Chemistry 350 Organic Chemistry I

Study Guide :: Unit 9

Alkynes: An Introduction to Organic Synthesis

Unit Preview

Addition reactions not only dominate the chemistry of alkenes, they are also the major class of reaction encountered in the chemistry of alkynes. This unit will discuss an important difference between (terminal) alkynes and alkenes, that is, the acidity of the former; it also addresses the problem of devising organic syntheses. Once you have completed this unit you will have increased the number of organic reactions in your repertoire, and should be able to design much more elaborate multistep syntheses. As you work through this unit, you should notice the many similarities among the reactions described here and those in Units 7 and 8.

Unit Objectives

After you have completed Unit 9, you should be able to

  1. fulfil all of the detailed objectives listed under each individual section.
  2. solve road-map problems involving any of the reactions introduced to this point.
  3. design multistep syntheses using any of the reactions introduced to this point, and determine the viability of a given synthesis.
  4. define, and use in context, the key terms introduced in this unit.

9.1  Naming Alkynes

Objectives

After completing this section, you should be able to

  1. provide the correct IUPAC name of an alkyne, given its Kekulé, condensed or shorthand structure.
  2. provide the correct IUPAC name of a compound containing both double and triple bonds, given its Kekulé, condensed or shorthand structure.
  3. draw the structure of a compound containing one or more triple bonds, and possibly one or more double bonds, given its IUPAC name.
  4. name and draw the structure of simple alkynyl groups, and where appropriate, use these names as part of the IUPAC system of nomenclature.

Learning Activities

Read 9.1 Naming Alkynes and do any associated exercises.

9.2  Preparation of Alkynes: Elimination Reactions of Dihalides

Objectives

After completing this section, you should be able to

  1. write an equation to describe the preparation of an alkyne by the dehydrohalogenation of a vicinal dihalide or vinylic halide.
  2. identify the alkyne produced from the dehydrohalogenation of a given vicinal dihalide or vinylic halide.
  3. write a reaction sequence to show how the double bond of an alkene can be transformed into a triple bond.
  4. identify the vicinal dihalide (or vinylic halide) needed to synthesize a given alkyne by dehydrohalogenation.

Learning Activities

Read 9.2 Preparation of Alkynes: Elimination Reactions of Dihalides and do any associated exercises.

9.3  Reactions of Alkynes: Addition of HX and X2

Objectives

After completing this section, you should be able to

  1. describe the bonding and geometry of the carbon-carbon triple bond in terms of the sp-hybridization of the carbon atoms involved.
  2. explain the reactivity of alkynes based on the known strengths of carbon-carbon single, double and triple bonds.
  3. write equations for the reaction of an alkyne with one or two equivalents of halogen (chlorine or bromine) or halogen acid (HCl, HBr or HI).
  4. draw the structure of the product formed when an alkyne reacts with one equivalent of the halogens and halogen acids listed in Objective 3.
  5. identify the alkyne which must have been used in an addition reaction with a halogen or halogen acid, given the product of such a reaction.

Learning Activities

Read 9.3 Reactions of Alkynes: Addition of HX and X2 and do any associated exercises.

9.4  Hydration of Alkynes

Objectives

After completing this section, you should be able to

  1. write the equation for the reaction of water with an alkyne in the presence of sulfuric acid and mercury(II) sulfate.
  2. describe keto-enol tautomerism.
  3. predict the structure of the ketone formed when a given alkyne reacts with sulfuric acid in the presence of mercury(II) sulfate.
  4. identify the reagents needed to convert a given alkyne to a given ketone.
  5. identify the alkyne needed to prepare a given ketone by hydration of the triple bond.
  6. write an equation for the reaction of an alkyne with borane.
  7. write the equation for the reaction of a vinylic borane with basic hydrogen peroxide or hot acetic acid.
  8. identify the reagents, the alkyne, or both, needed to prepare a given ketone or a given cis alkene through a vinylic borane intermediate.
  9. identify the ketone produced when a given alkyne is reacted with borane followed by basic hydrogen peroxide.
  10. identify the cis alkene produced when a given alkyne is reacted with borane followed by hot acetic acid.
  11. explain why it is necessary to use a bulky, sterically hindered borane when preparing vinylic boranes from terminal alkynes.
  12. predict the product formed when the vinylic borane produced from a terminal alkyne is treated with basic hydrogen peroxide.
  13. identify the alkyne needed to prepare a given aldehyde by a vinylic borane.

