Chemistry
Synthesis of Alkenes
The synthesis of alkenes involves the creation of carbon-carbon double bonds. This can be achieved through several methods, including elimination reactions of alkyl halides, dehydration of alcohols, and dehydrohalogenation of alkyl halides. These processes result in the formation of alkenes, which are important building blocks in organic chemistry.
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12 Key excerpts on "Synthesis of Alkenes"
- Armin de Meijere(Author)
- 2014(Publication Date)
- Thieme Chemistry(Publisher)
Each method and variation will be accompanied by reaction schemes, tables of examples, ex- perimental procedures, and a background discussion of the scope and limitations of the reaction described. The policy of the editorial board is to make Science of Synthesis the ultimate tool for the synthetic chemist in the 21st century. We would like to thank all of our authors for submitting contributions of such out- standing quality, and also for the dedication and commitment they have shown through- out the entire editorial process. The Editorial Board October 2000 D. Bellus (Basel, Switzerland) P. J. Reider (New Jersey, USA) E. N. Jacobsen (Cambridge, USA) E. Schaumann (Clausthal-Zellerfeld, Germany) S. V. Ley (Cambridge, UK) I. Shinkai (Tsukuba, Japan) R. Noyori (Nagoya, Japan) E. J. Thomas (Manchester, UK) M. Regitz (Kaiserslautern, Germany) B. M. Trost (Stanford, USA) VII Volume Editor’s Preface This volume of Science of Synthesis, dealing with the various approaches to alkenes, is meant to aid researchers around the world who are engaged in developing synthetic ap- proaches to new chemical entities or improving existing routes to known compounds of any importance. The carbon—carbon double bond with which every alkene is endowed, be it a hydrocarbon or not, is one of the most versatile functional groups in an organic molecule. Considering the multitude of other functionalities with which most methods for the Synthesis of Alkenes are compatible, alkenes gain even more importance in organic synthesis. The classical methods of alkene preparation comprise mainly elimination reac- tions of various kinds. However, the development of alkene syntheses, even of the elimi- nations, has never stopped.
- Syed Shahabuddin, Rama Gaur, Nandini Mukherjee(Authors)
- 2024(Publication Date)
- CRC Press(Publisher)
1 Introduction to Dehydrogenation Reactions of Organic CompoundsSyed Shahabuddin, Nandini Mukherjee, and Rama GaurDOI: 10.1201/9781003321934-11.1 INTRODUCTION
Dehydrogenation reactions include a wide variety of reactions where hydrogen is completely or partially eliminated from an organic compound to form a new compound (e.g., conversion of saturated into unsaturated compounds) [1 ]. Dehydrogenation reactions have applications in the production of hydrogen (H2 ), alkenes, cycloalkanes, aromatic compounds, imines, and oxygenates such as carbonyl compounds [2 ]. Alkenes, imines, and oxygenates are crucial chemical intermediates for producing solvents, polymers, rubbers, detergents, insecticides, and pharmaceuticals [3 ].Dehydrogenation is most important in the petrochemical industry especially in the refinery cracking process, where olefins are synthesized from saturated hydrocarbons [4 ]. Suitable hydrocarbon feed-stocks (e.g., naphtha) are subjected to cracking in order to produce industrially important olefins such as ethylene, propylene, butene derivatives, and butadiene derivatives in large volumes. Traditionally, steam cracking and fluid catalytic cracking (FCC) of C2+ hydrocarbons (ethane, propane, etc.) have been extensively used to produce alkenes. However, as there is an increasing demand for a specific alkene to be produced, building new specific steam crackers or FCC units for each individual alkene is not a frugal choice. Dehydrogenation reaction, in contrast, provides a comparatively flexible, cost-effective method for producing single alkenes. The dehydrogenation of ethylbenzene to produce an important monomer ‘styrene’ has become a preferred commercial route in the polymer industry.The benefits of building dehydrogenation reactors are the production not only of specific alkenes but also of aldehydes and ketones. Although the carbonyl compounds can be easily prepared via the oxidation of alkenes or alcohols, dehydrogenation routes may offer advantages such as a higher selectivity for products or the availability of a wider variety of feedstock [5 ]. Dehydrogenation reaction is also extremely useful in synthesizing α,β-unsaturated carbonyls as common organic building blocks in natural product and pharmaceutical synthesis. The introduction of a C=C group adjacent to an electron withdrawing group (EWG) provides greater synthetic opportunity that utilizes the resulting polarized double bond, since EWG is susceptible to regioselective functionalization by several methods [6 , 7 ]. Acceptorless dehydrogenation of alcohols, especially biomass-derived carbohydrates, provides an atom-economical method to synthesize carbonyl derivatives and a low-temperature route for selective H2 production [8 –14 ]. It also has synthetic applications in tandem coupling reactions involving C-N and C-C bond formation for the synthesis of imines and amides and the β-functionalization of alcohols [15 –24
