1. HOAÙ HOÏC HÖÕU CÔ Organic Chemistry CHÖÔNG 7 (t.t) HYDROCARBON CHÖA NO MAÏCH HỞ ALKYNES

2. Alkynes Alkynes have C-C triple bond. Alkynes have C-C triple bond. They are more unsaturated than alkenes. They are more unsaturated than alkenes. The general formula is C n H 2n-2. The general formula is C n H 2n-2. The first member of alkyne family is C 2 H 2. The first member of alkyne family is C 2 H 2. IUPAC name : ethyne IUPAC name : ethyne Common name : acetylene Common name : acetylene H-C C-H H-C C-H

3. Characteristics of alkynes The family name ends in –yne in the IUPAC nomenclature system. The family name ends in –yne in the IUPAC nomenclature system. Geometric isomerism is not possible for alkynes. Geometric isomerism is not possible for alkynes. Triple bond is due to the formation of sp- hybridized C. Triple bond is due to the formation of sp- hybridized C. The triple bond is composed of one strong sigma bond and two weak pi bonds. The triple bond is composed of one strong sigma bond and two weak pi bonds. Chemical reactions are same as alkenes but requires double the reagent as two pi bonds are present in alkynes. Chemical reactions are same as alkenes but requires double the reagent as two pi bonds are present in alkynes. Acetylene is an important industrial product. Acetylene torch used in welding to melt and vaporize steel and iron. Acetylene is an important industrial product. Acetylene torch used in welding to melt and vaporize steel and iron.

4. ALKYNES

7. I. Nomenclature of Alkynes -- “yne” Step 1: Name longest continuous chain containing the triple bond; parent is “yne” Step 1: Name longest continuous chain containing the triple bond; parent is “yne” Step 2: Number to give the carbon- carbon triple bond the lowest number. Step 2: Number to give the carbon- carbon triple bond the lowest number.

8. I. Nomenclature of Alkynes -- “yne” Step 3: If there is more than one triple bond, indicate by numbers where they are and use prefixes: Step 3: If there is more than one triple bond, indicate by numbers where they are and use prefixes:

9. Nomenclature of Compounds containing both double and triple carbon-carbon bonds: “enyne” Rule 1: Number from the end to give the multiple bond (double or triple) the lowest number and give each multiple bond a number. Rule 1: Number from the end to give the multiple bond (double or triple) the lowest number and give each multiple bond a number. Rule 2: The parent name is Rule 2: The parent name is #-alken-#-yne #-alken-#-yne

10. Nomenclature of Compounds containing both double and triple carbon-carbon bonds: “enyne” Rule 3: If number is the same the double and the triple bond, give the double bond the lower number. Rule 3: If number is the same the double and the triple bond, give the double bond the lower number.

11. Nomenclature of Alkynes: Substituents and Cycloalkynes Substituents: (ethynyl group) Substituents: (ethynyl group) ethynylcyclooctaneethynylcyclooctane Cycloalkyne Cycloalkyne cyclooctynecyclooctyne

13. Nomenclature IUPAC: use the infix -yn- to show the presence of a carbon-carbon triple bond IUPAC: use the infix -yn- to show the presence of a carbon-carbon triple bond Common names: prefix the substituents on the triple bond to the word “acetylene” Common names: prefix the substituents on the triple bond to the word “acetylene” Common name: IUPAC name: Dimethylacetylene Vinylacetylene 2-Butyne1-Buten-3-yne

14. Cycloalkynes Cyclononyne is the smallest cycloalkyne isolated Cyclononyne is the smallest cycloalkyne isolated it is quite unstable and polymerizes at room tempit is quite unstable and polymerizes at room temp the C-C-C bond angle about the triple bond is approximately 155°, indicating high angle strainthe C-C-C bond angle about the triple bond is approximately 155°, indicating high angle strain

15. Classification of Alkynes Terminal alkyne: monosubstituted alkyne: has triple bond at the end of the chain. Terminal alkyne: monosubstituted alkyne: has triple bond at the end of the chain. Internal alkyne: disubstituted alkyne: has triple bond inside chain. Internal alkyne: disubstituted alkyne: has triple bond inside chain.

