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Chapter 8. alkyne carbon-carbon triple An alkyne is a hydrocarbon that contains a carbon-carbon triple bond Introduction.

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Presentation on theme: "Chapter 8. alkyne carbon-carbon triple An alkyne is a hydrocarbon that contains a carbon-carbon triple bond Introduction."— Presentation transcript:

1 Chapter 8

2 alkyne carbon-carbon triple An alkyne is a hydrocarbon that contains a carbon-carbon triple bond Introduction

3 AcetyleneAcetylene is the simplest alkyne. pyrolysisIt is produced industrially from methane by high temperature decomposition (pyrolysis) It is the starting material of many organic molecules

4 I. Alkynes: An Overview I. Alkynes: An Overview A.Electronic Structure B.Naming Alkynes A.Electronic Structure B.Naming Alkynes

5 A.Electronic Structure A. Electronic Structure Carbon-carbon triple bond results from: –sp  –sp hybrid orbital on each C forming a  bond and –unhybridized p y p z  –unhybridized p y and p z orbitals forming a  bond

6 180º –The remaining sp orbitals form bonds to other atoms at 180º to C-C triple bond. shorterstronger –The triple bond is shorter and stronger than single or double bond

7 Breaking a  bond in acetylene (HCCH) requires 318 kJ/mole (in ethylene it is 268 kJ/mole)

8 B.Naming Alkynes B. Naming Alkynes International Union of Pure and Applied ChemistryIUPACLike alkanes and alkenes, alkynes are named according to the system devised by the International Union of Pure and Applied Chemistry (IUPAC).

9 Steps to naming alkynes 1 a.Find the longest continuous carbon chain containing the triple bond b.Name using the suffix –yne to indicate an alkyne. –The position of the triple bond is indicated by giving the number of the first alkyne carbon c.Numbering of chain with triple bond is set so that the smallest number possible includes the triple bond

10 Steps to naming alkynes 2 d.If more than one triple bond is present: –Indicate the position of each and use the suffixes - diyne, -triyne, … diyne –A compound with two triple bonds is a diyne triyne –A triyne has three triple bonds

11 Steps to naming alkynes 3 e.If a triple and a double bond are present: enyne –Name the compound an enyne –An enyne has a double bond and triple bond –Number from chain that ends nearest a double or triple bond - double bond is preferred if both are present in the same relative position

12 alkynylAlkynes as substituents are called “alkynyl”

13 Practice Problem: Name the following compounds:

14 Practice Problem: There are seven isomeric alkynes with the formula C 6 H 10. Draw and name them

15 II. Synthesis of Alkynes II. Synthesis of Alkynes A.Elimination Reactions of Dihalides B.Alkylation of Acetylide Anions A.Elimination Reactions of Dihalides B.Alkylation of Acetylide Anions

16 A.Elimination Reactions of Dihalides vicinal dihalideTreatment of a 1,2 dihaloalkane (vicinal dihalide) with KOH or NaNH 2 produces a two-fold elimination of HX and formation of an alkyne vinyl halide –Intermediate is a vinyl halide

17 –Vicinal dihalides –Vicinal dihalides are available from addition of bromine or chlorine to an alkene

18 –Vinyl halides –Vinyl halides give alkynes when treated with strong base.

19 Dehydrohalogenation of Vicinal dihalides Dehydrohalogenation of vicinal dihalides Dehydrohalogenation of vicinal dihalides results in twofold elimination (loss) of HX and formation of an alkyne strong base (KOH or NaNH 2 ) –uses strong base (KOH or NaNH 2 ) vinyl halide –Intermediate is a vinyl halide

20 B.Alkylation of Acetylide Anions

21 III. Reactions of Alkynes III. Reactions of Alkynes A.Addition of HX and X 2 B.Hydration of Alkynes C.Reduction of Alkynes D.Oxidative Cleavage of Alkynes A.Addition of HX and X 2 B.Hydration of Alkynes C.Reduction of Alkynes D.Oxidative Cleavage of Alkynes

