Chemistry
Reactions of Alkenes
Reactions of alkenes involve chemical transformations such as addition reactions, where the double bond is broken and new atoms or groups are added to the carbon atoms. Common addition reactions include hydrogenation, halogenation, and hydration. Alkenes can also undergo polymerization to form long chains of repeating units, which is important in the production of plastics and synthetic materials.
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9 Key excerpts on "Reactions of Alkenes"
eBook - PDF- William H. Brown, Thomas Poon(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
5.8 Summary of Key Questions 151 SUMMARY OF KEY QUESTIONS 5.1 What Are the Characteristic Reactions of Alkenes? • A characteristic reaction of alkenes is addition, during which a pi bond is broken and sigma bonds are formed to two new atoms or groups of atoms. Alkene addition reactions include addition of halogen acids, H Cl, acid‐catalyzed addition of H 2 O to form an alcohol, addition of halogens, X 2 , hydrobo- ration followed by oxidation to give an alcohol, and transi- tion metal‐catalyzed addition of H 2 to form an alkane. 5.2 What Is a Reaction Mechanism? • A reaction mechanism is a description of (1) how and why a chemical reaction occurs, (2) which bonds break and which new ones form, (3) the order and relative rates in which the various bond‐breaking and bond‐forming steps take place, and (4) the role of the catalyst if the reaction involves a catalyst. • Transition state theory provides a model for understanding the relationships among reaction rates, molecular struc- ture, and energetics. • A key postulate of transition state theory is that a transition state is formed in all reactions. • The difference in energy between reactants and the transi- tion state is called the activation energy. • An intermediate is an energy minimum between two transition states. • The slowest step in a multistep reaction, called the rate‐ determining step, is the one that crosses the highest energy barrier. • There are many patterns that occur frequently in organic reaction mechanisms. These include adding a proton, tak- ing a proton away, the reaction of a nucleophile and elec- trophile to form a new bond, and rearrangement of a bond. 5.3 What Are the Mechanisms of Electrophilic Additions to Alkenes? • An electrophile is any molecule or ion that can accept a pair of electrons to form a new covalent bond. All electro- philes are Lewis acids. • A nucleophile is an electron‐rich species that can donate a pair of electrons to form a new covalent bond.
eBook - PDF- John M. McIntosh(Author)
- 2018(Publication Date)
- De Gruyter(Publisher)
5 Reactions of Alkanes, Alkenes, and Alkynes 5.1 Introduction In the preceding four chapters you have been introduced to a wide range of basic prin-ciples that govern the structure, shape, and reactivity of organic molecules. We are fi-nally ready to start applying these principles to actual molecules and their reactions. In this chapter, we will look at some reactions of hydrocarbons, and particular atten-tion will be paid to alkenes . Some of the reactions we will see do not fit the general mechanistic types we will be developing and therefore must be learned separately. However, most will be considered from the viewpoint of what is actually happening as the molecules react: i.e., the mechanism. 5.2 Reactions of Alkanes This section will be quite brief simply because, on the usual scale of reactivities, alka-nes (saturated hydrocarbons) are quite unreactive. Furthermore, those reactions they do undergo do not fit the type of mechanistic pathways we will be considering. 5.2.1 Oxidation The most general reaction undergone by alkanes is combustion: i.e., their oxidation in air. For example CH 4 + 2O 2 → CO 2 + 2H 2 O + heat This, of course is the reaction that heats houses and powers internal combustion en-gines. It is also useful for determining the molecular formula of organic molecules (see Problem 2.4). The ultimate goal of any organic chemistry course is to be able to predict, from a knowledge of mechanism and/or by analogy with similar molecules, how a particular molecule will react under given conditions. It is strongly suggested that you start a list of the reactions we have discussed and keep it up-to-date, lecture by lecture. This will greatly simplify review. It is also important to realize that the reactions must be learned frontwards and backwards. That is – we will see a reac-tion where A gives B under certain conditions. You should remember this in terms of how A reacts and also how to prepare B. https://doi.org/10.1515/9783110565140-005
- Frederick Bettelheim, William Brown, Mary Campbell, Shawn Farrell(Authors)
- 2019(Publication Date)
- Cengage Learning EMEA(Publisher)
All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 12.5 Characteristic Reactions of Alkenes The most characteristic reaction of alkenes is an addition to their carbon– carbon double bond: the double bond is broken, and in its place, single bonds form between the carbons and two new atoms or groups of atoms. Table 12.1 shows several examples of alkene addition reactions along with the descriptive name(s) associated with each reaction. At this point, you might ask why a carbon–carbon double bond is a site of chemical reactivity, whereas carbon–carbon single bonds are quite CHEMICAL CONNECTIONS 12A Cis-Trans Isomerism in Vision The retina, the light-detecting layer in the back of our eyes, contains reddish compounds called visual pigments. Their name, rhodopsin, is derived from the Greek word meaning “rose-colored.” Each rhodopsin molecule is a combination of one molecule of a protein called opsin and one molecule of 11-cis-retinal, a deriv-ative of vitamin A in which the CH 2 OH group of carbon 15 is converted to an aldehyde group, i CH w O. When rhodopsin absorbs light energy, the less sta-ble 11-cis double bond is converted to the more stable 11-trans double bond. This isomerization changes the shape of the rhodopsin molecule, which in turn causes the neurons of the optic nerve to fire and produce a visual image. The retinas of vertebrates contain two kinds of rhodopsin-containing cells: rods and cones. Cones func-tion in bright light and are used for color vision; they are concentrated in the central portion of the retina, called the macula, and are responsible for the greatest visual acuity.
