Alkyne Reactions

9 Key excerpts on "Alkyne Reactions"

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  eBook - PDF

  Introduction to Organic Chemistry

  • William H. Brown, Thomas Poon(Authors)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  123 IN THE PREVIOUS CHAPTER , we learned about alkenes and alkynes, classes of hydrocar- bons that contain a carbon–carbon pi bond. Why are carbon‐carbon pi bonds so prevalent in organic chemistry? Why are they critical to so many biological and industrial processes? How do scientists take advantage of the special chemistry that they undergo? These questions will be answered in this chapter as we begin our systematic study of organic reactions and their reaction mechanisms. Reaction mechanisms are step‐by‐step descriptions of how reactions proceed and are one of the unifying concepts in organic chemistry. We will use the reactions of alkenes and alkynes as a vehicle to introduce this important concept. 5.1 What Are the Characteristic Reactions of Alkenes? The most characteristic reaction of alkenes is addition to the carbon–carbon double bond in such a way that the pi bond is broken and, in its place, sigma bonds are formed to two new atoms or groups of atoms. Several examples of reactions at the carbon–carbon double bond are shown in Table 5.1, along with the descriptive name(s) associated with each. From the perspective of the chemical industry, the single most important reaction of ethylene and other low‐molecular‐weight alkenes is the production of chain‐growth polymers (Greek: poly, many, and meros, part).

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  Chemistry, 5th Edition

  • Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  C C H 3 C H CH 3 H 842 Chemistry We have looked at molecules with one double bond. Molecules with more than one carbon–carbon double bond undergo the same addition reactions. However, there is a class of unsaturated molecules that do not undergo any of these addition reactions. These molecules are called aromatic compounds and are covered in section 16.7. 16.6 Reactions of alkynes LEARNING OBJECTIVE 16.6 Correlate the reactivity of alkynes with alkenes and describe how alkynes may be converted to alkenes. Much of the chemistry of alkynes mirrors the chemistry of alkenes; they undergo the same reduction reactions, as well as hydrogen halide addition and halogen addition reactions. Alkynes also undergo hydration, but unlike alkenes the synthetic outcome is a ketone and this is covered in the chapter on aldehydes and ketones. Reduction of alkynes is particularly important in the synthesis of complicated molecules used in making pharmaceuticals. Alkynes are easily reduced to alkanes by addition of hydrogen gas using a metal catalyst. This differs from the reduction of alkenes in that the reduction occurs in stages and the choice of catalyst can control the synthetic outcome. Complete reduction of the alkyne occurs when palladium coated onto carbon is used as the catalyst. Another catalyst involving deactivated palladium, called Lindlar catalyst, produces the cis alkene from the triple bond. The trans alkene can be generated using sodium or lithium dissolved in liquid ammonia. CH 3 CH 2 C CH 3 NH 3 2H 2 Li H 2 Pd/C CH 3 CH 2 CH 2 CH 2 CH 3 Lindlar catalyst CH 3 CH 2 C C CH 3 H H cis-pent-2-ene pentane trans-pent-2-ene CH 3 CH 2 C C CH 3 H H C 16.7 Aromatic compounds LEARNING OBJECTIVE 16.7 Describe how aromatic hydrocarbons differ from alkenes and alkynes through the presence of remarkably stable cyclic  bonds. The simplest example of an aromatic compound is benzene, C 6 H 6 .

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  Experimental Organic Chemistry

  A Miniscale & Microscale Approach

  • John Gilbert, Stephen Martin(Authors)
  • 2015(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  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. The carbon-carbon triple bond in an alkyne is formed by the overlap of two orthog-onal pairs of p -orbitals on adjacent sp -hybridized carbon atoms. Because the functional group in alkynes is related to the carbon-carbon double bond in alkenes (Chap. 10), the chemistry of these two classes of compounds is similar. For example, in much the same manner as one elimination reaction may be used to form the double bond in alkenes, two sequential elimination reactions yield the triple bond in alkynes (Eq. 11.1). 11 C C X H X H R R C R C R A 1,2-dihaloalkane An alkyne strong base ∇ (11.1) 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. 404 Experimental Organic Chemistry ■ Gilbert and Martin The p -electrons in both alkenes and alkynes provide a Lewis-base site for interaction with electrophilic reagents, which are Lewis acids, so one of the typi-cal reactions of alkenes and alkynes is electrophilic addition as illustrated in Equations 11.2 and 11.3. Such reactions of alkynes can be stopped after the addition of one equivalent of a reagent, but the use of excess reagent leads to the formation of saturated products in which a second equivalent has been added. Some aspects of the chemistry of alkynes will be explored in the experiments in this chapter.