22 III. Reactions of Alkynes III. Reactions of Alkynes E.Alkyne Acidity: Formation of Acetylide Anions F.Alkylation of Acetylide Anions G.An Introduction to Organic Synthesis E.Alkyne Acidity: Formation of Acetylide Anions F.Alkylation of Acetylide Anions G.An Introduction to Organic Synthesis

23 Addition reactions of alkynes are similar to those of alkenes Alkynes react with many electrophiles to give useful products by addition Introduction

24 A.Addition of HX and X 2 excessH-XdihalideAddition of excess H-X to an alkyne gives a dihalide product Markovnikov –Regiochemistry is Markovnikov (X attaches to the more highly substituted sp carbon and H adds to the less highly substituted side)

25 Addition of X 2 (where X = Br or Cl) excessX 2 tetrahalideAddition of excess X 2 to an alkyne gives a tetrahalide product. InitialX 2 (Br 2 Cl 2 ) transInitial addition of X 2 (Br 2 or Cl 2 ) to an alkyne gives trans intermediate

26 Addition of HX to Alkynes Involves Vinylic Carbocations

27 vinylic carbocation intermediateAddition of H-X to alkyne should produce a vinylic carbocation intermediate –Secondary vinyl carbocations form less readily than primary alkyl carbocations –Primary vinyl carbocations probably do not form at all Nonethelss, H-Br can add to an alkyne to give a vinyl bromide if the Br is not on a primary carbon

28 Vinylic carbocations are less stable than similarly substituted alkyl carbocations. –Vinyl carbocations have sp-hybridized carbons and thus lack stabilizing hyperconjugative interactions –Stability of carbocations: 3º alkyl > 2º alkyl > 1º alkyl ~ 2º vinyl > + CH 3 ~ 1º vinyl 3º alkyl > 2º alkyl > 1º alkyl ~ 2º vinyl > + CH 3 ~ 1º vinyl

29 Addition of HX and X 2 : Summary excessH-XdihalideAddition of excess H-X to an alkyne gives a dihalide –Markovnikov –Markovnikov Regiochemistry excessX 2 tetrahalideAddition of excess X 2 to an alkyne gives a tetrahalide product.

30 Practice Problem: What products would you expect from the following reactions?

31 B.Hydration of Alkynes H-OHHydration of alkynes is the addition H-OH to an alkyne –Mercury (II) catalyzes Markovnikov oriented addition –Hydroboration-oxidation gives the non-Markovnikov product

32 Mercury(II)-Catalyzed Hydration of Alkynes Mercuric ionMercuric ion (as the sulfate) is a Lewis acid catalyst that promotes addition of water in Markovnikov orientation enol ketone –The immediate product is a vinylic alcohol, or enol, which spontaneously rearranges to a ketone –Alkynes do not react with aqueous protic acids

33 Keto-enol Tautomerism tautomers tautomerismIsomeric compounds that can rapidily interconvert by the movement of a proton are called tautomers and the phenomenon is called tautomerism EnolsketoneEnols rearrange to the isomeric ketone by the rapid transfer of a proton from the hydroxyl to the alkene carbon ketostableThe keto form is usually so stable compared to the enol that only the keto form can be observed

34 Mechanism of Mercury (II) catalyzed Hydration vinylic carbocation intermediateAddition of Hg(II) to alkyne gives a vinylic carbocation intermediate Water adds and loses a proton A proton from aqueous acid replaces Hg(II)

35 Hydration of Unsymmetrical Alkynes Unsymmetrically substituted internal alkyne:Unsymmetrically substituted internal alkyne: –If the alkyl groups at either end of the C-C triple bond are not the same, both products can form and this is not normally useful

36 Hydration of Unsymmetrical Alkynes Terminal alkyne terminal alkyne –If the triple bond is at the first carbon of the chain (then H is what is attached to one side) this is called a terminal alkyne methyl ketone –Hydration of a terminal alkyne always gives the methyl ketone, which is useful

37 Mercury(II)-Catalyzed Hydration : Summary H-OH enol ketone H-OH adds to an alkyne to give an enol which converts to ketone –Markovnikov –Markovnikov regiochemistry –Mercury-containing vinylic carbocation –Mercury-containing vinylic carbocation intermediate

