Chemistry
Chemical Properties of Alkynes
Alkynes are hydrocarbons with a carbon-carbon triple bond, and their chemical properties include undergoing addition reactions to form new compounds. They can be hydrogenated to form alkanes or undergo halogenation, hydration, and other reactions to produce a variety of organic compounds. Alkynes also exhibit acidic properties due to the presence of a hydrogen atom bonded to a sp-hybridized carbon.
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9 Key excerpts on "Chemical Properties of Alkynes"
- Frederick Bettelheim, William Brown, Mary Campbell, Shawn Farrell(Authors)
- 2019(Publication Date)
- Cengage Learning EMEA(Publisher)
347 CONTENTS 12.1 Introduction to Alkenes and Alkynes 12.2 Structures of Alkenes and Alkynes 12.3 Naming Alkenes and Alkynes 12.4 Physical Properties of Alkenes and Alkynes 12.5 Characteristic Reactions of Alkenes 12.6 Important Polymerization Reactions of Ethylene and Substituted Ethylenes 12.7 Structure of Benzene 12.8 Naming Aromatic Compounds 12.9 Reactions of Benzene and Its Derivatives 12.10 Phenols Alkenes, Alkynes, and Aromatic Compounds 12 12.1 Introduction to Alkenes and Alkynes In this chapter, we begin our study of unsaturated hydrocarbons. Recall from Section 11.1 that unsaturated compounds contain one or more carbon–carbon double bonds, triple bonds, or benzene-like rings. In this chapter, we first study alkenes and alkynes . Alkenes are unsaturated hydrocarbons that contain one or more carbon–carbon double bonds. The simplest alkene is ethylene. Ethylene (an alkene) Acetylene (an alkyne) H H H H H ! C # C ! H C C Alkynes are unsaturated hydrocarbons that contain one or more carbon–carbon triple bonds. The simplest alkyne is acetylene. Because alkynes are not widespread in nature and have little importance in biochemistry, we will not study their chemistry in depth. Compounds containing carbon–carbon double bonds are especially widespread in nature. Furthermore, several low-molecular-weight Alkenes Unsaturated hydrocarbons that contain a carbon–carbon double bond Alkynes Unsaturated hydrocarbons that contains a carbon–carbon triple bond Charles D. Winters Carotene is a naturally occurring polyene in carrots and tomatoes (Problem 85). 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). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience.
eBook - PDF- William H. Brown, Thomas Poon(Authors)
- 2017(Publication Date)
- Wiley(Publisher)
• To name an alkene containing two or more double bonds, we change the infix to ‐adien‐, ‐atrien‐, and so forth. Compounds containing several double bonds are called polyenes. 4.3 What Are the Physical Properties of Alkenes and Alkynes? • Alkenes and alkynes are nonpolar compounds, and the only interactions between their molecules are dispersion forces. • The physical properties of alkenes and alkynes are similar to those of alkanes. 4.4 Why Are 1‐Alkynes (Terminal Alkynes) Weak Acids? • Terminal alkynes are weakly acidic (pK a 25) and can be con- verted to alkyne anions by strong bases such as sodium amide, NaNH 2 . QUICK QUIZ Answer true or false to the following questions to assess your general knowledge of the concepts in this chapter. If you have difficulty with any of them, you should review the appropriate section in the chapter (shown in parentheses) before attempting the more challenging end‐of‐chapter problems. 1. Ethylene and acetylene are constitutional isomers. (4.2) 2. Alkanes that are liquid at room temperature are insoluble in water and when added to water will float on water. (4.3) 3. The bulk of the ethylene used by the chemical industry worldwide is obtained from nonrenewable resources. (4.1) 4. Alkenes and alkynes are nonpolar molecules. (4.3) 5. The IUPAC name of CH 3 CH CHCH 3 is 1,2‐dimethylethyl- ene. (4.2) 6. Cyclohexane and 1‐hexene are constitutional isomers. (4.1) 7. The IUPAC name of an alkene is derived from the name of the longest chain of carbon atoms that contains the double bond. (4.2) 8. There are two classes of unsaturated hydrocarbons, alk- enes and alkynes. (4.1) 9. Both geraniol and menthol (pp. 243–244) show cis–trans isomerism. (4.4) 10. 1,2‐Dimethylcyclohexene shows cis–trans isomerism. (4.2) 11. 2‐Methyl‐2‐butene shows cis–trans isomerism. (4.2) 12. Both ethylene and acetylene are planar molecules. (4.1) 13. The physical properties of alkenes are similar to those of alkanes with the same carbon skeletons.