16. II. Structure Bonding: Bonding: geometry is lineargeometry is linear all atoms are in one lineall atoms are in one line hybridization is sphybridization is sp bond angles are 180 obond angles are 180 o triple bond is one sigma () bond and two pi () bondstriple bond is one sigma () bond and two pi () bonds

17. II. Structure Bond lengths: Bond lengths: H-C bond is 106 pmH-C bond is 106 pm C-C bond is 120 pmC-C bond is 120 pm both bond lengths are shorter than in alkene and alkaneboth bond lengths are shorter than in alkene and alkane percent s character is greater in alkynes (50%) vs 33% in alkenes vs 25% in alkanes:percent s character is greater in alkynes (50%) vs 33% in alkenes vs 25% in alkanes: electrons held closer to nucleus; results in shorter bond lengthselectrons held closer to nucleus; results in shorter bond lengths

18. ALKYNES

19. Alkynes stretch: weak absorption at 2260-2100 cm –1 - not observed for symmetrical alkynes (v. weak for ‘pseudo’ symmetric alkynes - terminal alkynes (R-C C-H) absorptions are stronger than internal (R-C C-R) absorptions C C–H stretch: - 3333–3267 cm –1 - strong, narrow (as compared to OH or NH) C C–H bend: - 700-610 cm –1 : broad, strong absorption - 1400-1220 cm –1, overtone of above

20. Terminal Alkynes Alkyne C-H stretch 3310 cm –1 Alkyne CC stretch 2119 cm –1 Alkyne C-H bend 630 cm –1 Alkyne C-H bend overtone 1260 cm –1

21. Alkyne Synthesis

24. Alkylation of Alkyne Anions Alkyne anions are both strong bases and good nucleophiles Alkyne anions are both strong bases and good nucleophiles They participate in nucleophilic substitution reactions with alkyl halides to form new C-C bonds to alkyl groups; they undergo alkylation They participate in nucleophilic substitution reactions with alkyl halides to form new C-C bonds to alkyl groups; they undergo alkylation because alkyne anions are also strong bases, alkylation is practical only with methyl and 1° halidesbecause alkyne anions are also strong bases, alkylation is practical only with methyl and 1° halides with 2° and 3° halides, elimination is the major reactionwith 2° and 3° halides, elimination is the major reaction

25. Alkylation of Alkyne Anions alkylation of alkyne anions is the most convenient method for the synthesis of terminal alkynesalkylation of alkyne anions is the most convenient method for the synthesis of terminal alkynes alkylation can be repeated and a terminal alkyne can be converted to an internal alkynealkylation can be repeated and a terminal alkyne can be converted to an internal alkyne

26. Preparation of Alkynes By double dehydrohalogenation of either By double dehydrohalogenation of either geminal dihalide (halides on same carbon) orgeminal dihalide (halides on same carbon) or vicinal dihalide (halides on adjacent carbons)vicinal dihalide (halides on adjacent carbons)

27. Alkyne Synthesis Double Elimination Double Elimination

28. Preparation of Alkynes: Double Elimination Vicinal dihalides (Cl or Br on adjacent carbons) Vicinal dihalides (Cl or Br on adjacent carbons) Requires two moles of very very strong base such as NaNH 2 in NH 3 (NH 2 - ) Requires two moles of very very strong base such as NaNH 2 in NH 3 (NH 2 - ) Triple bond forms between the carbons that had the halogen Triple bond forms between the carbons that had the halogen

29. Preparation of Alkynes Mechanism Mechanism Double dehydrohalogenationDouble dehydrohalogenation Preparation: Preparation: alkene-->dihalide-->alkynealkene-->dihalide-->alkyne

30. Alkyne Synthesis

32. Alkene to Alkyne Bromination and two consecutive dehydrohalogenation reactions Bromination and two consecutive dehydrohalogenation reactions

33. Preparation from Alkenes a side product may be an allene, a compound containing adjacent carbon-carbon double bonds, C=C=Ca side product may be an allene, a compound containing adjacent carbon-carbon double bonds, C=C=C A haloalkene (a vinylic halide) An allene An alkyne RCCCR R H R H X H NaNH 2 RC–C=CR-HBr CCC R RR H

34. Preparation from Alkenes for a terminal alkene to a terminal alkyne, 3 moles of base are requiredfor a terminal alkene to a terminal alkyne, 3 moles of base are required