eBook - PDFExperimental Organic Chemistry
A Miniscale & Microscale Approach
- John Gilbert, Stephen Martin(Authors)
- 2015(Publication Date)
- Cengage Learning EMEA(Publisher)
331 C H A P T E R Alkenes As discussed in the preceding chapter, alkanes are saturated hydrocarbons. You encounter such compounds in your daily life, especially in the form of fuels to power automobiles, trains, planes, and many power plants that generate electric-ity. These fuels also contain compounds that are known as alkenes, which are unsaturated hydrocarbons. In this chapter, you will discover that alkenes are much more interesting organic compounds because they undergo a variety of different chemical reactions, not just the radical reactions that are typical of alkanes. Thus, although you may have considered the chemistry of alkanes “boring,” you will find that the chemistry of alkenes is rich and characterized by a number of fascinating transformations of the carbon-carbon double bond, the functional group found in alkenes. 10.1 I N T R O D U C T I O N A functional group is an atom or group of atoms that governs the chemical and physical properties of a family of compounds. The introduction and manipulation of these functional groups are major objectives in modern organic chemistry. In this chapter, we will explore the chemistry of alkenes, 1 , which are organic compounds possessing a polarizable carbon-carbon double bond, a p -bond, as the functional group. Methods for introducing a carbon-carbon double bond into a molecule from alkyl halides and alcohols are presented first, and then some of the addition reac-tions that characterize this functional group are examined. Elimination reactions are among the most common ways to produce a carbon-carbon p -bond. For example, the elements of hydrogen halide, H–X, may be eliminated from an alkyl halide, 2 . The functional group of an alkyl halide is a carbon-halogen, single bond, C – X , and the process by which the carbon-halogen bond and an adjacent carbon-hydrogen bond are converted into a carbon-carbon p -bond via dehydrohalogenation is an example of a functional group transformation .
