8: Alkenes - Reactions and Synthesis
- Page ID
- 448606
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)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: Why This Chapter?
- Alkene addition reactions occur widely, both in the laboratory and in living organisms. Although we’ve studied only the addition of HX thus far, many closely related reactions also take place. In this chapter, we’ll see briefly how alkenes are prepared and we’ll discuss further examples of alkene addition reactions.
- 8.1: Preparation of Alkenes - A Preview of Elimination Reactions
- This section introduces the preparation of alkenes through elimination reactions, focusing on the dehydrohalogenation of alkyl halides and dehydration of alcohols. It emphasizes how these reactions typically yield alkenes and discusses their mechanisms, including the role of bases in facilitating the removal of hydrogen halides or water. The chapter sets the stage for understanding the synthesis of alkenes as vital intermediates in organic chemistry.
- 8.2: Halogenation of Alkenes - Addition of X₂
- Bromine and chlorine add rapidly to alkenes to yield 1,2-dihalides, a process called halogenation. For example, nearly 50 million tons of 1,2-dichloroethane (ethylene dichloride) are synthesized worldwide each year, much of it by addition of Cl2 to ethylene. The product is used both as a solvent and as starting material for the manufacture of poly(vinyl chloride), PVC, the third most widely synthesized polymer in the world afterpolyethelyne and polypropolyne.
- 8.3: Halohydrins from Alkenes - Addition of HO-X
- 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
- In addition to the oxymercuration–demercuration method, which yields the Markovnikov product, a complementary method that yields the non-Markovnikov product is also useful. Discovered in 1959 by H.C. Brown at Purdue University and called hydroboration, the reaction involves addition of a B−H bond of borane to an alkene to yield an organoborane intermediate. Oxidation of the organoborane by reaction with basic hydrogen peroxide, then gives an alcohol.
- 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
- Biological additions of radicals to alkenes play a crucial role in various biochemical processes. These radical reactions can lead to the formation of diverse products, influencing metabolic pathways. The mechanism often involves the generation of radical species that add to the double bond of alkenes, resulting in new chemical transformations. This chapter outlines the significance of these reactions in biological systems and highlights their implications in fields such as biochemistry.
- 8.12: Stereochemistry of Reactions - Addition of H₂O to an Achiral Alkene
- The addition of HO to an achiral alkene results in a stereochemical process that produces two possible products: syn and anti addition. This reaction typically yields a racemic mixture, reflecting the symmetry of the alkene and the lack of chirality in the starting material. Understanding this stereochemistry is crucial for predicting the outcomes of reactions involving alkenes in organic synthesis.
- 8.13: Stereochemistry of Reactions - Addition of H₂O to a Chiral Alkene
- The addition of HO to a chiral alkene results in stereochemical outcomes that reflect the configuration of the alkene. This reaction can lead to the formation of different stereoisomers, influenced by how the nucleophile approaches the chiral center. The stereochemistry of the products is essential in organic synthesis, impacting the properties and reactivity of the resulting compounds.
- 8.14: Chemistry Matters—Terpenes- Naturally Occurring Alkenes
- Terpenes are naturally occurring alkenes primarily found in plants and are responsible for their fragrance. These compounds, which include monoterpenes and sesquiterpenes, play essential roles in ecological interactions and have applications in medicine and perfumery. The synthesis of terpenes involves various reactions, showcasing the diverse reactivity of alkenes. Their structure and functional groups significantly influence their properties and biological activity.
- 8.16: Summary
- This section reviews key alkene reactions, including addition reactions and their regioselectivity and stereochemistry. It emphasizes the use of alkenes in organic synthesis and their predictable reactivity, especially in terms of Markovnikov and anti-Markovnikov addition. The chapter highlights the versatility of alkenes as building blocks in designing complex organic syntheses.
- 8.17: Summary of Reactions
- This section summarizes the key reactions involving alkenes, including electrophilic additions, oxidation, and polymerization. It highlights important mechanisms and stereochemistry in reactions such as hydrohalogenation, hydration, and hydrogenation. The chapter emphasizes the versatility of alkenes in organic synthesis and the significant role of regioselectivity and stereoselectivity in determining product outcomes. Understanding these reactions is crucial for applying alkenes in various chem