Learning Activities

Read 9.4 Hydration of Alkynes and do any associated exercises.

9.5  Reduction of Alkynes

Objectives

After completing this section, you should be able to

  1. write equations for the catalytic hydrogenation of alkynes to alkanes and cis alkenes.
  2. identify the reagent and catalyst required to produce a given alkane or cis alkene from a given alkyne.
  3. identify the product formed from the reaction of a given alkyne with hydrogen and a specified catalyst.
  4. identify the alkyne that must be used to produce a given alkane or cis alkene by catalytic hydrogenation.
  5. write the equation for the reduction of an alkyne with an alkali metal and liquid ammonia.
  6. predict the structure of the product formed when a given alkyne is reduced with an alkali metal and liquid ammonia.
  7. identify the alkyne that must be used to produce a given alkene by reduction with an alkali metal and ammonia.

Learning Activities

Read 9.5 Reduction of Alkynes and do any associated exercises.

9.6  Oxidative Cleavage of Alkynes

Objectives

After completing this section, you should be able to

  1. write an equation to represent the oxidative cleavage of an alkyne with potassium permanganate or ozone.
  2. identify the products that result from the oxidative cleavage of a given alkyne.
  3. identify the reagents needed to carry out the oxidative cleavage of an alkyne.
  4. use the results of an oxidative cleavage to determine the identity of an alkyne of unknown structure.

Learning Activities

Read 9.6 Oxidative Cleavage of Alkynes and do any associated exercises.

9.7  Alkyne Acidity: Formation of Acetylide Anions

Objectives

After completing this section, you should be able to

  1. write an equation for the reaction that occurs between a terminal alkyne and a strong base, such as sodamide, NaNH2.
  2. rank a given list of compounds, including water, acetylene and ammonia, in order of increasing or decreasing acidity.
  3. rank a given list of hydrocarbons, such as acetylene, ethylene and ethane, in order of increasing or decreasing acidity.
  4. describe a general method for determining which of two given compounds is the stronger acid.
  5. provide an acceptable explanation of why terminal alkynes are more acidic than alkanes or alkenes.

Learning Activities

Read 9.7 Alkyne Acidity: Formation of Acetylide Anions and do any associated exercises.

9.8  Alkylation of Acetylide Anions

Objectives

After completing this section, you should be able to

  1. write an equation to describe the reaction of an acetylide ion with an alkyl halide.
  2. discuss the importance of the reaction between acetylide ions and alkyl halides as a method of extending a carbon chain.
  3. identify the alkyne (and hence the acetylide ion) and the alkyl halide needed to synthesize a given alkyne.
  4. determine whether or not the reaction of an acetylide ion with a given alkyl halide will result in substitution or elimination, and draw the structure of the product formed in either case.

Learning Activities

Read 9.8 Alkylation of Acetylide Anions and do any associated exercises.

9.9  An Introduction to Organic Synthesis

Objective

After completing this section, you should be able to design a multistep synthesis to prepare a given product from a given starting material, using any of the reactions introduced in the textbook up to this point.

Learning Activities

Read 9.9 An Introduction to Organic Synthesis and do any associated exercises.

Summary

Probably the most effective way of determining how much you have remembered and understood is to attempt some of the problems found in each section. We cannot over-emphasize the fact that the best way of achieving success in an organic chemistry course is to attempt lots of problems. You may not want to try all of these problems at this stage. One possible approach is to do some of them now, some more when you are preparing for your midterm examination, and the remainder when you review the whole course in preparation for the final examination.

At this point, complete the first assignment, which you will find in the Assignment Drop Boxes on the course homepage, and submit your work for grading. You may also wish to make arrangements to write the midterm—see “Procedures for Applying for and Writing Examinations” in the Student Manual.

When you are confident that you understand the material presented in Unit 9, please complete Self Test 3.

You should now be almost ready to write your midterm examination (Units 3-9). You can prepare for your midterm by

  1. reviewing the objectives of each section, making sure you can achieve each one.
  2. reviewing the feedback from your academic expert on Assignment 1.
  3. reviewing Self Tests 1-3.
  4. writing the practice midterm examination.