eBook - PDFChemistry for Today
General, Organic, and Biochemistry
- Spencer Seager, Michael Slabaugh, Maren Hansen, , Spencer Seager, Spencer Seager, Michael Slabaugh, Maren Hansen(Authors)
- 2021(Publication Date)
- Cengage Learning EMEA(Publisher)
Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Unsaturated Hydrocarbons 391 Example 12.9 Naming Alkynes Since the first alkyne was isolated in 1826, well over a thousand alkynes have been dis- covered in biologically occurring materials, making the naming of alkynes an important skill. Give an IUPAC name for the following alkynes: a. CH 3 CH 2 H C C b. H CH CH 3 CH 3 C C Solution a. Alkynes are named in exactly the same way as alkenes, except the ending -yne is used. The longest chain containing the triple bond has four carbon atoms. The four-carbon alkyne is butyne. CH 3 CH 2 H C 2 1 3 C 4 To give lower numbers to the carbons bound by the triple bond, the chain is numbered from the right. The position of the triple bond is indicated by the lower-numbered car- bon atom that is triple-bonded, carbon 1 in this case. The name is 1-butyne. b. To give lower numbers to the carbons bound by the triple bond, the chain is numbered from the left: CH CH 3 H C 3 2 4 C 1 CH 3 CH CH 3 H C 2 3 1 C 4 CH 3 not Thus, the compound is a 1-butyne with an attached methyl group on carbon 3. There- fore, the name is 3-methyl-1-butyne. ✔ LEARNING CHECK 12.9 Give the IUPAC name for each of the following: CH 3 CH 2 C C CH 3 CH 3 CH CH 2 a. CH 3 C C CH 3 b. The physical properties of the alkynes are nearly the same as those of the corresponding alkenes and alkanes. They are insoluble in water, less dense than water, and have relatively low melting and boiling points. Alkynes also resemble alkenes in their addition reactions. The same substances (Br 2 , H 2 , HCl, etc.) that add to double bonds also add to triple bonds. The one significant difference is that alkynes consume twice as many moles of addition reagent as alkenes in addition reactions that go on to completion. 12.6 Aromatic Compounds and the Benzene Structure Learning Objective 8 Classify organic compounds as aliphatic or aromatic.
eBook - PDF- H. Stephen Stoker(Author)
- 2015(Publication Date)
- Cengage Learning EMEA(Publisher)
1-17 Chemical Properties of Alkanes and Cycloalkanes 35 Copyright 2016 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). 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. 36 CHAPTER 1 Saturated Hydrocarbons Chemistry at a Glance—Properties of Alkanes and Cycloalkanes—summarizes the physical properties and chemical reactions of alkanes and cycloalkanes. Halogenation usually results in the formation of a mixture of products rather than a single product. More than one product results because more than one hydro-gen atom on an alkane can be replaced with halogen atoms. To illustrate this concept, let us consider the chlorination of methane, the simplest alkane. Methane and chlorine, when heated to a high temperature or in the presence of light, react as follows: CH 4 1 Cl 2 CH 3 Cl 1 HCl Heat or light The reaction does not stop at this stage, however, because the chlorinated methane product can react with additional chlorine to produce polychlorinated products. Heat or light Heat or light Heat or light CH 3 Cl 1 Cl 2 CHCl 3 1 Cl 2 CH 2 Cl 2 1 Cl 2 CH 2 Cl 2 1 HCl CHCl 3 1 HCl CCl 4 1 HCl By controlling the reaction conditions and the ratio of chlorine to methane, it is possible to favor formation of one or another of the possible chlorinated methane products. The chemical properties of cycloalkanes are similar to those of alkanes. Cycloalkanes readily undergo combustion, as well as chlorination and bromination. With unsubstituted cycloalkanes, monohalogenation produces a single product because all hydrogen atoms present in the cycloalkane are equivalent to one another.