38 Practice Problem: What product would you obtain by hydration of the following alkynes?

39 Practice Problem: What alkynes would you start with to prepare the following ketones?

40 Hydroboration/Oxidation of Alkynes BH 3 vinylic boraneBH 3 (borane) adds to alkynes to give a vinylic borane OxidationH 2 O 2 enol ketonealdehydeOxidation with H 2 O 2 produces an enol that converts to the ketone or aldehyde Non-MarkovnikovProcess converts alkyne to ketone or aldehyde with Non-Markovnikov orientation This is opposite to mercuric ion catalyzed hydration

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42 Comparison of Hydration of Terminal Alkynes Hydroboration/oxidation aldehydesHydroboration/oxidation converts terminal alkynes to aldehydes because addition of water is non-Markovnikov Mercury(II) catalyzed hydration methyl ketonesMercury(II) catalyzed hydration converts terminal alkynes to methyl ketones

43 Hydroboration-oxidation: Summary Borane vinylic borane alkaline H 2 O 2 enol ketonealdehyde Borane adds to an alkyne to give a vinylic borane which is then oxidized by alkaline H 2 O 2 to give an enol which converts to ketone or aldehyde –Non-Markovnikov –Non-Markovnikov regiochemistry

44 Practice Problem: What alkyne would you start with to prepare each of the following compounds by a hydroboration/oxidation reaction?

45 C.Reduction of Alkynes H-HReduction of alkynes is the addition H-H to an alkyne –Complete Catalytic hydrogenation using a metal catalyst (Pd/C) or –Partial Catalytic hydrogenation using Lindlar catalyst –Reduction with Lithium/ammonia

46 H 2 over a metal catalyst alkanesAddition of H 2 over a metal catalyst (such as palladium on carbon, Pd/C) converts alkynes to alkanes (complete reduction) –The addition of the first equivalent of H 2 produces an alkene intermediate, which is more reactive than the alkyne so the alkene is not observed Catalytic Hydrogenation

47 H 2 using the Lindlar catalyst cis alkeneAddition of H 2 using the Lindlar catalyst (chemically deactivated palladium on calcium carbonate) produces a cis alkene syn –The two hydrogens add syn (from the same side of the triple bond) Conversion of Alkynes to cis-Alkenes

48 trans alkenessodium or lithium in liquid ammoniaAlkynes are reduced to trans alkenes with sodium or lithium in liquid ammonia –Anhydrous ammonia (NH 3 ) is liquid below -33 ºC –Alkali metals dissolve in liquid ammonia and function as reducing agents radical anion intermediateThe reaction involves a radical anion intermediate Conversion of Alkynes to trans-Alkenes

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50 Reduction of Alkynes : Summary H 2 over a metal catalyst alkaneAddition of H 2 over a metal catalyst (Pd/C) to an alkyne produces an alkane

51 H 2 using the Lindlar catalyst cis alkeneAddition of H 2 using the Lindlar catalyst produces a cis alkene –syn –syn stereochemistry

52 trans alkenessodium or lithium in liquid ammoniaAlkynes are reduced to trans alkenes with sodium or lithium in liquid ammonia radical anion –a radical anion intermediate –anti –anti stereochemistry

53 Practice Problem: Using any alkyne needed, how would you prepare the following alkenes? a)trans-2-Octene b)cis-3-Heptene c)3-Methyl-1-pentene

54 D.Oxidative Cleavage of Alkynes Strong oxidizing reagentsStrong oxidizing reagents (O 3 or KMnO 4 ) cleave –internal two carboxylic acids –internal alkynes, producing two carboxylic acids –terminala carboxylic acid carbon dioxide –terminal alkynes, producing a carboxylic acid and carbon dioxide

55 Neither process is useful in modern synthesis. –Historically, these were used to elucidate structures because the products indicate the structure of the alkyne precursor

56 Oxidative Cleavage of Alkynes: Summary

57 Practice Problem: Propose structures for alkynes that give the following products on oxidative cleavage by KMnO 4 :

58 E.Alkyne Acidity: Formation of Acetylide Anions strong anhydrous bases acetylide ionReaction of strong anhydrous bases (Na +- NH 2 ) with a terminal alkyne produces an acetylide ion –Terminal alkynes are relatively acidic

59 Terminal alkynes are weak Brønsted acids (pK a ~ 25) –Alkenes and alkanes are much less acidic.

60 Acetylide anions are more stable than either alkyl anions or vinylic anions because: –They have sp-hybridized carbon and their negative charge is in a hybrid orbital with 50% s character, allowing the charge to be closer to the nucleus.

