Chemistry
Reactions of Cycloalkanes
Reactions of cycloalkanes involve various chemical transformations such as ring-opening reactions, substitution reactions, and addition reactions. These reactions can be influenced by factors such as ring strain and steric hindrance. Understanding the reactivity of cycloalkanes is important in organic chemistry for the synthesis of complex molecules and understanding the behavior of cyclic compounds.
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7 Key excerpts on "Reactions of Cycloalkanes"
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2017(Publication Date)
- Wiley(Publisher)
Adamantane In the chapter closer we shall see several examples of other unusual and highly strained, cyclic hydrocarbons. 4.15 CHEMICAL REACTIONS OF ALKANES Alkanes, as a class, are characterized by a general inertness to many chemical reagents. Carbon–carbon and carbon–hydrogen bonds are quite strong; they do not break unless alkanes are heated to very high temperatures. Because carbon and hydrogen atoms have nearly the same electronegativity, the carbon–hydrogen bonds of alkanes are only slightly polarized. As a consequence, they are generally unaffected by most bases. Molecules of alkanes have no unshared electrons to offer as sites for attack by acids. This low reactivity of alkanes toward many reagents accounts for the fact that alkanes were originally called paraffins ( parum affinis, Latin: little affinity). The term paraffin, however, was probably not an appropriate one. We all know that alkanes react vigorously with oxygen when an appropriate mixture is ignited. This combustion occurs, for example, in the cylinders of automobiles, in furnaces, and, more gently, with paraffin candles. When heated, alkanes also react with chlorine and bromine, and they react explosively with fluorine. We shall study these reactions in Chapter 10. 4.16 SYNTHESIS OF ALKANES AND CYCLOALKANES A chemical synthesis may require, at some point, the conversion of a carbon–carbon double or triple bond to a single bond. Synthesis of the following compound, used as an ingredient in some perfumes, is an example. (used in some perfumes) CO 2 CH 3 CO 2 CH 3 4.16 SYNTHESIS OF ALKANES AND CYCLOALKANES 183 This conversion is easily accomplished by a reaction called hydrogenation. There are several reaction conditions that can be used to carry out hydrogenation, but among the common ways is use of hydrogen gas and a solid metal catalyst such as platinum, pal- ladium, or nickel. Equations in the following section represent general examples for the hydrogenation of alkenes and alkynes.
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
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.
- Shuanhu Gao, Shengming Ma(Authors)
- 2016(Publication Date)
- Thieme(Publisher)
1.5 Cyclization Reactions of Alkenes and Alkynes L. Zhang General Introduction Alkenes and alkynes are two common classes of substrates of ubiquitous usage in metal catalysis, be it involving soft late transition, hard early transition, or main group metals. Intramolecular reactions of these ð -systems bearing a diverse range of appropriately teth-ered functional groups serve as essential tools for studying reactivities of metal com-plexes, and are a main avenue for developing synthetically versatile methodologies. A ma-jority of these reactions belong to metal-catalyzed cyclization reactions, which are typi-cally kinetically more facile than the corresponding intermolecular reactions due to de-creased entropy penalty and provide efficient access to synthetically versatile ring struc-tures. To avoid overlapping with other chapters, this chapter is focused on metal-cata-lyzed cyclization reactions involving polar intermediates with the exception of the intra-molecular Heck and the oxidative Heck reactions (Section 1.2), intramolecular allylation reactions (Section 1.3), cycloisomerization reactions with multiple unsaturated bonds (Section 1.7), amination and C — O bond-forming reactions (Section 1.8), and cycloaddition reactions (Sections 2.4–2.8). Cyclizations involving radical intermediates are discussed in Section 2.9. The reactions discussed here are grouped into two types: one where an unac-tivated carbon–carbon double/triple bond reacts as nucleophile to attack tethered electro-philes (Section 1.5.1), and the other where the ð -system is activated by a metal-based ð -acid and subsequently attacked by carbon nucleophiles (Section 1.5.2). 1.5. 1 Unactivated Alkenes and Alkynes as Nucleophiles Electron-rich and electron-neutral alkenes and alkynes are broadly used as nucleophiles for various transformations, many of which are cyclization reactions.
