1. Chapter 9 Alkynes
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2. 9.1 Sources of Alkynes 1

3. Industrial preparation of acetylene is by dehydrogenation of ethylene.+
CH3CH3
H2C
CH2
1150°C H2 +
H2C
CH2 HC CH
Cost of energy makes acetylene a more expensive industrial chemical than ethylene. 2

4. Naturally Occurring Alkynes
Some alkynes occur naturally. For example,
C(CH2)4COH
CH3(CH2)10C O
Tariric acid: occurs in seed of a Guatemalan plant.
Histrionicotoxin: defensive toxin in poison dart frogs of Central and South America 2

5. 9.2 Nomenclature 3

6. Acetylene and ethyne are both acceptable IUPAC names for HC CH
Nomenclature
Acetylene and ethyne are both acceptable IUPAC names for HC CH
Higher alkynes are named in much the same way as alkenes except using an -yne suffix instead of -ene. HC
CCH3
Propyne HC
CCH2CH3
1-Butyne or But-1-yne
(CH3)3CC
CCH3
4,4-Dimethyl-2-pentyne or 4,4-Dimethyl-pent-2-yne 4

7. 9.3 Physical Properties of Alkynes
The physical properties of alkynes are similar to those of alkanes and alkenes. 5

8. 9.4 Structure and Bonding in Alkynes: sp Hybridization5

9. Linear geometry for acetylene
Structure
Linear geometry for acetylene C H
120 pm
106 pm
CH3
121 pm
146 pm 6

10. Cyclooctyne polymerizes on standing.
Cycloalkynes
Cyclononyne is the smallest cycloalkyne stable enough to be stored at room temperature for a reasonable length of time.
Cyclooctyne polymerizes on standing. C 14

11. sp Hybridization in Acetylene
Mix together (hybridize) the 2s orbital and one of the three 2p orbitals.
Atomic orbitals
Hybridized 2p 2p
2sp
Each carbon has two half-filled sp orbitals available to form  bonds. 2s 8

12.  Bonds in Acetylene
Each carbon is connected to a hydrogen by a  bond. The two carbons are connected to each other by a  bond and two  bonds.
Figure 9.2 (a) 9

13.  Bonds in Acetylene
One of the two  bonds in acetylene is shown here. The second  bond is at right angles to the first.
Figure 9.2 (b) 9

14. This is the second of the two  bonds in acetylene.
Figure 9.2 (c) 9

15. Figure 9.3 Electrostatic Potential in Acetylene
The region of highest negative charge encircles the molecule around its center in acetylene.
The region of highest negative charge lies above and below the molecular plane in ethylene. 18

16. Table 9.1 Structural Features of Ethane, Ethylene, and Acetylene
Ethane Ethylene Acetylene
C—C distance
153 pm
134 pm
120 pm
C—H distance
111 pm
110 pm
106 pm
H—C—C angles
111.0°
121.4°
180°
C—C BDE
368 kJ/mol
611 kJ/mol
820 kJ/mol
C—H BDE
410 kJ/mol
452 kJ/mol
536 kJ/mol
hybridization of C
sp3
sp2 sp
% s character
25%
33%
50%
pKa 62 45 26 18

17. 9.5 Acidity of Acetylene and Terminal Alkynes:H C 5

18. Acidity of Hydrocarbons
In general, hydrocarbons are exceedingly weak acids,
but alkynes are not nearly as weak as alkanes or alkenes.
Compound pKa 26 45
CH4 60 HC CH
H2C
CH2 20

19. Carbon: Hybridization and Electronegativity
pKa = 62
sp3 : –
H+ +
sp2 H C : –
pKa = 45
H+ + sp C H : –
pKa = 26
Electrons in an orbital with more s character are closer to the nucleus and more strongly held. 21

20. Sodium Acetylide Objective:
Prepare a solution containing sodium acetylide Will treatment of acetylene with NaOH be effective?
NaC CH
H2O
NaOH + HC CH
NaC NO 22

21. Hydroxide is not a strong enough base to deprotonate acetylene.
Sodium Acetylide
Hydroxide is not a strong enough base to deprotonate acetylene. HO .. : H C CH – +
weaker acid pKa = 26
stronger acid pKa = 15.7
In acid-base reactions, the equilibrium lies to the side of the weaker acid. 23

22. 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 +
stronger acid pKa = 26
weaker acid pKa = 36
Ammonia is a weaker acid than acetylene. The position of equilibrium lies to the right. 23

23. 9.6 Preparation of Alkynes by Alkylation of Acetylene and Terminal Alkynes5

24. Preparation of Alkynes
There are two main methods for the preparation of alkynes:
Carbon-carbon bond formation alkylation of acetylene and terminal alkynes
Functional-group transformations elimination 25

25. Alkylation of Acetylene and Terminal Alkynes
H—C
C—H
Remove 1st H with NaNH2,
then alkylate
with RX.
R—C
C—H
R—C
C—R
Remove 2nd H with NaNH2,
then alkylate
with RX. 27

