1. Alkynes C 17

2. Synthesis of Acetylene
Heat coke with lime in an electric to form calcium carbide.
Then drip water on the calcium carbide.
coke
lime
*This reaction was used to produce light for miners’ lamps and for the stage. *

3. The Structure of Alkynes
A triple bond is composed of a s bond and two p bonds

4. Question
Arrange ethane, ethene, and ethyne in order of increasing C-C bond length.
A) ethane < ethene < ethyne
B) ethene < ethane < ethyne
C) ethyne < ethene < ethane
D) ethane < ethyne < ethene

5. Acidity of Acetylene and Terminal AlkynesH C 17

6. Acidity of Hydrocarbons
In general, hydrocarbons are very weak acids
Compound pKa
HF 3.2
H2O 16
NH3 36 45
CH4 60
H2C
CH2 20

7. Acetylene
Acetylene is a weak acid, but not nearly as weak as alkanes or alkenes.
Compound pKa
HF 3.2
H2O 16
NH3 36 45
CH4 60 26 HC CH
H2C
CH2 20

8. Question
Which one of the following is the strongest acid?
A) water
B) ammonia
C) 1-butene
D) 1-butyne

9. Figure Number: UN
Title: Relative Acidities
Caption: Electrostatic potential maps of various acids.
Notes: Increasing amounts of blue color on hydrogen in the stronger acids indicate more positive charge on hydrogen due to increasing electronegativities of atoms attached to hydrogen.

10. Carbon: Hybridization and Electronegativity
10-60 C H
H+ + :
sp3 C H C :
sp2
10-45
H+ + C C
10-26 C H C : sp
H+ +
Electrons in an orbital with more s character are closer to the nucleus and more strongly held. 21

11. Question
Which one of the following statements best explains the greater acidity of terminal alkynes (RCºCH) compared with monosubstituted alkenes (RCH=CH2)?
A) The sp-hybridized carbons of the alkyne are less electronegative than the sp2 carbons of the alkene.
B) The two p bonds of the alkyne are better able to stabilize the negative charge of the anion by resonance.
C) The sp-hybridized carbons of the alkyne are more electronegative than the sp2 carbons of the alkene.
D) The question is incorrect - alkenes are more acidic than alkynes.

12. The stronger the acid, the weaker its conjugate base
top 252

13. Sodium Acetylide
Solution: Use a stronger base. Sodium amide is a stronger base than sodium hydroxide.
NH3
NaNH2 + HC CH
NaC .. .. – –
H2N : + + H C CH
H2N H : C CH
stronger acid pKa = 26
weaker acid pKa = 36
Ammonia is a weaker acid than acetylene. The position of equilibrium lies to the right. 23

14. Question
Which of the following bases is strong enough to completely deprotonate propyne?
A) NH3
B) CH3OH
C) NaNH2
D) NaOH

15. Preparation of Various Alkynes by alkylation reactions with Acetylide or Terminal Alkynes26

16. Synthesis Using Acetylide Ions: Formation of C–C Bond

17. Alkylation of Acetylene and Terminal Alkynes
H—C
C—H
R—C
C—H
R—C
C—R 27

18. Alkylation of Acetylene and Terminal Alkynes– :
H—C
SN2 + X– : R X
C—R
H—C +
The alkylating agent is an alkyl halide, and the reaction is nucleophilic substitution.
The nucleophile is sodium acetylide or the sodium salt of a terminal (monosubstituted) alkyne. 28

19. Example: Alkylation of Acetylene
NaNH2 HC CH HC
CNa
NH3
CH3CH2CH2CH2Br HC C
CH2CH2CH2CH3
(70-77%) 29

20. Question
Which alkyl halide will react faster with the acetylide ion (HCºCNa) in an SN2 reaction?
A) bromopropane
B) 2-bromopropane
C) tert-butyl iodide
D) 1-bromo-2-methylbutane

21. Example: Alkylation of a Terminal AlkyneCH
(CH3)2CHCH2C
NaNH2, NH3
CNa
(CH3)2CHCH2C
CH3Br
(81%)
C—CH3
(CH3)2CHCH2C 30

22. Example: Dialkylation of Acetylene
H—C
C—H
1. NaNH2, NH3
2. CH3CH2Br
C—H
CH3CH2—C
1. NaNH2, NH3
2. CH3Br
C—CH3
CH3CH2—C
(81%) 31

23. Effective only with primary alkyl halides
Limitation
Effective only with primary alkyl halides
Secondary and tertiary alkyl halides undergo elimination 32

24. What is the product of the following reaction?
Question
What is the product of the following reaction?

25. What is the product of the following reaction?
Answer
What is the product of the following reaction?
B) SEE: Skillbuilder 10.5.