35. Physical Properties Similar to alkanes and alkenes of comparable molecular weight and carbon skeleton Similar to alkanes and alkenes of comparable molecular weight and carbon skeleton

36. Acidity of Alkynes

38. ALKYNES

46. A Look to Why Alkynes are Less Reactive Than Alkenes

49. Predicting direction of equilibrium: Equilibrium lies toward weaker species Equilibrium lies toward weaker species

50. Reduction of Alkynes Hydrogenation reaction Hydrogenation reaction Heterogeneous catalystsHeterogeneous catalysts Complete reductionComplete reduction Partial reductionPartial reduction Lindlar’s catalyst – syn additionLindlar’s catalyst – syn addition

51. Alkyne Reductions Alkyne Reductions

52. Lindlar is a special catalyst that allows the hydrogenation of an alkyne to stop after one mole of hydrogen is added. quinoline syn addition Most amines, and compounds containing sulfur, reduce the activity of catalysts or “poison” them. Lindlar Catalyst: syn addition

53. Reduction of Alkynes

54. Dissolving metal reduction Dissolving metal reduction anti additionanti addition Mechanism Mechanism

55. Dissolving Metal Reduction anti This reaction proceeds with anti addition (trans compound). Catalytic reduction proceeds with syn addition, hence we have a choice of methods.

56. MECHANISM OF Na-LIQUID-NH 3 REDUCTIONS ( from (NH 3 ) n ). - anti addition electron transfer 1 electron transfer 2 +2e +2H + :NH 2 - All intermediates prefer the trans geometry. radical- anion radical anion

57. SODIUM IN LIQUID AMMONIA SODIUM AMIDE IN LIQUID AMMONIA TWO DIFFERENT REAGENTS ! Reducing Agent Strong Base = = NaNH 2 / NH 3 (l) Na / NH 3 (l)

58. Addition of Hydrogen Addition of Hydrogen

60. Addition of Hydrogen Halides and Halogens

61. Addition of hydrogen halide to alkynes

63. Addition of HX Alkynes undergo regioselective addition of either 1 or 2 moles of HX, depending on the ratios in which the alkyne and halogen acid are mixed Alkynes undergo regioselective addition of either 1 or 2 moles of HX, depending on the ratios in which the alkyne and halogen acid are mixed 2,2-Dibromopropane2-BromopropenePropyne CH 3 CCHBrBrBrHBrCH 3 C=CH 2 CH 3 CCH 3 HBr

64. Addition of HX the intermediate in addition of HX is a 2° vinylic carbocationthe intermediate in addition of HX is a 2° vinylic carbocation reaction of the vinylic cation (an electrophile) with halide ion (a nucleophile) gives the productreaction of the vinylic cation (an electrophile) with halide ion (a nucleophile) gives the product

65. Addition of HX in the addition of the second mole of HX, Step 1 is reaction of the electron pair of the remaining pi bond with HBr to form a carbocationin the addition of the second mole of HX, Step 1 is reaction of the electron pair of the remaining pi bond with HBr to form a carbocation of the two possible carbocations, the favored one is the resonance-stabilized 2° carbocationof the two possible carbocations, the favored one is the resonance-stabilized 2° carbocation

68. Hydration of alkynes

71. Addition of Water

72. Something to Watch For !

73. Addition of H 2 O: hydration In the presence of sulfuric acid and Hg(II) salts, alkynes undergo addition of water In the presence of sulfuric acid and Hg(II) salts, alkynes undergo addition of water + Propanone (Acetone) 1-Propen-2-ol (an enol) Propyne CH 3 CCHOH O HgSO 4 H 2 SO 4 H 2 OCH 3 C=CH 2 CH 3 CCH 3

74. Addition of X 2 Alkynes add one mole of bromine to give a dibromoalkene Alkynes add one mole of bromine to give a dibromoalkene addition shows anti stereoselectivityaddition shows anti stereoselectivity

75. Addition halogen to alkynes

76. Addition of X 2 the intermediate in bromination of an alkyne is a bridged bromonium ionthe intermediate in bromination of an alkyne is a bridged bromonium ion

77. Ozonlysis of alkynes

78. Addition of Borane: Addition of Borane:Hydroboration-Oxidation

81. Good For Terminal Alkyne

82. Hydroboration Addition of borane to an internal alkyne gives a trialkenylborane Addition of borane to an internal alkyne gives a trialkenylborane addition is syn stereoselectiveaddition is syn stereoselective

