkenes to the chemical industry.
  3. use the concept of “degree of unsaturation” in determining chemical structures.
  4. describe the electronic structure and geometry of alkenes.
  5. describe the factors that influence alkene stability, and determine the relative stability of a number of given alkenes.
  6. write the IUPAC name of a given alkene, and draw the structure of any alkene, given its IUPAC name.
  7. determine whether a given alkene has an E configuration or a Z configuration.
  8. explain why alkenes are more reactive than alkanes.
  9. describe the reaction between an alkene and a hydrogen halide, and explain why one product is formed rather than another, basing your explanation on the concept of carbocation stability and the Hammond postulate.
  10. define, and use in context, the new key terms.

7.0  Introduction

Objective

After completing this section, you should be able to give an example of a naturally occurring compound that contains at least one double bond.

Learning Activities

Read 7.0 Introduction and do any associated exercises.

7.1  Industrial Preparation and Use of Alkenes

Objectives

After completing this section, you should be able to

  1. discuss the industrial importance of ethylene (ethene) and propylene (propene).
  2. describe, briefly, the industrial process known as thermal cracking.

Learning Activities

Read 7.1 Industrial Preparation and Use of Alkenes and do any associated exercises.

7.2  Calculating Degree of Unsaturation

Objectives

After completing this section, you should be able to

  1. determine the degree of unsaturation of an organic compound, given its molecular formula, and hence determine the number of double bonds, triple bonds and rings present in the compound.
  2. draw all the possible isomers that correspond to a given molecular formula containing only carbon (up to a maximum of six atoms) and hydrogen.
  3. draw a specified number of isomers that correspond to a given molecular formula containing carbon, hydrogen, and possibly other elements, such as oxygen, nitrogen and the halogens.

Learning Activities

Read 7.2 Calculating Degree of Unsaturation and do any associated exercises.

7.3  Naming Alkenes

Objectives

After completing this section, you should be able to

  1. provide the correct IUPAC name for an acyclic or cyclic alkene, given its Kekulé, condensed or shorthand structure.
  2. draw the Kekulé, condensed or shorthand structure of an alkene (cyclic or acyclic), given its IUPAC name.
  3. give the IUPAC equivalent of the following trivial names: ethylene, propylene, isobutylene and isoprene.
  4. draw the structure of a vinyl (ethenyl) and allyl (2-propenyl) group, and use these names in alkene nomenclature.

Learning Activities

Read 7.3 Naming Alkenes and do any associated exercises.

7.4  Cis-Trans Isomerism in Alkenes

Objectives

After completing this section, you should be able to

  1. discuss the formation of carbon-carbon double bonds using the concept of sp2 hybridization.
  2. describe the geometry of compounds containing carbon-carbon double bonds.
  3. compare the molecular parameters (bond lengths, strengths and angles) of a typical alkene with those of a typical alkane.
  4. explain why free rotation is not possible about a carbon-carbon double bond.
  5. explain why the lack of free rotation about a carbon-carbon double bond results in the occurrence of cis‑trans isomerism in certain alkenes.
  6. decide whether or not cis‑trans isomerism is possible for a given alkene, and where such isomerism is possible, draw the Kekulé structure of each isomer.

Learning Activities

Read 7.4 Cis-Trans Isomerism in Alkenes and do any associated exercises.

7.5  Sequence Rules: The E,Z Designation

Objectives

After completing this section, you should be able to

  1. illustrate, by means of a suitable example, the limitations of the terms cis and trans in naming isomeric alkenes.
  2. use the E,Z designation to describe the geometry of a given alkene structure.
  3. incorporate the E,Z designation into the IUPAC name of a given alkene.
  4. draw the correct Kekulé, condensed or shorthand structure of an alkene, given its E,Z designation plus other necessary information (e.g., molecular formula, IUPAC name).

Learning Activities

Read 7.5 Alkene Stereochemistry and the E,Z Designation and do any associated exercises.

7.6  Stability of Alkenes

Objectives

After completing this section, you should be able to

  1. explain why cis alkenes are generally less stable than their trans isomers.
  2. explain that catalytic reduction of a cis alkene produces the same alkane as the catalytic reduction of the trans isomer.
  3. explain how heats of hydrogenation (ΔH°hydrog) can be used to show that cis alkenes are less stable than their trans isomers, and discuss, briefly, the limitations of this approach.
  4. arrange a series of alkenes in order of increasing or decreasing stability.
  5. describe, briefly, two of the hypotheses proposed to explain why alkene stability increases with increased substitution. [Note: This problem is a typical example of those instances in science where there is probably no single “correct” explanation for an observed phenomenon.

Learning Activities

Read 7.6 Stability of Alkenes and do any associated exercises.

7.7  Electrophilic Addition Reactions of Alkenes

Objectives

After completing this section, you should be able to

  1. explain the term “electrophilic addition reaction,” using the reaction of a protic acid, HX, with an alkene as an example.
  2. write the mechanism for the reaction of a protic acid, HX, with an alkene.
  3. sketch a reaction energy diagram for the electrophilic addition of an acid, HX, to an alkene.

Learning Activities

Read 7.7 Electrophilic Addition Reactions of Alkenes and do any associated exercises.

7.8  Orientation of Electrophilic Addition: Markovnikov’s Rule

Objectives

After completing this section, you should be able to

  1. use Markovnikov’s rule to predict the product formed when a protic acid, HX, reacts with an alkene.
  2. identify the protic acid, HX, and the alkene that must be reacted together to produce a given alkyl halide. Note: Special conditions are needed if an alkyl iodide is to be produced.
  3. distinguish among primary, secondary and tertiary carbocations.

Learning Activities

Read 7.8 Orientation of Electrophilic Addition: Markovnikov’s Rule and do any associated exercises.

7.9  Carbocation Structure and Stability

Objectives

After completing this section, you should be able to

  1. describe the geometry of a given carbocation.
  2. arrange a given series of carbocations in order of increasing or decreasing stability.
  3. explain the relative stability of methyl, primary, secondary and tertiary carbocations in terms of hyperconjugation and inductive effects.

Learning Activities

Read 7.9 Carbocation Structure and Stability and do any associated exercises.

7.10  The Hammond Postulate

Objective

After completing this section, you should be able to use the Hammond postulate to explain the formation of the most stable carbocation during the addition of a protic acid, HX, to an alkene.

Learning Activities

Read 7.10 The Hammond Postulate and do any associated exercises.

7.11  Evidence for the Mechanism of Electrophilic Addition: Carbocation Rearrangements

Objective

After completing this section, you should be able to explain the “unusual” products formed in certain reactions in terms of the rearrangement of an intermediate carbocation.

Learning Activities

Read 7.11 Evidence for the Mechanism of Electrophilic Addition: Carbocation Rearrangements and do any associated exercises.

Summary

Note that we will frequently refer to many of the concepts from the reaction mechanisms of hydrogen halides with alkenes throughout this course.