26. E2 predominates over SN2 when alkyl halide is secondary or tertiary.
Acetylide Ion as a Base
E2 predominates over SN2 when alkyl halide is secondary or tertiary. H C X C – :
H—C E2 + C
H—C —H X– : 33

27. Question
Consider the reaction of each of the following with cyclohexyl bromide. For which one is
the ratio of substitution to elimination highest?
A) NaOCH2CH3, ethanol, 60°C
B) NaSCH2CH3, ethanol-water, 25°C
C) NaNH2, NH3, -33°C
D) NaCºCH, NH3, -33°C

28. Preparation of Alkynes by Elimination Reactions34

29. Preparation of Alkynes by "Double Dehydrohalogenation"X C H X C H
Geminal dihalide
Vicinal dihalide
The most frequent applications are in preparation of terminal alkynes. 35

30. Geminal dihalide  Alkyne
(CH3)3CCH2—CHCl2
1. 3NaNH2, NH3
2. H2O
(56-60%)
(CH3)3CC CH 36

31. Geminal dihalide  Alkyne
(CH3)3CCH2—CHCl2
NaNH2, NH3
(slow)
(CH3)3CCH
CHCl
NaNH2, NH3
(CH3)3CC CH
(slow)
H2O
NaNH2, NH3
(CH3)3CC
CNa
(fast) 37

32. Question
In addition to NaNH2, what other base can be used to convert 1,1-dichlorobutane into
1-butyne?
A) NaOCH3
B) NaOH
C) NaOCH2CH3
D) KOC(CH3)3

33. Vicinal dihalide  Alkyne
CH3(CH2)7CH—CH2Br Br
1. 3NaNH2, NH3
2. H2O
(54%)
CH3(CH2)7C CH 38

34. Question
Which of the following compounds yield 1-heptyne on being treated with three moles of sodium amide (in liquid ammonia as the solvent) followed by adding water to the reaction mixture?
A) 1,1,2,2-tetrachloroheptane
B) 1-bromo-2-chloroheptane
C) 1,1,2-trichloropentane
D) all of the above

35. Reactions of Alkynes 1

36. Reactions of Alkynes Acidity Hydrogenation Metal-Ammonia Reduction
Addition of Hydrogen Halides
Hydration
Addition of Halogens
Ozonolysis 2

37. Hydrogenation of Alkynes3

38. Atomic Force Microscopy of Acetylene Lawrence Berkeley Laboratory (LBL)H

39. Imaging: acetylene on Pd(111) at 28 KH
Molecular Image
Tip cruising altitude ~700 pm
Δz = 20 pm
Why don’t we see the Pd atoms?
Because the tip needs to be very close to image the Pd atoms and would knock the molecule away
p orbital pz
TIP H O +
Surface atomic profile
Tip cruising altitude ~500 pm
Δz = 2 pm
Calculated image
(Philippe Sautet)
If the tip was made as big as an airplane, it would be flying at 1 cm from the surface and waving up an down by 1 micrometer
The STM image is a map of the pi-orbital of distorted acetylene
1 cm
(± 1 μm)
M. Salmeron (LBL)

40. Excitation of frustrated rotational modes in acetylene molecules on Pd(111) at T = 30 K
Tip e-
((( ) ( )))
M. Salmeron (LBL)

41. Hydrogenation of Alkynes
cat RC
CR' +
2H2
RCH2CH2R'
catalyst = Pt, Pd, Ni, or Rh
alkene is an intermediate 4

42. Partial HydrogenationRC
CR'
cat H2
RCH
CHR'
cat H2
RCH2CH2R'
Alkenes could be used to prepare alkenes if a catalyst were available that is active enough to catalyze the hydrogenation of alkynes, but not active enough for the hydrogenation of alkenes. 8

43. Lindlar Palladium RC CR' cat H2 RCH CHR' cat H2 RCH2CH2R'
There is a catalyst that will catalyze the hydrogenation of alkynes to alkenes, but not that of alkenes to alkanes.
It is called the Lindlar catalyst and consists of palladium supported on CaCO3, which has been poisoned with lead acetate and quinoline.
syn-Hydrogenation occurs; cis alkenes are formed. 9

