Chemistry 350 Organic Chemistry I

Study Guide :: Unit 8

Alkenes: Reactions and Synthesis

Unit Preview

As you have seen, addition reactions dominate the chemistry of alkenes. This unit will show how a variety of reagents can add to alkenes; how hydrogen bromide can be made to add to alkenes in a non-Markovnikov manner; and how alkene molecules can be cleaved into easily identifiable parts. The unit will begin by examining the preparation of alkenes by elimination reactions.

Unit Objectives

After you have completed Unit 8, you should be able to

  1. fulfill all of the detailed objectives listed under each individual section.

  2. design a relatively simple, multistep synthesis using the reactions introduced in this unit, given the structure, name, or both, of the starting material and product. For example, show how you would convert 1-bromobutane to

  3. deduce the structures of a number of compounds involved in a certain reaction sequence, given sufficient information. In other words, solve so-called road-map problems of the type shown here at the end of the unit.
  4. define, and use in context, the key terms introduced in Chapter 8.

8.1  Preparation of Alkenes: A Preview of Elimination Reactions

Objectives

After completing this section, you should be able to

  1. explain the relationship between an addition reaction and an elimination reaction.
  2. write an equation to describe the dehydrohalogenation of an alkyl halide.
  3. identify the reagents required to bring about dehydrohalogenation of an alkyl halide.
  4. write an equation to represent the dehydration of an alcohol.
  5. identify the reagents required to dehydrate a given alcohol.

Learning Activities

Read section 8.1 Preparation of Alkenes: A Preview of Elimination Reactions in the LibreText and do any associated exercises.

8.2  Halogenation of Alkenes: Addition of X2

Objectives

After completing this section, you should be able to

  1. write the equation for the reaction of chlorine or bromine with a given alkene.
  2. identify the conditions under which an addition reaction occurs between an alkene and chlorine or bromine.
  3. draw the structure of the product formed when a given alkene undergoes an addition reaction with chlorine or bromine.
  4. write the mechanism for the addition reaction that occurs between an alkene and chlorine or bromine, and account for the stereochemistry of the product.

Learning Activities

Read section 8.2 Halogenation of Alkenes: Addition of X2 and do any associated exercises.

8.3  Halohydrins from Alkenes: Addition of HOX

Objectives

After completing this section, you should be able to

  1. write the equation for the formation of a halohydrin from an alkene.
  2. write the mechanism for the formation of a halohydrin from an alkene and a mixture of halogen and water.
  3. predict the mechanism of the addition reaction that occurs between a given reagent and an alkene, basing your prediction on mechanisms you have studied in this unit.
  4. identify the alkene, the reagents, or both, that should be used to produce a given halohydrin by an addition reaction.
  5. identify N-bromosuccinimide in aqueous dimethyl sulfoxide as an alternative source of bromine for producing bromohydrins.

Learning Activities

Read section 8.3 Halohydrins from Alkenes: Addition of HOX and do any associated exercises.

8.4  Hydration of Alkenes: Addition of H2O by Oxymercuration

Objectives

After completing this section, you should be able to

  1. write an equation for the hydration of an alkene with sulfuric acid.
  2. write an equation for the formation of an alcohol from an alkene by the oxymercuration-demercuration process.
  3. identify the alkene, the reagents, or both, that should be used to produce a given alcohol by the oxymercuration-demercuration process.
  4. write the mechanism for the reaction of an alkene with mercury(II) acetate in aqueous tetrahydrofuran (THF).

Learning Activities

Read section 8.4 Hydration of Alkenes: Addition of H2O by Oxymercuration and do any associated exercises.

8.5  Hydration of Alkenes: Addition of H2O by Hydroboration

Objectives

After completing this section, you should be able to

  1. identify hydroboration (followed by oxidation) as a method for bringing about the (apparently) non-Markovnikov addition of water to an alkene.
  2. write an equation for the formation of a trialkylborane from an alkene and borane.
  3. write an equation for the oxidation of a trialkylborane to an alcohol.
  4. draw the structure of the alcohol produced by the hydroboration, and subsequent oxidation, of a given alkene.
  5. determine whether a given alcohol should be prepared by oxymercuration-demercuration or by hydroboration-oxidation, and identify the alkene and reagents required to carry out such a synthesis.
  6. write the detailed mechanism for the addition of borane to an alkene, and explain the stereochemistry and regiochemistry of the reaction.

Learning Activities

Read section 8.5 Hydration of Alkenes: Addition of H2O by Hydroboration and do any associated exercises.

8.6  Reduction of Alkenes: Hydrogenation

Objectives

After completing this section, you should be able to

  1. write an equation for the catalytic hydrogenation of an alkene.
  2. identify the product obtained from the hydrogenation of a given alkene.
  3. identify the alkene, the reagents, or both, required to prepare a given alkane by catalytic hydrogenation.
  4. describe the mechanism of the catalytic hydrogenation of alkenes.
  5. explain the difference between a heterogeneous reaction and a homogeneous reaction.
  6. recognize that other types of compounds containing multiple bonds, such as ketones, esters, nitriles and aromatic compounds, do not react with hydrogen under the conditions used to hydrogenate alkenes.