61 Formation of Acetylide Anions: Summary strong anhydrous bases acetylide ionReaction of strong anhydrous bases (Na +- NH 2 ) with a terminal alkyne produces an acetylide ion

62 Practice Problem: The pK a of acetone, CH 3 COCH 3, is Which of the following bases is strong enough to deprotonate acetone? a)KOH (pK a of H 2 O =15.7) b)Na + - CΞCH (pK a of C 2 H 2 =25) c)NaHCO 3 (pK a of H 2 CO 3 = 6.4) d)NaOCH 3 (pK a of CH 3 OH = 15.6)

63 F.Alkylation of Acetylide Anions acetylide anionprimary alkyl halide larger alkyneReaction of an acetylide anion with a primary alkyl halide produces a larger alkyne –Acetylide anionsnucleophiles –Acetylide anions can react as nucleophiles as well as bases –Acetylide anions can displace a halide ion from a 1 o alkyl halide

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66 Limitations of Alkylation of Acetylide Ions efficient with 1º alkyl bromides and alkyl iodides Reactions only are efficient with 1º alkyl bromides and alkyl iodides Acetylide anions can behave as bases as well as nucelophiles Reactions with 2º and 3º alkyl halides gives dehydrohalogenation, converting alkyl halide to alkene

67 Alkylation of Acetylide Anions: Summary acetylide anionprimary alkyl halide larger alkyneReaction of an acetylide anion with a primary alkyl halide produces a larger alkyne

68 Practice Problem: Show the terminal alkyne and alkyl halide from which the following products can be obtained. If two routes look feasible, list both:

69 Practice Problem: How would you prepare cis-2-butene starting from propyne, an alkyl halide, and any other reagents needed? This problem can’t be worked in a single step. You’ll have to carry out more than one reaction.

70 G. An Introduction to Organic Synthesis Organic synthesis may be used to –produce new molecules that are needed as drugs or materials –design, test and improve efficiency and safety for making known molecules –test ideas and methods, answering challenges

71 Synthesis as a Tool for Learning Organic Chemistry In order to propose a synthesis, one must be familiar with reactions: –What they begin with –What they lead to –How they are accomplished –What the limitations are A synthesis combines a series of proposed steps to go from a defined set of reactants to a specified product

72 Strategies for Synthesis 1.Compare the target and the starting material 2.Consider reactions that efficiently produce the outcome. 3.Look at the product and think of what can lead to it

73 Example –Problem –Problem: prepare octane from 1-pentyne –Strategy –Strategy: use acetylide coupling

74 Practice Problem: Beginning with 4-octyne as your only source of carbon and using any inorganic reagents necessary, how would you synthesize the following compounds? a)Butanoic acid b)cis-4-Octene c)4-Bromooctane d)4-Octanol (4-hydroxyoctane) e)4,5-Dichlorooctane

75 Practice Problem: Beginning with acetylene and any alkyl halides needed, how would you synthesize the following compounds? a)Decane b)2,2-Dimethylhexane c)Hexanal d)2-Heptanone

76 Chapter 8

77 Addition of HX and X 2 : Summary

78 Hydration of Alkynes Mercury (II) catalyzes Markovnikov oriented addition Hydroboration-oxidation gives the non-Markovnikov product

79 Reduction of Alkynes : Summary

80 Oxidative Cleavage of Alkynes: Summary

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82 Formation of Acetylide Anions: Summary

83 Alkylation of Acetylide Anions: Summary acetylide anionprimary alkyl halide larger alkyneReaction of an acetylide anion with a primary alkyl halide produces a larger alkyne

84 Alkylation of Acetylide Anions: Summary acetylide anionprimary alkyl halide larger alkyneReaction of an acetylide anion with a primary alkyl halide produces a larger alkyne


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