Chemistry 350 Organic Chemistry I

Study Guide :: Unit 10

Organohalides

Unit Preview

You have already encountered alkyl halides several times in this course. In this unit, you will examine in some detail the methods that are used to prepare these important compounds. Alkyl halides are extremely useful to chemists involved in the synthesis of new organic compounds, particularly because of their ability to react with certain metals and form organometallic compounds. However, a detailed discussion of the reactions of alkyl halides will be delayed until Unit 11.

Unit Objectives

After you have completed Unit 10, you should be able to

  1. fulfil all of the detailed objectives listed under each individual section.
  2. design a multistep synthesis to prepare a given compound from a given starting material using any of the reactions studied up to this point in the course, including those which involve alkyl halides.
  3. solve road-map problems requiring a knowledge of any of the reactions or concepts studied up to this point, including those introduced in this unit.
  4. define, and use in context, the key terms introduced.

10.0  Introduction

Objectives

After completing this section, you should be able to

  1. list the industrial uses of some important halogenated hydrocarbons including 1,1,1-trichloroethane, tetrafluoroethylene and dichlorodifluoromethane.
  2. outline, briefly, how the chemistry of vinyl halides and aryl halides differs from that of the alkyl halides discussed in this unit.

Learning Activities

Read 10.0 Introduction and do any associated exercises.

10.1  Names and Properties of Alkyl Halides

Objectives

After completing this section, you should be able to

  1. write the IUPAC name of a halogenated aliphatic hydrocarbon, given its Kekulé, condensed or shorthand structure.
  2. draw the Kekulé, condensed or shorthand structure of a halogenated aliphatic hydrocarbon, given it IUPAC name.
  3. write the IUPAC name and draw the Kekulé, condensed or shorthand structure of a simple alkyl halide, given a systematic, non-IUPAC name (e.g., sec-butyl iodide).
  4. arrange a given series of carbon-halogen bonds in order of increasing or decreasing length and strength.

Learning Activities

Read 10.1 Names and Properties of Alkyl Halides and do any associated exercises.

10.2  Preparing Alkyl Halides from Alkanes: Radical Halogenation

Objectives

After completing this section, you should be able to

  1. explain why the radical halogenation of alkanes is not usually a particularly good method of preparing pure samples of alkyl halides.

  2. use $\ce{\sf{C–H}}$ bond energies to account for the fact that in radical chlorinations, the reactivity of hydrogen atoms decreases in the order

    tertiary > secondary > primary.

  3. predict the approximate ratio of the expected products from the monochlorination of a given alkane.

Learning Activities

Read 10.2 Preparing Alkyl Halides from Alkanes: Radical Halogenation and do any associated exercises.

10.3  Preparing Alkyl Halides from Alkenes: Allylic Bromination

Objectives

After completing this section, you should be able to

  1. write the equation for the bromination of a symmetrical alkene using N‑bromosuccinimide.
  2. predict the product formed when a given symmetrical alkene is treated with N-bromosuccinimide.
  3. identify the reagent, the symmetrical alkene, or both, needed to produce a given allyl halide by allylic bromination.
  4. list the following radicals in order of increasing or decreasing stability: allyl, vinyl, primary alkyl, secondary alkyl, tertiary alkyl, methyl.
  5. explain the ease of forming an allyl radical, and the difficulty of forming a vinyl radical, in terms of the relative $\ce{\sf{C–H}}$ bond dissociation energies.

Learning Activities

Read 10.3 Preparing Alkyl Halides from Alkenes: Allylic Bromination and do any associated exercises.

10.4  Stability of the Allyl Radical: Resonance Revisited

Objectives

After completing this section, you should be able to

  1. explain the stability of the allyl radical in terms of resonance.
  2. explain the difference between resonance and tautomerism. [Note: Review Section 8.4 if necessary.]
  3. write an equation for the reaction of an unsymmetrical alkene with N‑bromosuccinimide.
  4. draw the structure of each of the possible products that could be obtained from the reaction of a given unsymmetrical alkene with N-bromosuccinimide, and predict which product will predominate.
  5. explain the formation of more than one product from the reaction of N‑bromosuccinimide with a given unsymmetrical alkene.
  6. explain the observed product ratio when a given unsymmetrical alkene is treated with N-bromosuccinimide.

Learning Activities

Read 10.4 Stability of the Allyl Radical: Resonance Revisited and do any associated exercises.

10.5  Preparing Alkyl Halides from Alcohols

Objectives

After completing this section, you should be able to

  1. write an equation for the conversion of an alcohol to an alkyl halide.
  2. list a given series of alcohols in increasing or decreasing order of reactivity with hydrogen halides.
  3. identify the alkyl halide formed when a given alcohol reacts with thionyl chloride, phosphorus tribromide, or a hydrogen halide.
  4. identify the alcohol which should be used to prepare a given alkyl halide using one of the reagents specified in Objective 3.
  5. select the most appropriate reagent for converting a given alcohol to a given alkyl halide.

Learning Activities

Read 10.5 Preparing Alkyl Halides from Alcohols and do any associated exercises.

10.6  Reactions of Alkyl Halides: Grignard Reagents

Objectives

After completing this section, you should be able to

  1. write an equation to describe the formation of a Grignard reagent.
  2. give examples of Grignard reagents formed from aryl and vinyl halides as well as from alkyl halides.
  3. explain the reactivity of Grignard reagents in terms of the polarity of the carbon-magnesium bond.
  4. write an equation for the reaction of a Grignard reagent with a proton donor, such as water.
  5. predict the product formed from the reaction of a given organohalide with magnesium followed by a proton donor.
  6. identify the organohalide, the reagents, or both, needed to prepare a given alkane.
  7. describe how a deuterium atom may be introduced at a specific location in an organic molecule through use of a Grignard reagent.
  8. describe at least one limitation on the use of Grignard reagents in organic synthesis.
  9. write an equation for the direct conversion of an alkyl halide to an alkane using a hydride donor, such as lithium aluminum hydride.

Learning Activities

Read 10.6 Reactions of Alkyl Halides: Grignard Reagents and do any associated exercises.

10.7  Organometallic Coupling Reactions

Objectives

After completing this section, you should be able to

  1. write an equation for the formation of an alkyllithium from an alkyl halide.
  2. write an equation for the formation of a lithium dialkylcopper (Gilman) reagent from an alkyllithium and copper(I) iodide.
  3. write an equation for the coupling of a lithium dialkylcopper reagent with an alkyl halide (i.e., a Corey-House synthesis).
  4. draw the structure of the product formed from a given Corey-House synthesis.
  5. identify the reagents needed to convert two given organohalides to a specified hydrocarbon through a Corey-House synthesis.

Learning Activities

Read 10.7 Organometallic Coupling Reactions and do any associated exercises).

10.8  Oxidation and Reduction in Organic Chemistry

Objectives

After completing this section, you should be able to

  1. identify organic reactions as being oxidations, reductions, or neither.
  2. rank given compounds in order of their oxidation level.

Learning Activities

Read 10.8 Oxidation and Reduction in Organic Chemistry and do any associated exercises.

Summary

The main purpose of the unit was to discuss the preparation and nomenclature of alkyl halides. The concept of resonance was introduced, one which will be used frequently throughout the remainder of the course. Finally, you saw how alkyl halides can react to form two important classes of organometallic compounds: Grignard reagents and organolithiums. The role of the latter in the Corey-House synthesis of alkanes was emphasized; the reactions of Grignard reagents will be discussed later.