83. Hydroboration to prevent dihydroboration with terminal alkynes, it is necessary to use a sterically hindered dialkylborane, such as (sia) 2 BHto prevent dihydroboration with terminal alkynes, it is necessary to use a sterically hindered dialkylborane, such as (sia) 2 BH treatment of a terminal alkyne with (sia) 2 BH results in stereoselective and regioselective hydroborationtreatment of a terminal alkyne with (sia) 2 BH results in stereoselective and regioselective hydroboration

84. Hydroboration Treating an alkenylborane with H 2 O 2 in aqueous NaOH gives an enol Treating an alkenylborane with H 2 O 2 in aqueous NaOH gives an enol enol: a compound containing an OH group on one carbon of a carbon-carbon double bondenol: a compound containing an OH group on one carbon of a carbon-carbon double bond an enol is in equilibrium with a keto form by migration of a hydrogen from oxygen to carbon and the double bond from C=C to C=Oan enol is in equilibrium with a keto form by migration of a hydrogen from oxygen to carbon and the double bond from C=C to C=O keto forms generally predominate at equilibriumketo forms generally predominate at equilibrium keto and enol forms are tautomers and their interconversion is called tautomerismketo and enol forms are tautomers and their interconversion is called tautomerism

85. Hydroboration hydroboration/oxidation of an internal alkyne gives a ketonehydroboration/oxidation of an internal alkyne gives a ketone hydroboration/oxidation of a terminal alkyne gives an aldehydehydroboration/oxidation of a terminal alkyne gives an aldehyde 3-Hexanone3-Hexyne 1. BH 3 2. H 2 O 2, NaOH O

87. ALKYNES

88. Diels-Alder Reaction Six-membered Ring formation (4+2) Six-membered Ring formation (4+2) Diene + Dienophile Diene + Dienophile

89. Cleavage Reactions of Alkynes Ozonolysis Ozonolysis Hot Potassium Permanganate Hot Potassium Permanganate

99. ALKYNES

100. Organic Synthesis A successful synthesis must A successful synthesis must provide the desired product in maximum yieldprovide the desired product in maximum yield have the maximum control of stereochemistry and regiochemistryhave the maximum control of stereochemistry and regiochemistry do minimum damage to the environment (it must be a “green” synthesis)do minimum damage to the environment (it must be a “green” synthesis) Our strategy will be to work backwards from the target molecule Our strategy will be to work backwards from the target molecule

101. Organic Synthesis We analyze a target molecule in the following ways We analyze a target molecule in the following ways the carbon skeleton: how can we put it together. Our only method to date for forming new a C-C bond is the alkylation of alkyne anions (Section 7.5)the carbon skeleton: how can we put it together. Our only method to date for forming new a C-C bond is the alkylation of alkyne anions (Section 7.5) the functional groups: what are they, how can they be used in forming the carbon-skeleton of the target molecule, and how can they be changed to give the functional groups of the target moleculethe functional groups: what are they, how can they be used in forming the carbon-skeleton of the target molecule, and how can they be changed to give the functional groups of the target molecule

102. Organic Synthesis We use a method called a retrosynthesis and use an open arrow to symbolize a step in a retrosynthesis We use a method called a retrosynthesis and use an open arrow to symbolize a step in a retrosynthesis Retrosynthesis: a process of reasoning backwards from a target molecule to a set of suitable starting materials Retrosynthesis: a process of reasoning backwards from a target molecule to a set of suitable starting materials target molecule starting materials

103. Organic Synthesis Target molecule: cis-3-hexene Target molecule: cis-3-hexene

104. Organic Synthesis starting materials are acetylene and bromoethanestarting materials are acetylene and bromoethane

105. Organic Synthesis Target molecule: 2-heptanone Target molecule: 2-heptanone

106. Organic Synthesis starting materials are acetylene and 1- bromopentanestarting materials are acetylene and 1- bromopentane

107. Alkynes

108. ALKYNES

109. Some biologically important alkynes: Interesting properties: Interesting properties: antibioticsantibiotics birth control agentbirth control agent

110. THANKYOU FOR YOUR KIND ATTENTION HAVE A NICE DAY