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8: Structure and Synthesis of Alkenes

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  • Learning Objectives:

    After reading the chapter and completing the exercises and homework, a student can be able to:

    • describe the electronic structure of alkenes using Molecular Orbital (MO) Theory and Orbital Hybridization - refer to section 8.1
    • memorize the common names for vinylic and allylic groups including isoprene and styrene refer to section 8.2
    • predict the relative physical properties of alkenes - refer to section 8.2
    • recognize and classify the stereochemistry of alkenes using the cis/trans and E/Z systems - refer to section 8.3
    • calculate the Degrees of Unsaturation (DU) and apply it to alkene structure - refer to section 8.4
    • give the IUPAC names for alkenes given their structure & vice versa including E/Z isomers - refer to section 8.5 and chapter 3
    • use heats of hydrogenation to compare the stabilities of alkenes - refer to section 8.6
    • interpret and draw reaction energy diagrams for dehydrohalogenation of R-X’s and alcohol dehydration reactions - refer to sections 8.7 and 8.8 respectively and chapter 7
    • propose mechanisms for a dehydrohalogenation or dehydration reactions - refer to sections 8.7 and 8.8 respectively and chapter 7
    • predict the products and specify the reagents for alkene synthesis from dehydrohalogenation of R-X’s and alcohol dehydration reactions - refer to sections 8.7 and 8.8 respectively
    • predict and explain the stereochemistry of E2 eliminations to form alkenes, especially from cyclohexanes - refer to sections 8.7 and 8.8 and chapter 7
    • discuss the uses and sources of alkenes including catalytic cracking - refer to section 8.9

    • 8.1: Alkene Structure
      Alkenes are a class of hydrocarbons (i.e., containing only carbon and hydrogen). They are unsaturated compounds with at least one carbon-to-carbon double bond.
    • 8.2: Physical Properties and Important Common Names
      Alkenes are non-polar hydrocarbons with physical properties similar to alkanes.  At room temperature, alkenes exist in all three phases, solid, liquids, and gases.  The stereochemistry of the geometric isomers (cis/trans) can influence the physical properties.
    • 8.3: The Alkene Double Bond and Stereoisomerism
      The two lobes of the pi bond in the alkenes prevent rotation and are responsible for their rigid nature.  The lack of rotation creates the potential for geometric isomers (cis/trans).
    • 8.4: Degrees of Unsaturation
      Calculating the degrees of unsaturation (DU) can provide useful information about the chemical structure from the molecular formula.  The  DU indicates the presence of rings and π bonds, but cannot distinguish between them.
    • 8.5: The E/Z System (when cis/trans does not work)
      Some alkenes cannot be unambiguously named using the cis/trans system.  The Cahn-Ingold-Prelog (CIP) rules were used to develop the E/Z system for naming the stereoisomers of alkenes.
    • 8.6: Stability of Alkenes
      The energy released during alkene hydrogenation is called the heat of hydrogenation and indicates the relative stability of the double bond in the molecule.
    • 8.7: Alkene Synthesis by Elimination of Alkyl Halides
      The alkyl halide elimination reactions (E1 and E2) to synthesize alkenes are briefly reviewed.  Refer to chapter 7 sections 13 through 18 for a complete explanation.
    • 8.8: Alkene Synthesis by Dehydration of Alcohols
      The dehydration reaction of alcohols to generate alkene proceeds by heating the alcohols in the presence of a strong acid, such as sulfuric or phosphoric acid, at high temperatures.
    • 8.9: Uses & Sources of Alkenes
      Among the most important and most abundant organic chemicals produced worldwide are the two simple alkenes, ethylene and propylene. Thermal cracking is briefly explained.
    • 8.10: Additional Exercises
      This section has additional exercises for the key learning objectives of this chapter.
    • 8.11: Solutions to Additional Exercises
      This section has the solutions to the additional exercises from the previous section.