10: Properties and Reactions of Alkenes

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10.1: 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.
10.2: 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.
10.3: 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).
10.4: 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.
10.5: 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.
10.6: 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.
10.7: 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.
10.8: 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.
10.9: Additional Exercises. Part 1
This section has additional exercises for the key learning objectives of this chapter.
10.10: Solutions to Additional Exercises, Part 1
This section has the solutions to the additional exercises from the previous section.
10.11: 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.
10.12: 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.
10.13: Alkene Asymmetry and Markovnikov's Rule
The regioselectivity of electrophilic addition reactions is determined by carbocation stability and is summarized by Markovnikov's Rule.
10.14: 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.
10.15: Hydration- Oxymercuration-Demercuration
Oxymercuration is a stereospecific, regioselective electrophilic addition reaction because there are no carbocation rearrangements due to stabilization of the reactive intermediate. The Markovnikov products are reliably synthesized by this pathway.
10.16: 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.
10.17: 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).
10.18: 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.
10.19: 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).
10.20: 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.
10.21: 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.
10.22: Oxidation of Alkenes - Epoxidation
Oxidation of alkenes is introduced and the epoxidation of alkenes is discussed.
10.23: Dihydroxylation of Alkenes
Alkenes can react to produce glycols (two adjacent hydroxyl groups) through either an anti- or syn- addition mechanism that is stereospecific.
10.24: 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.
10.25: Additional Exercises, Part 2
This section has additional exercises for the key learning objectives of this chapter.
10.26: Solutions to Additional Exercises, Part 2
This section has the solutions to the additional exercises from the previous section.