- Frederick Bettelheim, William Brown, Mary Campbell, Shawn Farrell(Authors)
- 2019(Publication Date)
- Cengage Learning EMEA(Publisher)
From the perspective of the chemical industry, the single most important reaction of ethylene and other low-molecular-weight alkenes is polymer-ization, which we discuss in Section 12.6. The crucial point to recognize is that ethylene and all of the commercial and industrial products synthesized from it are derived from either natural gas or petroleum—both nonrenew-able natural resources! 12.2 Structures of Alkenes and Alkynes A. Alkenes Using the VSEPR model (Section 3.10), we predict bond angles of 120° about each carbon in a double bond. The observed H i C i C bond angle in ethylene, for example, is 121.7°, close to the predicted value. In other alkenes, deviations from the predicted angle of 120° may be somewhat larger because of interactions between alkyl groups bonded to the dou-bly bonded carbons. The C i C i C bond angle in propene, for example, is 124.7°. Ethylene 121.7 8 Propene 124.7 8 H H H H C C H 3 C H H H C C If we look at a molecular model of ethylene, we see that the two carbons of the double bond and the four hydrogens bonded to them all lie in the same plane—that is, ethylene is a flat or planar molecule. Furthermore, chemists have discovered that under normal conditions, no rotation is possible about the carbon–carbon double bond of ethylene or, for that matter, of any other alkene. Whereas free rotation occurs about each carbon–carbon single bond in an alkane (Section 11.7A), rotation about the carbon–carbon double bond in an alkene does not normally take place because the bond is so rigid. For an important exception to this generalization about carbon–carbon double bonds, see Chemical Connections 12D on cis-trans isomerism in vision. 348 | Chapter 12 Alkenes, Alkynes, and Aromatic Compounds Copyright 2020 Cengage Learning. 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).
eBook - PDFOrganic Synthesis
Strategy and Control
- Paul Wyatt, Stuart Warren(Authors)
- 2007(Publication Date)
- Wiley(Publisher)
Introduction: Alkenes: framework or functional groups? The carbon-carbon double bond (olefin or alkene) is a structural feature on the border of frame- work and functionality. Unlike the carboxylic acid in 1 it is inside the skeleton of the molecule and involves no atoms except carbon and hydrogen. It has no polarity and yet it has the potential of producing highly polar functionality at two carbon atoms in a single reaction, as in the iodolactonisation to give 2 from 1. In the same reaction stereochemistry may be generated: two stereogenic centres are in fact produced in this reaction and in the epoxidation of 3. The sense of the stereochemistry produced in these reactions depends critically on two factors: the inherent stereoselectivity of the reactions (anti in the iodolactonisation and syn in the epoxidation) and the geometry of the alkene. Inside a six-membered ring of 1 the alkene must of course by cis or Z. In the open chain allylic alcohol 3 it can be E or Z depending on the method of synthesis. This chapter explores ways to control the stereochemistry of alkenes as an essential preliminary to the control of three-dimensional stereo- chemistry in chapter 25 where you will meet a method to make single enantiomers of 4 from the alkenes in E- and Z-3. Control of Alkene Geometry by Equilibrium Methods In many reactions there is an equilibrium between reactants and products so that only the more stable of the two alkenes is produced. In the Claisen ester condensation of cyclohexanone 8 with ethyl formate, the true product under the conditions of the reaction is the stable enolate 9 and this is reversibly protonated on workup to give the more stable H-bonded enol of the ketoaldehyde Z- 10. In the aldol reaction between the same ketone and benzaldehyde, the initial product 7 gives the enolate 6 and dehydration is reversible: only the more sterically favourable E isomer of 5 is formed.
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.
