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8: Alkenes - Reactions and Synthesis

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Sep 28, 2021
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- 7.S: Alkenes- Structure and Reactivity (Summary)
- 8.0: Chapter Objectives

Steven Farmer & Dietmar Kennepohl

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As you have seen, addition reactions dominate the chemistry of alkenes. This chapter shows how a variety of reagents can add to alkenes; how hydrogen bromide can be made to add to alkenes in a non-Markovnikov manner; and how alkene molecules can be cleaved into easily identifiable parts. First, you will examine the preparation of alkenes by elimination reactions.

8.0: Chapter Objectives
8.1: Preparation of Alkenes - A Preview of Elimination Reactions
8.2: Halogenation of Alkenes - Addition of X₂
8.3: Halohydrins from Alkenes - Addition of HOX
Lewis acids like the halogens, boron hydrides and certain transition metal ions are able to bond to the alkene pi-electrons, and the resulting complexes rearrange or are attacked by nucleophiles to give addition products.The electrophilic character of the halogens is well known. Chlorine (Cl2) and bromine(Br2) react selectively with the double bond of alkenes, and these reactions are what we will focus on. Fluorine adds uncontrollably with alkenes,and the addition of iodine is unfavorable, so th
8.4: Hydration of Alkenes - Addition of H₂O by Oxymercuration
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.
8.5: Hydration of Alkenes - Addition of H₂O by Hydroboration
Hydroboration-oxidation is a two step pathway used to produce alcohols. The reaction proceeds in an anti-Markovnikov manner, where the hydrogen (from BH3 or BHR2) attaches to the more substituted carbon and the boron attaches to the least substituted carbon in the alkene double bond. Furthermore, the borane acts as a Lewis acid by accepting two electrons in its empty p orbital from an alkene that is electron rich.
8.6: Reduction of Alkenes - Hydrogenation
Addition of hydrogen to a carbon-carbon double bond is called hydrogenation. The overall effect of such an addition is the reductive removal of the double bond functional group. Regioselectivity is not an issue, since the same group (a hydrogen atom) is bonded to each of the double bond carbons. The simplest source of two hydrogen atoms is molecular hydrogen, but mixing alkenes with hydrogen does not result in any discernible reaction.
8.7: Oxidation of Alkenes - Epoxidation and Hydroxylation
Oxacyclopropane rings, also called epoxide rings, are useful reagents that may be opened by further reaction to form anti vicinal diols. One way to synthesize oxacyclopropane rings is through the reaction of an alkene with peroxycarboxylic acid.
8.8: Oxidation of Alkenes - Cleavage to Carbonyl Compounds
Ozonolysis is a method of oxidatively cleaving alkenes or alkynes using ozone ( O 3 O3 ), a reactive allotrope of oxygen. The process allows for carbon-carbon double or triple bonds to be replaced by double bonds with oxygen. This reaction is often used to identify the structure of unknown alkenes. by breaking them down into smaller, more easily identifiable pieces. Ozonolysis also occurs naturally and would break down repeated units used in rubber and other polymers.
8.9: Addition of Carbenes to Alkenes - Cyclopropane Synthesis
The highly strained nature of cyclopropane compounds makes them very reactive and interesting synthetic targets. Additionally cyclopropanes are present in numerous biological compounds. One common method of cyclopropane synthesis is the reaction of carbenes with the double bond in alkenes or cycloalkenes. Methylene, H2C, is simplest carbene, and in general carbenes have the formula $R_2C$ .
8.10: Radical Additions to Alkenes - Chain-Growth Polymers
All the monomers from which addition polymers are made are alkenes or functionally substituted alkenes. The most common and thermodynamically favored chemical transformations of alkenes are addition reactions. Many of these addition reactions are known to proceed in a stepwise fashion by way of reactive intermediates, and this is the mechanism followed by most polymerizations.
8.11: Biological Additions of Radicals to Alkenes
8.12: Stereochemistry of Reactions - Addition of H₂O to an Achiral Alkene
8.13: Stereochemistry of Reactions - Addition of H₂O to a Chiral Alkene
8.S: Alkenes - Reactions and Synthesis (Summary)

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