Learning Activities

Read section 8.6 Reduction of Alkenes: Hydrogenation and do any associated exercises.

8.7  Oxidation of Alkenes: Epoxidation and Hydroxylation

Objectives

After completing this section, you should be able to

  1. write the equation for the epoxidation of an alkene using meta-chloroperoxybenzoic acid.
  2. identify the alkene, reagents, or both, that must be used to prepare a given epoxide.
  3. write the equation for the hydroxylation of an alkene using osmium tetroxide, and draw the structure of the cyclic intermediate.
  4. draw the structure of the diol formed from the reaction of a given alkene with osmium tetroxide.
  5. identify the alkene, the reagents, or both, that must be used to prepare a given 1,2‑diol.

Learning Activities

Read section 8.7 Oxidation of Alkenes: Epoxidation and Hydroxylation and do any associated exercises.

8.8  Oxidation of Alkenes: Cleavage to Carbonyl Compounds

Objectives

After completing this section, you should be able to

  1. write an equation to describe the cleavage of an alkene by ozone, followed by reduction of the ozonide so formed with either sodium borohydride or zinc and acetic acid.
  2. predict the products formed from the ozonolysis-reduction of a given alkene.
  3. write an equation to describe the cleavage of an alkene by potassium permanganate.
  4. predict the products from the oxidative cleavage of a given alkene by potassium permanganate.
  5. use the results of ozonolysis-reduction, or cleavage with permanganate, to deduce the structure of an unknown alkene.
  6. identify the reagents that should be used in the oxidative cleavage of an alkene to obtain a given product or products.
  7. write the equation for the cleavage of a 1,2‑diol by periodic acid, and draw the structure of the probable intermediate.
  8. predict the product or products that will be formed from the treatment of a given 1,2‑diol with periodic acid.
  9. use the results of hydroxylation/1,2‑diol cleavage to deduce the structure of an unknown alkene.

Learning Activities

Read section 8.8 Oxidation of Alkenes: Cleavage to Carbonyl Compounds and do any associated exercises.

8.9  Addition of Carbenes to Alkenes: Cyclopropane Synthesis

Objectives

After completing this section, you should be able to

  1. describe, and write the detailed mechanism for, the formation of a carbene, such as dichlorocarbene.
  2. describe the structure of a carbene in terms of the hybridization of the central carbon atom.
  3. write an equation for the formation of a substituted cyclopropane from an alkene and a carbene.
  4. identify the reagents, the alkene, or both, needed to prepare a given substituted cyclopropane by addition of a carbene to a double bond.
  5. identify the substituted cyclopropane formed from the reaction of a given alkene with the reagents necessary to form a carbene.

Learning Activities

Read section 8.9 Addition of Carbenes to Alkenes: Cyclopropane Synthesis and do any associated exercises.

8.10  Radical Additions to Alkenes: Chain-Growth Polymers

Objectives

After completing this section, you should be able to

  1. write the detailed mechanism for the radical polymerization of an alkene.
  2. give examples of some common alkene monomers used in the manufacture of chain-growth polymers.
  3. identify the alkene monomer used to prepare a specific chain-growth polymer, given the structure of the polymer.

Learning Activities

Read section 8.10 Radical Additions to Alkenes: Chain-Growth Polymers and do any associated exercises.

8.11  Biological Additions of Radicals to Alkenes

Objective

After completing this section, you should be able to discuss, briefly, some of the addition reactions that take place in nature, and the role of enzymes in such processes.

Learning Activities

Read section 8.11 Biological Additions of Radicals to Alkenes and do any associated exercises.

8.12  Stereochemistry of Reactions: Addition of H2O to an Achiral Alkene

Objective

After completing this section, you should be able to account for the stereochemistry of the product of the addition of water to an alkene in terms of the formation of a planar carbocation.

Learning Activities

Read section 8.12 Stereochemistry of Reactions: Addition of H2O to an Achiral Alkene and do any associated exercises.

8.13  Stereochemistry of Reactions: Addition of H2O to a Chiral Alkene

Objective

After completing this section, you should be able to explain why the addition of H2O to a chiral alkene leads to unequal amounts of diastereomeric products.

Learning Activities

Read section 8.13 Stereochemistry of Reactions: Addition of H2O to a Chiral Alkene and do any associated exercises.

Summary

In examinations, you will be asked to write a large number of equations, so you should practise by writing equations for the reactions of a variety of alkenes with each of the reagents identified in the section. Next, read about solving “Road-map Problems.”

When you are confident that you understand the material presented in Units 6, 7 and 8, please complete Self Test 2.