44. Example CH3(CH2)3C C(CH2)3CH3 + H2 Lindlar Pd CH3(CH2)3 (CH2)3CH3 C H
(87%) 10

45. Figure: UN
Caption:

46. Metal-Ammonia Reduction of Alkynes
Alkynes  trans-Alkenes 11

47. Partial Reduction RC CR' RCH CHR' RCH2CH2R'
Another way to convert alkynes to alkenes is by reduction with sodium (or lithium or potassium) in ammonia.
trans-Alkenes are formed. 8

48. Figure: UN
Caption:
To form a trans alkene, two hydrogens must be added to the alkyne with anti stereochemistry, so this reduction is used to convert alkynes to trans alkenes.

49. Example
CH3CH2C
CCH2CH3
Na, NH3
CH3CH2 H C
CH2CH3 H
(82%) 10

50. Figure: UN
Caption:
To form a trans alkene, two hydrogens must be added to the alkyne with anti stereochemistry, so this reduction is used to convert alkynes to trans alkenes.

51. Question
How would you accomplish the following conversion?
A) NaNH2
B) H2, Lindlar Pd
C) Na, NH3
D) either B or C

52. Mechanism
Metal (Li, Na, K) is reducing agent; H2 is not involved; proton comes from NH3
four steps
(1) electron transfer
(2) proton transfer
(3) electron transfer
(4) proton transfer 13

53. Question
Select the most effective way to synthesize cis-2-pentene from 1-propyne.
A) 1) NaNH2 2) CH3CH2Br 3) H2, Pd
B) 1) NaNH2 2) CH3Br 3) H2, Lindlar Pd
C) 1) NaNH2 2) CH3CH2I 3) H2, Lindlar Pd
D) 1) NaNH2 2) CH3CH2Br 3) Na,NH3

54. Question
Which reagent would accomplish the transformation of 3-hexyne into trans-3-hexene?
A) H2/Ni
B) H2, Lindlar Pd
C) Na, NH3
D) NaNH2, NH3

55. Problem
Suggest an efficient syntheses of (E)- and (Z)-2- heptene from propyne and any necessary organic or inorganic reagents. 18

56. Problem Strategy 19

57. Problem Strategy 19

58. Problem Synthesis
1. NaNH2
2. CH3CH2CH2CH2Br
H2, Lindlar Pd
Na, NH3 19

59. Question
Which would be the best sequence of reactions to use in order to prepare cis-3-nonene
from 1-butyne?
A) 1. NaNH2 in NH3; 2. 1-bromopentane; 3. H2, Lindlar Pd
B) 1. NaNH2 in NH3; 2. 1-bromopentane; 3. Na, NH3
C) 1. H2, Lindlar Pd; 2. NaNH2 in NH3; bromopentane
D) 1. Na, NH3; 2. NaNH2 in NH3; bromopentane

60. Addition of Hydrogen Halides to Alkynes21

61. Follows Markovnikov's Rule
HBr
CH3(CH2)3C CH
CH3(CH2)3C
CH2 Br
(60%)
Alkynes are slightly less reactive than alkenes 22

62. Two Molar Equivalents of Hydrogen Halide
CH3CH2C
CCH2CH3
2 HF F C H
CH3CH2
CH2CH3
(76%) 24

63. Free-radical Addition of HBr
CH3(CH2)3C CH
CH3(CH2)3CH
CHBr
peroxides
(79%)
regioselectivity opposite to Markovnikov's rule 25

65. Hydration of Alkynes expected reaction: H+ RC CR' H2O + OH RCH CR'
enol
observed reaction: H+ + RC
CR'
H2O
RCH2CR' O
ketone 27

66. enols are regioisomers of ketones, and exist in equilibrium with them OH
RCH
CR'
RCH2CR' O
enol
ketone
enols are regioisomers of ketones, and exist in equilibrium with them
keto-enol equilibration is rapid in acidic media
ketones are more stable than enols and predominate at equilibrium 28

67. Figure: UN
Caption:
To form a trans alkene, two hydrogens must be added to the alkyne with anti stereochemistry, so this reduction is used to convert alkynes to trans alkenes.