eBook - ePub- David John Ager, Michael B. East(Authors)
- 2020(Publication Date)
- CRC Press(Publisher)
Chapter 8PREPARATION AND Reactions of Alkenes
This chapter is concerned with the preparation of simple and functionalized alkenes together with reactions of carbon–carbon unsaturation. However, only the reactions of simple alkenes have been covered. The plethora of methods that relates to the chemistry of functionalized alkenes is discussed elsewhere [Chapter 9 ]. Due to the importance of the oxidations of carbon–carbon unsaturation, this topic is covered separately. The oxidations of simple alkenes will be found in Chapter 10 , while those of functionalized alkenes are covered in Chapter 11 .8.1. ALKENE SYNTHESIS
Many of the asymmetric methods cited in this book rely on the addition of various reagents to an alkene in a stereoselective manner.†5 ,6 ,7 ,8To provide good stereoselection for asymmetric transformations, only one isomer of the alkene must be present in the substrate. It is pertinent, therefore, to address reactions that can be used for the stereoselective preparation of alkenes.§ These methods fall into four main categories: reduction of an acetylene; condensation of an organometallic species with either a carbonyl or vinyl compound; and elimination reactions.9 ,10 ,11 ,12 ,13 ,14 ,15 ,16 ,17 ,18 ,19 ,208.1.1. ALKYNE REDUCTIONS AND ADDITIONS
Alkynes provide access to a wide variety of alkenes. It should be noted, that to avoid a mixture of diastereomers, many of the examples in this section rely on the use of terminal or symmetrical alkynes.8.1.1.1. Simple Alkenes
Acetylenes can provide E-alkenes by reduction with lithium aluminum hydride21 ,22or alkali metals (Scheme 8.1 ).23 The use of hydrogenation or diisobutylaluminum hydride with an alkyne provides the Z-isomer. Hydroboration gives access to either E- or Z-alkenes (Scheme 8.2 ).17 ,24 ,25 ,26 ,27 ,28 ,29 ,30 ,31 ,32 ,33 ,34 ,35 ,36 ,37 ,38 ,391-Bromo-1-alkynes can also be used as substrates to access either alkene isomer.24 ,25 ,33Trisubstituted alkenes can also be obtained by hydroboration of internal alkynes with dialkyl or alkylbromoboranes.27 ,32 ,40
- G. Liu(Author)
- 2023(Publication Date)
- Thieme(Publisher)
427 47.1.5.7 Synthesis of Alkenes via Radical Addition Reactions P. Chen and G. Liu General Introduction This review represents the addition of a new topic to the Science of Synthesis coverage on methods for alkene synthesis; radical addition reactions of alkynes were not covered in the chapter on the synthesis of alkenes by addition reactions of alkynes (Section 47.1.5) that was published in 2010. The radical-involved functionalization of alkynes and allenes is an efficient strategy for the synthesis of functionalized alkenes, and has experienced rapid growth over the past ten years with the development of photocatalysis and transi- tion-metal catalysis. [1–4] This section focuses on alkene synthesis initiated by radical addi- tion to alkynes, and also allenes, with a particular focus on intermolecular reactions. In- tramolecular radical cascade reactions usually provide cyclization products bearing an al- kene moiety; however, in most cases these initial products then lead to formation of an aromatic ring. [5] Thus, this pattern of reactions is not included in this chapter. 47.1.5.7.1 Radical Addition to Alkynes Alkynes are useful motifs that contain at least one C”C bond in which the carbon atoms are sp-hybridized. Radical additions to alkynes are highly valuable because of the forma- tion of reactive vinyl radicals that can be trapped by a coupling reaction or fast cycliza- tion, resulting in an alkene moiety. These reactions always provide Z/E stereoisomeric product mixtures because of the nature of vinyl radicals. Vinyl radicals have either a bent or a linear structure and are usually designated as s- and p-type radicals, respectively (Scheme 1). [6] These appellations refer to the presence or absence of any s-character in the singly occupied molecular orbital (SOMO). The configuration of vinyl radicals depends on the nature of the a-substituent R 1 . Vinyl radicals with an a-hydrogen, -alkyl, -alkoxy, or -halo substituent usually show a bent structure.
eBook - PDF- David R. Klein(Author)
- 2021(Publication Date)
- Wiley(Publisher)
Several examples are listed in Table 8.1. 358 CHAPTER 8 Addition Reactions of Alkenes The first process illustrates that π bonds can be readily protonated, while the second process illus- trates that π bonds can attack electrophilic centers. Both processes will appear many times through- out this chapter. 8.2 Alkenes in Nature and in Industry Alkenes are abundant in nature. Here are just a few examples, all acylic compounds (compounds that do not contain a ring): Allicin Responsible for the odor of garlic S S O Geraniol Isolated from roses and used in perfumes OH α-Farnesene Found in the natural waxy coating on apple skins Nature also produces many cyclic, bicyclic, and polycyclic alkenes: Limonene Responsible for the strong smell of orange peels Cholesterol Produced by all animals; this compound plays a pivotal role in many biological processes HO H H H H α -Pinene Isolated from pine resin; a primary constituent of turpentine (paint thinner) Double bonds are also often found in the structures of pheromones. Recall that pheromones are chem- icals used by living organisms to trigger specific behavioral responses in other members of the same species. For example, alarm pheromones are used to signal danger, while sex pheromones are used to attract the opposite sex for mating. Below are several examples of pheromones that contain double bonds: Muscalure Sex pheromone of the common housefly Ectocarpene A pheromone released by the eggs of the seaweed Ectocarpus siliculosus to attract sperm cells β-Farnesene An aphid alarm pheromone The greatest threat to the productivity of apple orchards is an infestation of codling moths. A bad infestation can destroy up to 95% of an apple crop. A female codling moth can lay up to 100 eggs. Once hatched, the larvae dig into the apples, where they are shielded from insecticides. The so-called “worm” in an apple is generally the larva of a codling moth.
