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9: Reactions of Alkenes

  • Page ID
    136932
  • Learning Objectives

    After reading this chapter and completing ALL the exercises, a student can be able to

    • draw the general Electrophilic Addition Reaction (EAR) mechanism for an alkene - refer to section 9.1
    • predict the products/specify the reagents for EAR of hydrohalic acids (HX) with symmetrical alkenes - refer to section 9.2
    • predict the products/specify the reagents for EAR of hydrohalic acids (HX) with asymmetrical alkenes using Markovnikov's Rule for Regioselectivity - refer to section 9.3
    • apply the principles of regioselectivity and stereoselectivity to the addition reactions of alkenes - refer to sections 9.3 - 9.14
    • predict the products, specify the reagents, and discern most efficient reaction for hydration of alkenes (acid catalyzed hydration; or oxymercuration/demercuration; or hydroboration/oxidation) - refer to sections 9.4, 9.5, and 9.6 respectively
    • discern the stereochemical differences between the EAR of chiral and achiral alkenes - refer to sections 9.7 and 9.8
    • predict the products/specify the reagents for halogenation and hydrohalogenation of alkenes - refer to sections 9.9 and 9.10 respectively
    • recognize organic oxidation and reduction reactions - refer to sections 9.11 and 9.12
    • predict the products/specify the reagents for hydrogenation (reduction) of alkenes - refer to section 9.11
    • predict the products/specify the reagents for epoxidation of alkenes - refer to section 9.12
    • predict the products/specify the reagents for dihydroxylation of alkenes - refer to sections 9.13 and 9.14
    • predict the products/specify the reagents for oxidative cleavage of alkenes - refer to section 9.15
    • predict the products of carbene additions to alkenes - refer to section 9.16
    • predict the polymer/specify the monomer for radical, chain -growth polymers of alkenes - refer to section 9.17
    • discuss an example biological addition reactions - refer to section 9.18

    • 9.1: Electrophilic Addition Reactions (EARs)
      Electrophilic addition reactions can occur in compounds containing pi bonds like the alkenes.  Depending on the structure of the alkene and the specific reagents, the reactions can be regioselective and/or stereoselective.
    • 9.2: Addition of Hydrogen Halides to Symmetrical Alkenes
      The regioselective reaction of the carbon-carbon double bond in alkenes with hydrohalogens (HX) is a controlled by carbocation stability.  Consequently, the symmetry of the alkene must be considered for this mechanistic pathway.
    • 9.3: Alkene Asymmetry and Markovnikov's Rule
      The regioselectivity of electrophilic addition reactions is determined by carbocation stability and is summarized by Markovnikov's Rule.
    • 9.4: Hydration: Acid Catalyzed Addition of Water
      Electrophilic hydration is the act of adding electrophilic hydrogen from a non-nucleophilic strong acid (a reusable catalyst, examples of which include sulfuric and phosphoric acid) and applying appropriate temperatures to break the alkene's double bond. After a carbocation is formed, water bonds with the carbocation to form a 1º, 2º, or 3º alcohol on the alkane.
    • 9.5: Hydration: Oxymercuration-Demercuration
      Oxymercuration is a stereospecific, regioselective electrophilic addition raction because there are no carbocation rearrangements due to stabilization of the reactive intermediate.  The Markovnikov products are reliably synthesized by this pathway.
    • 9.6: Hydration: Hydroboration-Oxidation
      Hydroboration-oxidation of alkenes has been a very valuable laboratory method for the stereoselectivity and regioselectivity of alkenes that are the non-Markovnikov products for alkene hydration.
    • 9.7: Stereochemistry of Reactions - Hydration of Achiral Alkenes
      For achiral alkenes, the symmetrical trigonal planar geometry of the carbocation leads to equivalent synthesis of both R and S products giving a racemic (50/50).
    • 9.8: Stereochemistry of Reactions - Hydration of Chiral Alkenes
      Chiral alkenes form electrophilic addition products in non 50:50 ratios due to the differences in steric effects between the enantiomeric starting materials.
    • 9.9: Addition of Halogens
      Halogens can act as electrophiles due to polarizability of their covalent bond and react with the pi bond of alkenes.  This electrophilic addition mechanism is stereospecific.  The orientation of the electrophile during a stereospecific electrophilic addition reaction will determine the stereochemistry of the product(s).
    • 9.10: Formation of Halohydrins
      When the halogenation reaction of alkenes is performed in a nucleophilic solvent like water or alcohol, then the solvent becomes the nucleophile to give halohydrin or haloalkoxy products.
    • 9.11: Reduction of Alkenes - Catalytic Hydrogenation
      Catalytic hydrogenation of a carbon-carbon double bond is a reduction reaction.  The alkene orientation required for interaction with the surface of the catalyst means that t his reaction is stereospecific.
    • 9.12: Oxidation of Alkenes - Epoxidation
      Oxidation of alkenes is introduced and the epoxidation of alkenes is discussed.
    • 9.13: Dihydroxylation of Alkenes
      Alkenes can react to produce glycols (two adjacent hydroxyl groups) through either an anti- or  syn- addition mechanism that is stereospecific.
    • 9.14: Opening of Epoxides - Acidic versus Basic Conditions
      Ring-opening reactions can proceed by either SN2 or SN1 mechanisms, depending on the nature of the epoxide and on the reaction conditions.
    • 9.15: Oxidative Cleavage of Alkenes
      Oxidative cleavage of alkenes can occur by several different pathways.  The most common reagents and pathways are discussed in this section.
    • 9.16: Addition of Carbenes to Alkenes - Cyclopropane Synthesis
      A carbene such as methlyene will react with an alkene breaking the pi bond to form a cyclopropane.  Since methylene is highly reactive, it is prepared in situ immediately preceding the addition of the alkene.
    • 9.17: Radical Chain-Growth Polymerization
      All the monomers from which addition polymers are made are alkenes or functionally substituted alkenes.  The free radical mechanism for chain growth polymers is explained.
    • 9.18: 9.18 Biological Additions of Radicals to Alkenes
      Some electrophilic addition reactions that take place in nature and the role of enzymes in such processes are introduced.
    • 9.19: Additional Exercises
      This section has additional exercises for the key learning objectives of this chapter.
    • 9.20: Solutions to Additional Exercises
      This section has the solutions to the additional exercises from the previous section.