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  Methods 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).

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  Organic Chemistry

  • David R. Klein(Author)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  • Alkynes exhibit linear geometry and can function either as bases or as nucleophiles. SECTION 9.2 • Alkynes are named much like alkanes, with the following additional rules: • The suffix “ane” is replaced with “yne.” • The parent is the longest chain that includes the C≡C bond. • The triple bond should receive the lowest number possible. • The position of the triple bond is indicated with a single locant placed either before the parent or the suffix. • Monosubstituted acetylenes are terminal alkynes, while disubstituted acetylenes are internal alkynes. SECTION 9.3 • The conjugate base of acetylene, called an acetylide ion, is relatively stabilized because the lone pair occupies an sp-hybridized orbital. • The conjugate base of a terminal alkyne is called an alkynide ion, which can only be formed with a sufficiently strong base, such as NaNH 2 . SECTION 9.4 • Alkynes can be prepared from either geminal or vicinal dihalides via two successive E2 reactions. SECTION 9.5 • Catalytic hydrogenation of an alkyne yields an alkane. • Catalytic hydrogenation in the presence of a poisoned cata- lyst (Lindlar’s catalyst or Ni 2 B) yields a cis alkene. • A dissolving metal reduction will convert an alkyne into a trans alkene. The reaction involves an intermediate radi- cal anion and employs fishhook arrows, which indicate the movement of only one electron. SECTION 9.6 • Alkynes react with HX via a Markovnikov addition. • One possible mechanism for the hydrohalogenation of alkynes involves a vinylic carbocation, while another pos- sible mechanism is termolecular. 428 CHAPTER 9 Alkynes • Addition of HX to alkynes probably occurs through a vari- ety of mechanistic pathways all of which are occurring at the same time and competing with each other. • Treatment of a terminal alkyne with HBr and peroxides gives an anti-Markovnikov addition of HBr.

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  Organic Chemistry

  • 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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  Klein's Organic Chemistry

  • David R. Klein(Author)
  • 2020(Publication Date)
  • Wiley

    (Publisher)

  468 CHAPTER 10 Alkynes 10.8 HALOGENATION OF ALKYNES In the previous chapter, we saw that alkenes will react with Br 2 or Cl 2 to produce a dihalide. In much the same way, alkynes are also observed to undergo halogenation. The one major difference is that alkynes have two π bonds rather than one and can, therefore, add two equivalents of the halogen to form a tetrahalide: C C R R (X = Cl or Br) R C C R X X X X (60–70%) excess X 2 CCl 4 In some cases, it is possible to add just one equivalent of halogen to produce a dihalide. Such a reac- tion generally proceeds via an anti addition (just as we saw with alkenes), producing the E isomer as the major product: R R + R R X X Major R X R X Minor X 2 (one equivalent) CCl 4 The mechanism of alkyne halogenation is not entirely understood. 10.9 OZONOLYSIS OF ALKYNES When treated with ozone followed by water, alkynes undergo oxidative cleavage to produce carboxylic acids: R C OH O R′ C HO O + C C R R′ 1) O 3 2) H 2 O PRACTICE the skill APPLY the skill need more PRACTICE? This transformation requires a Markovnikov addition, which can be accomplished via an acid-catalyzed hydration: H 2 SO 4 , H 2 O HgSO 4 O 10.22 Identify reagents that you could use to achieve each of the following transformations: (a) O (b) O H 10.23 Identify the reagents you would use to achieve each of the following transformations: STEP 2 Identify the reagents that achieve the desired regiochemical outcome. (a) O H Br Br (b) O Cl Cl Try Problems 10.52b, 10.62 10.10 Alkylation of Terminal Alkynes 469 When a terminal alkyne undergoes oxidative cleavage, the terminal side is converted into carbon dioxide: R C OH O + C O O C C R H 1) O 3 2) H 2 O Decades ago, chemists used oxidative cleavage to help with structural determinations. An unknown alkyne would be treated with ozone followed by water, and the resulting carboxylic acids would be identified. This technique allowed chemists to identify the location of a triple bond in an unknown alkyne.