eBook - PDFMethods for Oxidation of Organic Compounds V1
Alcohols, Alcohol Derivatives, Alky Halides, Nitroalkanes, Alkyl Azides, Carbonyl Compounds Hydroxyarenes and Aminoarenes
- Alan Haines(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
-3 -Alkenes 3.1. Formation of Alkynes (i) The conversion of an alkene into an alkyne [Scheme 1 (1 -• 3)] generally requires the formation of an intermediate (2), in which the substituents X and Y are good leaving groups. Elimination of HX and HY from 2 by treatment with a suitable base then yields alkyne 3. Clearly, if an alkene possesses a good leaving group on the carbon-carbon double bond, as in 4 [Scheme 1], elimination of HX will afford alkyne 3 directly, but such alkyne forming reactions are not considered here. In the alkene to alkyne conversion (1 3), it is convenient if X = Y = halogen, and this type of reaction is considered below. The actual oxidative step in the process is the halogenation reaction performed on the alkene (1). R 1 C H = C H R 2 F ^ C H X — C H Y R 2 R 4 C = C R 2 R ' C H ^ C X R 2 1 2 3 4 Scheme 1 R E F E R E N C E S 1. For review articles containing information on the formation of alkynes, see D. A. Ben-Efraim, in The Chemistry of the Carbon-Carbon Triple Bond (S. Patai, ed.), p. 755. Wiley, New York, 1978, and refs. 1 and 2 therein. For detailed laboratory procedures for the preparation of alkynes, see L. Brandsma, Preparative Acetylenic Chemistry. Elsevier, Amsterdam, 1971. 3.1.1. Dehydrogenation through a Halogenation-Dehydrohalogenation Sequence The halogenation step in this type of synthesis is conveniently performed with bromine (7). The dehydrohalogenation reaction is frequently accom-plished with sodamide in liquid ammonia (Table 3.1, entries 1-3), with 71 72 3 . ALKENES potassium hydroxide in alcoholic solvents (entries 4-6), or more recently, with aqueous sodium hydroxide in the presence of a phase-transfer agent, tetra-n-butylammonium hydrogen sulphate (entry 7).
eBook - ePub- Metin Balcı(Author)
- 2021(Publication Date)
- Wiley-VCH(Publisher)
As σ-bonds are more stable than π-bonds, the most common reaction of C=C double bonds is the transformation of π-bonds into σ-bonds. When alkene and hydrogen gases are reacted in the presence of a catalyst, such as platinum, palladium, or nickel, two hydrogen atoms add to the C=C double bond to yield alkanes [94]. This reaction is called hydrogenation. The reaction is exothermic and the heat released is called the heat of hydrogenation and is about −20 to −30 kcal/mol (−80 to 125 kJ/mol), indicating that the product formed is more stable than the alkene. In this section, we will focus only on the reduction of carbon–carbon double bonds as a wide variety of functional groups can be reduced by catalytic hydrogenation. Depending on the type of the catalyst and reaction conditions, the reduction reactions are categorized into two groups: Heterogeneous catalytic reduction Homogeneous catalytic reduction 4.7.1 Heterogeneous Catalytic Reduction Heterogeneous catalysts have their advantages and disadvantages. After completion of the hydrogenation reaction, the heterogeneous catalysts are separated by filtration from the reaction medium because they do not dissolve in solvents. Hydrogenation reactions are generally carried out with heterogeneous catalysts. Alkenes do not react directly with hydrogen gas under normal conditions. A temperature of at least 500 °C is required for the reaction to take place. The activation energy required for adding hydrogen is quite high. The hydrogen–hydrogen bond must be weakened for reduction. Otherwise, it is also a challenging process. The activation energy of hydrogenation reactions decreases when a catalyst is used. The task of the catalyst is to lower the activation energy (Figure 4.2). Generally, the alkene is dissolved in alcohol, hydrocarbon, or acetic acid. A small portion of the catalyst is added to the reaction mixture. The reaction proceeds using hydrogen at atmospheric pressure
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