68. Mechanism of conversion of enol to ketone.. : O H C H + : O H H 30

69. Mechanism of conversion of enol to ketone+ : .. 30

70. Mechanism of conversion of enol to ketone.. : O H H H C C + : O : H 30

71. Mechanism of conversion of enol to ketone.. O : : O H H H C C + 30

72. Key Carbocation Intermediate
Carbocation is stabilized by electron delocalization (resonance). .. O C H .. + : O H H C C + 30

73. Mechanism of conversion of enol to ketone: + .. 30

74. Useful for symmetrical starting alkynes
to produce a single product.
Unsymmetrical starting alkynes that are not terminal produce a mixture of ketones…non-regioselectively.

75. Markovnikov's rule followed in formation of enol,
Regioselectivity
Markovnikov's rule followed in formation of enol,
Useful with terminal alkynes. O
H2O, H2SO4
CH3(CH2)5C CH
CH3(CH2)5CCH3
HgSO4
(91%)
via
CH3(CH2)5C
CH2 OH 32

76. Aldehyde vs. Ketone

77. Question
What is the product of the acid catalyzed hydration of 1-hexyne?
A) B)
C) D)

78. I) Which reactions give ketones? II) Which reactions give aldehydes?
Question
I) Which reactions give ketones? II) Which reactions give aldehydes?
A. I = C, D, E, F; II = A, B
B. I = B, D, E, F; II = A, C
C. I = B, C, D, F; II = A, E
D. I = A, D, E, F; II = B, C
E. I = A, C, D, F; II = B, E

79. Addition of Halogens to Alkynes26

80. Example Cl
(63%) C
Cl2CH
CH3 HC
CCH3
+ 2 Cl2 36

81. Addition is anti
CH3CH2 Br
Br2 C
CH3CH2C
CCH2CH3 Br
CH2CH3
(90%) 37

82. NBS = N-bromosuccinimide
NBS Example
AgNO3 HC
CCH3
+ NBS
Br-C
CCH3 O
NBr
NBS = N-bromosuccinimide 36

83. gives two carboxylic acids by cleavage of triple bond
Ozonolysis of Alkynes
gives two carboxylic acids by cleavage of triple bond 38

84. Example
CH3(CH2)3C CH
1. O3
2. H2O +
CH3(CH2)3COH
(51%) O
HOCOH 39

85. Question
What product is formed when 2-butyne is subjected to ozonolysis?
A) B)
C) D)

86. Answer
What product is formed when 2-butyne is subjected to ozonolysis?
A) B)
C) D)

87. Alkynes Synthesis & Functions26

88. Can you identify and name the function?
Figure: UN
Caption:
To form a trans alkene, two hydrogens must be added to the alkyne with anti stereochemistry, so this reduction is used to convert alkynes to trans alkenes.

93. Example

96. Question
What is the structure of Compound Y in the following synthetic sequence?
A) pentane
B) cis-2-pentene
C) trans-2-pentene
D) 2-pentyne

97. Answer
What is the structure of Compound Y in the following synthetic sequence?
A) pentane
B) cis-2-pentene
C) trans-2-pentene
D) 2-pentyne

98. Natural Products 26

99. Polyynes

100. Compound 1 is isolated from the root and bark of mistletoe, Paramacrolobium caeruleum (Loranthaceae). The stems and leaves of members of this family have been used for the treatment of cancer in Indonesia.

101. Compound 2, thiarubrine B, has been isolated from Giant Ragweed, Ambrosia trifida. Native cultures in Canada and Africa use plants with similar natural products to treat skin infections and intestinal parasites.

102. Compound 3, oplopandiolacetate, is found in the bark and roots of Devil's club, Oplopanax horridus. It is used medicinally by native Americans to treat a variety of ailments. Other polyynes found in plants include oenanthotoxin, cicutoxin, and falcarinol .

103. Compound 4, dihydromatricaria acid, is found in the soldier beetle, Cantharidae, who are related to the Lampyridae or firefly family, but unable to produce light. They provide biological control of a number of insect pests including grasshoppers, aphids, caterpillars and other soft bodied insects.

104. Histrionicotoxin is isolated from the skin of frogs in the Dendrobatidae family. It comes from insects in their diet and is used by indigenous South American tribes as a poison on arrows.

105. Polyyne Cu (I) Coupling Reactions
AgNO3 HC
CCH3
+ NBS
Br-C
CCH3 36