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  Organic Chemistry

  • David R. Klein(Author)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  SECTION 9.10 • Alkynide ions undergo alkylation when treated with an alkyl halide (methyl or primary). • Acetylene possesses two terminal protons and can undergo two separate alkylations. SECTION 9.11 • An alkene can be converted into an alkyne via bromination followed by elimination with excess NaNH 2 . SKILLBUILDER REVIEW 9.1 ASSEMBLING THE SYSTEMATIC NAME OF AN ALKYNE STEP 1 Identify the parent: choose the longest chain that includes the triple bond. Heptyne STEP 2 Identify and name substituents. propyl methyl ethyl STEP 3 Number the parent chain and assign a locant to each substituent. 1 2 3 4 5 6 7 STEP 4 Assemble the substituents alphabetically. 4-Ethyl-5-methyl-3-propyl-1-heptyne Try Problems 9.1–9.4, 9.32, 9.33, 9.55 9.2 SELECTING A BASE FOR DEPROTONATING A TERMINAL ALKYNE STEP 3 The equilibrium favors the weaker acid and the weaker base. In this case, the equilibrium favors the left side. Therefore, hydroxide is not a suitable base to deprotonate a terminal alkyne. STEP 2 Compare the two acids and determine which is the weaker acid (higher pK a ). C R C H (pK a ∼ 25) H 2 O (pK a = 15.7) STEP 1 Draw the products of the proton transfer reaction and identify the acid and base on each side of the equilibrium. C R C H Acid + + Acid H 2 O Base OH Base C R C − − Try Problems 9.5, 9.6, 9.35, 9.38 9.3 DRAWING A MECHANISM FOR ACID-CATALYZED KETO-ENOL TAUTOMERIZATION STEP 2 Draw the resonance structure of the intermediate. STEP 3 Remove a proton to form the ketone. STEP 1 Protonate the π bond of the enol (do not protonate the hydroxyl group). OH O H H H O H O H O H O H + + + Try Problems 9.16, 9.17, 9.44b, 9.71, 9.72, 9.75 SkillBuilder Review 447 9.4 CHOOSING THE APPROPRIATE REAGENTS FOR THE HYDRATION OF AN ALKYNE STEP 1 Identify the regiochemical outcome. STEP 2 Choose the reagents that achieve that outcome.

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  Modern Acetylene Chemistry

  • Peter J. Stang, François Diederich, Peter J. Stang, François Diederich(Authors)
  • 2008(Publication Date)
  • Wiley-VCH

    (Publisher)

  308 8 Cyclic Alkynes: Preparation and Properties clodecadiyne system (137) results in the formation of a nonaromatic 1,Cdehydrobutadiene system (Scheme 8-18) [70]. The scope and limitations of this reaction are not yet fully ex- plored. The importance of transannular electronic interactions is beautifully shown in a reaction found by S. Misumi, T. Ogawa and T. Kaneda [Eq. (22)], when transannular interaction be- tween a diyne system and an aromatic ring results in completely unusual reactivity [3a]. 137 138 139 140 Scheme 8-18 1 NC 142 8.4.3 Addition Reactions of Cyclic Alkynes The triple bonds in cyclic alkynes can, of course, be subjected to all known addition reactions of acetylenes. Here, we will discuss examples which either lead to particularly interesting addi- tion products or demonstrate unusual reactivity of bent triple bonds. 8.4.3.1 Homonuclear Addition Reactions The addition of dihydrogen to triple bonds, which can be readily achieved either using ionic reagents such as LiAlH, to give trans double bonds or catalytically affording cis-alkenes, has found important applications in the synthesis of cyclic alkenes with interesting n-parameters. Thus, Sondheimer's famous synthesis of [18]annulene (145) employs the hydrogenation of an acetylenic precursor in the last step (Scheme 8-19). Recently, hydrogenation of cyclic diene- diynes has been used for the preparation of interesting cyclic tetraenes, such as tetrahomocy- clooctatetraene 146 [Eq. (23)l 15 a]. The high reactivity of bent triple bonds toward both nucleophiles and electrophiles is demonstrated by the reaction of cyclooctyne with lithium and iodine, both reactions affording a homonuclear addition product [3b] (Scheme 8-20).

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