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2017(Publication Date)
- Wiley(Publisher)
312 CHAPTER 7 ALKENES AND ALKYNES I: Properties and Synthesis. Elimination Reactions of Alkyl Halides This carbocation, since it is internal to the chain, will be secondary or tertiary, depending on the specific substrate. Various processes that you have already learned can now occur from this carbocation: (1) a different β hydrogen may be removed, leading to a more stable alkene than the initially formed terminal alkene; (2) a hydride or alkanide rearrangement may occur leading to a yet more stable carbocation (e.g., moving from a 2° to a 3° carbocation) or to a carbocation of approximately equal stability, after which the elimination may be completed; or (3) a nucleophile may attack any of these carbocations to form a substitution product. Under the high-temperature conditions for alcohol dehydration the principal products will be alkenes rather than substitution products. 7.12 THE ACIDITY OF TERMINAL ALKYNES The hydrogen bonded to the carbon of a terminal alkyne, called an acetylenic hydrogen atom, is considerably more acidic than those bonded to carbons of an alkene or alkane (see Section 3.8A). The pK a values for ethyne, ethene, and ethane illustrate this point: C C A terminal alkyne is 10 20 times more acidic than an alkene or alkane. H H H H H H C C H H C C H H H H pK a 25 pK a 44 pK a 50 The order of basicity of their anions is opposite that of their relative acidity: Relative Basicity CH 3 CH 2 ⋅ ⋅ − > CH 2 = CH ⋅ ⋅ − > HC≡C ⋅ ⋅ − If we include in our comparison hydrogen compounds of other first-row elements of the periodic table, we can write the following orders of relative acidities and basicities. This comparison is useful as we consider what bases and solvents to use with terminal alkynes.
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
312 CHAPTER 7 ALKENES AND ALKYNES I: Properties and Synthesis. Elimination Reactions of Alkyl Halides This carbocation, since it is internal to the chain, will be secondary or tertiary, depending on the specific substrate. Various processes that you have already learned can now occur from this carbocation: (1) a different β hydrogen may be removed, leading to a more stable alkene than the initially formed terminal alkene; (2) a hydride or alkanide rearrangement may occur leading to a yet more stable carbocation (e.g., moving from a 2° to a 3° carbocation) or to a carbocation of approximately equal stability, after which the elimination may be completed; or (3) a nucleophile may attack any of these carbocations to form a substitution product. Under the high-temperature conditions for alcohol dehydration the principal products will be alkenes rather than substitution products. 7.12 THE ACIDITY OF TERMINAL ALKYNES The hydrogen bonded to the carbon of a terminal alkyne, called an acetylenic hydrogen atom, is considerably more acidic than those bonded to carbons of an alkene or alkane (see Section 3.8A). The pK a values for ethyne, ethene, and ethane illustrate this point: C C A terminal alkyne is ∼10 20 times more acidic than an alkene or alkane. H H H H H H C C H H C C H H H H pK a = 25 pK a = 44 pK a = 50 The order of basicity of their anions is opposite that of their relative acidity: Relative Basicity CH 3 CH 2 ⋅ ⋅ − > CH 2 = CH ⋅ ⋅ − > HC≡C ⋅ ⋅ − If we include in our comparison hydrogen compounds of other first-row elements of the periodic table, we can write the following orders of relative acidities and basicities. This comparison is useful as we consider what bases and solvents to use with terminal alkynes.