- 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- John M. McIntosh(Author)
- 2018(Publication Date)
- De Gruyter(Publisher)
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 62 | 5 Reactions of Alkanes, Alkenes, and Alkynes 5.2.2 Halogenation The replacement of hydrogen atoms by halogen (usually chlorine) is another common reaction of alkanes. The products are called alkyl halides . (Alkyl is the term used to describe a general structure of the type C n H 2 n + 1 ). The reaction is used frequently in industrial processes CH 3 CH 3 + Cl 2 → CH 3 CH 2 Cl + HCl The products, particularly if they are polyhalogenated (i.e., they contain several halo-gen atoms), are useful as flame retardants, insecticides, herbicides, and solvents. When alkanes of more complex structures are used, it is found that tertiary hydro-gens are replaced at a faster rate than secondary which, in turn, are replaced faster than primary hydrogens. It is frequently difficult to get clean replacement of one type to the complete exclusion of others and as a result, mixtures of products are com-monly obtained. If these mixtures can be used directly, this poses no problem, but frequently very undesirable properties are associated with the impurities. An exam-ple of this can be found in the chlorination of an organic molecule called phenol. The desired product – 2,4,6-trichlorophenol is contaminated with another product called dioxin, which has the reputation, perhaps undeserved, of being one of the most toxic compounds known. 5.3 Electrophilic Addition to Alkenes: Our First Mechanism E + E X X E Fig. 5.1 Alkenes (olefins) are electron-rich molecules; that is they contain more electrons than are required to hold the atoms together in the molecule. Therefore, they can be con-sidered to be nucleophilic compounds.
eBook - ePubUnderstanding Advanced Organic and Analytical Chemistry
The Learner's ApproachRevised Edition
- Kim Seng Chan, Jeanne Tan;;;(Authors)
- 2016(Publication Date)
- WS EDUCATION(Publisher)
CHAPTER 5
Alkenes
5.1 Introduction
Alkenes are unsaturated hydrocarbons containing the C=C double bond. They have the general formula Cn H2n. Sharing this same general formula are the cycloalkanes, which are constitutional/structural isomers to the corresponding alkenes. Apart from functional group isomerism, constitutional/structural isomerism in alkenes can also arise due to the different degree of branching in the main carbon chain of their molecules (chain isomerism) and the location of the C=C bond (positional isomerism).Alkenes also exhibit cis-trans isomerism, otherwise known as geometrical isomerism (see Chapter 2 ). This form of stereoisomerism is attributed to the restricted rotation about the C=C bond.As the doubly bonded carbon atoms are sp2 hybridized, the geometry about each of them is trigonal planar. To show the trigonal planar geometry, the structure of an alkene is normally drawn in such a way as to depict an angle of 120° about the double bond, as follows:This form of drawing is especially important in illustrating both the cis and trans isomers.5.2 Nomenclature
For alkenes, their chemical names end with the suffix −ene. Table 5.1 lists the names of the first few members of the alkene family. For an alkene with four carbon atoms onwards, there exists different ways in which the carbon atoms can be connected to each other, hence giving rise to the notion of constitutional/structural isomerism (see Chapter 2 ). For instance, both but-1-ene and but-2-ene have a chain of four carbon atoms. The difference between them is the location of the C=C bond.Table 5.1The word “butene” does not just wrongfully account for the two constitutional/structural isomers mentioned above, ambiguity is also found in the word “but-2-ene,”which actually constitutes a pair of cis-trans (geometrical) isomers:Hence, “butene” actually stands for a total of three distinct compounds – but-1-ene, cis-but-2-ene and trans
eBook - PDF- Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
Polymers arise from the sequential 830 Chemistry addition of many low-molar-mass molecules to create very large molecules of high molar mass, as illustrated here by the formation of polyethylene from ethene (ethylene). nCH 2 CH 2 initiator ← → ( CH 2 CH 2 ) n To achieve such an addition, alkenes first react with a specific reagent called an initiator and then with each other to form a steadily growing chain. In alkene-derived polymers of industrial and commercial importance, n is a large number, typically several thousand. We discuss the formation of polymers from alkenes in more detail in the chapter on polymers. Another reaction of alkenes is reduction to alkanes, which is essentially the addition of H 2 across the double bond. We will discuss this after we look at the addition reactions. Electrophilic addition reactions The basis of reactivity is the attraction between positive and negative species. The double bond in alkenes is an electron-rich (i.e. negative) target for positive species. These positive species are called electrophiles (which literally means ‘attracted to electrons’). Alkenes undergo addition reactions with electrophiles to produce saturated compounds. In this section we examine the three most important types of electrophilic addition reactions: the addition of hydrogen halides (HCl, HBr and HI), water (H 2 O) and halogens (Br 2 , Cl 2 ). We first study some of the experimental observations about each addition reaction and then its mechanism. By examining these particular reactions, we develop a general understanding of how alkenes undergo addition reactions. Addition of hydrogen halides The hydrogen halides HCl, HBr and HI (commonly abbreviated HX) add to alkenes to give haloalkanes (alkyl halides). These additions may be carried out either with the pure reagents or in a polar solvent such as acetic acid.
