8: Structure and Synthesis of Alkenes
- Page ID
- 45187
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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 and 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.