eBook - PDFExperimental Organic Chemistry
A Miniscale & Microscale Approach
- John Gilbert, Stephen Martin(Authors)
- 2015(Publication Date)
- Cengage Learning EMEA(Publisher)
403 C H A P T E R Alkynes You may have been introduced to the simplest alkyne, acetylene, because it is frequently used as a fuel in welding. Indeed, when acetylene is burned with oxygen, the flame reaches temperatures of about 3300 °C (6000 °F), hotter than all but two or three other mixtures of combustible gases. You already know that flames and organic labs can be a dangerous combination, so we will explore tamer properties of alkynes, avoiding studies of their combustibility. Like the alkenes discussed in Chapter 10, alkynes are unsaturated hydrocarbons, and their chemistry is also dominated by the presence of carbon-carbon multiple or p -bonds. Whereas alkenes have a double bond, alkynes are characterized by a triple bond, which is composed of two orthogonal carbon-carbon p -bonds. Hence, if you understand the reactions that lead to the formation of alkenes, you will be able to apply this knowledge to preparing alkynes. Similarly, you will be able to extend your knowledge of the reactions of alkenes to predicting products of reactions of alkynes. Because alkynes have two double bonds, however, they basically just do everything twice. Well, it’s almost that simple. 11.1 I N T R O D U C T I O N Unsaturated organic compounds that contain a carbon-carbon triple bond as the functional group are called alkynes . Acetylene (ethyne), H–C ≡ C–H, is the simplest alkyne and is widely used in industry as a fuel and as a chemical feedstock for the preparation of other organic compounds such as acetic acid (CH 3 CO 2 H), vinyl chlo-ride (CH 2 = CHCl), a monomer used in the manufacture of polyvinyl chloride, and chloroprene (CH 2 = CCl–CH = CH 2 ), which polymerizes to give neoprene, the material from which your protective gloves may be made. The value of acetylene to Germany during World War II is described in the Historical Highlight Acetylene: A Valuable Small Molecule , which is available online.
eBook - PDF- William H. Brown, Thomas Poon(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
Therefore, an alkyne anion is the most stable of the series, and an alkyne is the strongest acid of the series. By similar reasoning, the alkane carbon (sp 3 hybridized and 25% s character) is the least electronegative, and an alkane is the weakest acid of the series. An alkene, with 33% s character, is intermediate. Finally, it is only the hydrogen of a 1‐alkyne that shows this type of acidity. No other hydogens of an alkyne have comparable acidity, and no other hydrogens are removed by NaNH 2 . CH 3 CH 2 CH 2 C C H these hydrogens are much lower in acidity and are not deprotonated by NaNH 2 only this hydrogen is acidic enough to be deprotonated by NaNH 2 SUMMARY OF KEY QUESTIONS 4.1 What Are the Structures and Shapes of Alkenes and Alkynes? • An alkene is an unsaturated hydrocarbon that contains a carbon–carbon double bond. • Alkenes have the general formula C n H 2n . • An alkyne is an unsaturated hydrocarbon that contains a carbon–carbon triple bond. • Alkynes have the general formula C n H 2n−2 . • According to the orbital overlap model, a carbon–carbon double bond consists of one sigma bond formed by the overlap of sp 2 hybrid orbitals and one pi bond formed by the overlap of parallel 2p atomic orbitals. It takes approximately 264 kJ/mol (63 kcal/mol) to break the pi bond in ethylene. • A carbon–carbon triple bond consists of one sigma bond formed by the overlap of sp hybrid orbitals and two pi bonds formed by the overlap of pairs of parallel 2p orbitals. • The structural feature that makes cis–trans isomerism pos- sible in alkenes is restricted rotation about the two carbons of the double bond. • To date, trans‐cyclooctene is the smallest trans‐cycloalkene that has been prepared in pure form and is stable at room temperature. 4.2 How Do We Name Alkenes and Alkynes? • According to the IUPAC system, we show the presence of a carbon–carbon double bond by changing the infix of the parent hydrocarbon from ‐an‐ to ‐en‐.
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
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