eBook - ePubIntroductory Organic Chemistry and Hydrocarbons
A Physical Chemistry Approach
- Caio Lima Firme(Author)
- 2019(Publication Date)
- CRC Press(Publisher)
Chapter SeventeenAlkenes (reactions)
INTRODUCTION
Unlike alkanes, alkenes undergo polar reactions of basically three types: pericyclic reactions, polymerization reactions, and electrophilic addition reactions. Most polar reactions in alkenes are stepwise, but alkenes also undergo concerted reactions. Similarly to alkanes, alkenes also undergo radical reactions, following radical addition instead of radical substitution as in alkanes.An important point in representing the mechanism of these reactions is the use of the most appropriate, correct view of the alkene. As shown before (Fig. 16.1 in chapter sixteen ), there are two views for representing alkenes: upper view where the π-bond electrons/orbital is shown above the plane containing vinylic carbon and their substituents (i.e., π-bond electrons are outside the plane of the paper); and side view where π-bond electrons/orbital is within the plane of the paper and the vinylic substituents are behind and at the front of the plane of the paper. In all alkene reactions, the π-bond electrons play the main role and then any appropriate mechanistic representation should take into account the correct approach of π-bond electrons to the reactant within the plane of the paper (since we are limited to a bi-dimensional representation in the paper) yielding the effective collision. As it was discussed in chapter ten, the effective collision requires reactants to approach each other at the correct orientation and with the minimum energy.The only alkene’s view that can correctly represent the approach between alkene’s π-bond electrons and its reactant within the plane of the paper is the side view. On the other hand, from the upper view, the approach between π-bond electrons and its reactant might result in a non-effective collision and does not lead to the expected transition state. In other words, the alkene’s side view is the correct orientation for alkene’s reaction yielding an effective collision, while the alkene’s upper view is the incorrect orientation for a bi-dimensional representation of a reaction since it yields a non-effective collision
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
eBook - ePub- Michael North(Author)
- 2017(Publication Date)
- Routledge(Publisher)
Scheme 9.13 . The elimination of selenoxides is of particular synthetic utility, since the reaction occurs spontaneously at room temperature. In each case, the elimination occurs in a single step and in the transition state three pairs of electrons are moving around a five or six-membered ring. It is the requirement for the formation of this cyclic transition state that accounts for the observation of a syn-elimination, and the reactions are stereospecific, with each diastereomer of the starting material giving a single diastereomer of the alkene product.Scheme 9.12If the elimination is occurring through a six atom cyclic transition state (as in the elimination of esters), then the transition state will adopt a chair conformation (Scheme 9.13 ), and the two groups being eliminated need not adopt an exactly synperiplanar conformation; a synclinal conformation is also compatible with the formation of the chair transition state. However, for elimination to occur through a five-membered ring transition state then the two groups being eliminated must be exactly synperiplanar.Scheme 9.139.4 Addition reactions to alkenesAddition reactions to alkenes can be either syn-additions or anti-additions. In a syn-addition, both groups being added are attached to the same face of the double bond, whilst in an anti-addition the two groups are attached to opposite faces of the double bond. Both syn- and anti-additions are stereospecific with each diastereomer of the alkene giving a single diastereomer of the product as shown in Scheme 9.14 for the anti-addition of bromine to 2-butene, and in Scheme 9.15 for the syn-epoxidation of 2-butene.Scheme 9.14Scheme 9.15A large number of reagents are known to undergo anti-addition to alkenes and, in most of these cases, a cyclic intermediate such as the bromonium ions 9.24a,b are formed as intermediates. The formation of this cyclic intermediate occurs by a syn-addition, that is both carbon–bromine bonds of the bromonium ions 9.24a,b are formed on the same face of the alkene. These cyclic intermediates then undergo an SN 2 reaction to give the final products and it is the inversion of configuration that is known to occur during an SN 2 reaction (cf. section 9.2.1) that accounts for the observed overall anti-addition. The product if chiral will be racemic, since the two faces of the achiral alkene are enantiotopic and so cannot be distinguished by an achiral reagent (cf. Chapter 7
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