eBook - PDF- David R. Klein(Author)
- 2020(Publication Date)
- Wiley(Publisher)
This chapter will introduce only the most basic principles of conformational analysis, which we will use to analyze the flexibility of molecules. To simplify our discussion, we will explore compounds that lack a functional group, called alkanes and cycloalkanes. Analysis of these compounds will enable us to understand how molecules achieve flexibility. Specifically, we will explore how alkanes and cycloalkanes change their three-dimensional shape as a result of the rota- tion of C C single bonds. Our discussion of conformational analysis will involve the comparison of many different compounds and will be more efficient if we can refer to compounds by name. A system of rules for naming alkanes and cycloalkanes will be developed prior to our discussion of molecular flexibility. 4.2 Nomenclature of Alkanes 133 4.1 INTRODUCTION TO ALKANES Recall that hydrocarbons are compounds comprised of only C and H; for example: C C H H H H H H Ethane C 2 H 6 C C H H H H Ethylene C 2 H 4 C C H H Acetylene C 2 H 2 Benzene C 6 H 6 Ethane is unlike the other examples in that it has no π bonds. Hydrocarbons that lack π bonds are called saturated hydrocarbons, or alkanes. The names of these compounds usually end with the suffix “-ane,” as seen in the following examples: Propane Butane Pentane This chapter will focus on alkanes, beginning with a procedure for naming them. The system of nam- ing chemical compounds, or nomenclature, will be developed and refined throughout the remaining chapters of this book. 4.2 NOMENCLATURE OF ALKANES An Introduction to IUPAC Nomenclature In the early nineteenth century, organic compounds were often named at the whim of their discover- ers.
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
Carbon–carbon and carbon–hydrogen bonds are quite strong; they do not break unless alkanes are heated to very high temperatures. Because carbon and hydrogen atoms have nearly the same electronegativity, the carbon–hydrogen bonds of alkanes are only slightly polarized. As a consequence, they are generally unaffected by most bases. Molecules of alkanes have no unshared electrons to offer as sites for attack by acids. This low reactivity of alkanes toward many reagents accounts for the fact that alkanes were originally called paraffins (parum affinis, Latin: little affinity). The term paraffin, however, was probably not an appropriate one. We all know that alkanes react vigorously with oxygen when an appropriate mixture is ignited. This combus- tion occurs, for example, in the cylinders of automobiles, in furnaces, and, more gently, with paraffin candles. When heated, alkanes also react with chlorine and bromine, and they react explosively with fluorine. We shall study these reactions in Chapter 10. 4.16 Synthesis of Alkanes and Cycloalkanes A chemical synthesis may require, at some point, the conversion of a carbon–carbon double or triple bond to a single bond. Synthesis of the following compound, used as an ingredient in some perfumes, is an example. (used in some perfumes) CO 2 CH 3 CO 2 CH 3 4.16 Synthesis of Alkanes and Cycloalkanes 187 This conversion is easily accomplished by a reaction called hydrogenation. There are several reaction conditions that can be used to carry out hydrogenation, but among the common ways is use of hydrogen gas and a solid metal catalyst such as platinum, palladium, or nickel. Equa- tions in the following section represent general examples for the hydrogenation of alkenes and alkynes. 4.16A Hydrogenation of Alkenes and Alkynes • Alkenes and alkynes react with hydrogen in the presence of metal catalysts such as nickel, palladium, and platinum to produce alkanes.
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
Carbon–carbon and carbon–hydrogen bonds are quite strong; they do not break unless alkanes are heated to very high temperatures. Because carbon and hydrogen atoms have nearly the same electronegativity, the carbon–hydrogen bonds of alkanes are only slightly polarized. As a consequence, they are generally unaffected by most bases. Molecules of alkanes have no unshared electrons to offer as sites for attack by acids. This low reactivity of alkanes toward many reagents accounts for the fact that alkanes were originally called paraffins (parum affinis, Latin: little affinity). The term paraffin, however, was probably not an appropriate one. We all know that alkanes react vigorously with oxygen when an appropriate mixture is ignited. This combustion occurs, for example, in the cylinders of automobiles, in furnaces, and, more gently, with paraffin candles. When heated, alkanes also react with chlorine and bromine, and they react explosively with fluorine. We shall study these reactions in Chapter 10. 4.16 SYNTHESIS OF ALKANES AND CYCLOALKANES A chemical synthesis may require, at some point, the conversion of a carbon–carbon double or triple bond to a single bond. Synthesis of the following compound, used as an ingredient in some perfumes, is an example. (used in some perfumes) CO 2 CH 3 CO 2 CH 3 4.16 SYNTHESIS OF ALKANES AND CYCLOALKANES 183 This conversion is easily accomplished by a reaction called hydrogenation. There are several reaction conditions that can be used to carry out hydrogenation, but among the common ways is use of hydrogen gas and a solid metal catalyst such as platinum, pal- ladium, or nickel. Equations in the following section represent general examples for the hydrogenation of alkenes and alkynes. 4.16A Hydrogenation of Alkenes and Alkynes • Alkenes and alkynes react with hydrogen in the presence of metal catalysts such as nickel, palladium, and platinum to produce alkanes.
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