26. Alkylation of Acetylene and Terminal Alkynes
SN2 C – :
H—C + R X
C—R
H—C + X– :
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 (these are strong bases).
Effective only with methyl and 1o alkyl halides, 2o and 3o alkyl halides undergo elimination. 28

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

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

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

30. Limitation on Alkylation of Acetylides
Alkylation of an acetylide is effective only with methyl or primary alkyl halides
Secondary and tertiary alkyl halides undergo elimination. 32

31. 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

32. 9.7 Preparation of Alkynes by Elimination Reactions
Elimination of vicinal and geminal dihalides yield alkynes. 5

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

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

35. Geminal dihalide  Alkyne
(CH3)3CCH2—CHCl2
NaNH2, NH3
(slow)
(Dehydrohalogenation)
(CH3)3CCH
CHCl
NaNH2, NH3
(CH3)3CC CH
(slow)
(Dehydrohalogenation)
H2O
NaNH2, NH3
(CH3)3CC
CNa
(fast)
(Loss of a proton)
(Regain
proton) 37

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

37. 9.8 Reactions of Alkynes 5

38. Hydrogenation (Section 9.9) Metal-Ammonia Reduction (Section 9.10)
Reactions of Alkynes
Acidity (Section 9.5)
Hydrogenation (Section 9.9)
Metal-Ammonia Reduction (Section 9.10)
Addition of Hydrogen Halides (Section 9.11)
Hydration (Section 9.12)
Hydroboration-oxidation (not in text)
Addition of Halogens (Section 9.13)
Ozonolysis (Section 9.14) 2

39. 9.9 Hydrogenation of Alkynes5

40. Hydrogenation of Alkynes
cat RC
CR' +
2H2
RCH2CH2R'
catalyst = Pt, Pd, Ni, or Rh
Twice the H2 is needed here compared to reaction with an alkene since there are two pi bonds in the alkyne. An alkene is an intermediate. 4

41. Heats of Hydrogenation
CH3CH2C CH
CH3C
CCH3
292 kJ/mol
275 kJ/mol
Alkyl groups stabilize triple bonds in the same way that they stabilize double bonds. Internal triple bonds are more stable than terminal ones. 6

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

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

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

45. 9.10 Metal-Ammonia Reduction of Alkynes
Alkynes  trans-Alkenes 5

46. 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. This reaction goes by a multistep mechanism.
In this reaction, trans-alkenes are formed. 8

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

48. Mechanism
The metal (Li, Na, K) is the reducing agent; H2 is not involved in this reaction.
Four steps:
(1) electron transfer
(2) proton transfer
(3) electron transfer
(4) proton transfer 13

49. Mechanism M . + R R' C .. – M+ . R' R C H – . R C C R' .. .. NH2 .. –
Step (1): Transfer of an electron from a metal atom to the alkyne to give an anion radical. M . + R R' C .. – M+
Step (2): Transfer of a proton from the solvent (liquid ammonia) to the anion radical. . R' R C H – . R C C R' .. ..
NH2 .. – : H
NH2 15

50. Mechanism M+ R' R C H .. – R . . C C R' + M H
Step (3): Transfer of an electron from a 2nd metal atom to the alkenyl radical to give a carbanion. M+ R' R C H .. – R . . C C R' + M H 16

51. Mechanism .. H NH2 R' H C R NH2 .. – : R' R C H .. –
Step (4): Transfer of a proton from the solvent (liquid ammonia) to the carbanion. .. H
NH2 R' H C R
NH2 .. – : R' R C H .. – 17

52. Propose efficient syntheses of (E)- and (Z)-2- heptene from propyne and other necessary reagents.
Strategy 19

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

54. 9.11 Addition of Hydrogen Halides to Alkynes5

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

56. Termolecular Rate-determining Step.. Br H : RC CH .. Br H :
Observed rate law: rate = k[alkyne][HX]2 23

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

58. Free-radical Addition of HBr
CH3(CH2)3C CH
(79%)
CH3(CH2)3CH
CHBr
peroxides
Regioselectivity opposite to Markovnikov's rule. As with alkenes, the anti-Markovnikov product is formed in presence of peroxide. 25

59. 9.12 Hydration of Alkynes 5

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

61. Enols are tautomers of ketones, and exist in equilibrium with them. OH
RCH
CR'
RCH2CR' O
enol
ketone
Enols are tautomers 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

62. Mechanism of Conversion of Enol to Ketone: H + O C .. .. : O H H H C C + : O : H 30

63. Mechanism of Conversion of Enol to Ketone.. : H O + : O H H C C O H C : + .. 30

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

65. Example of Alkyne Hydration
CH3(CH2)2C
C(CH2)2CH3
via OH
CH3(CH2)2CH
C(CH2)2CH3
Hg2+
H2O, H+
(89%) O
CH3(CH2)2CH2C(CH2)2CH3 32

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

67. 9.13 Addition of Halogens to Alkynes5

68. Both pi bonds react with excess X2.
Example Cl
(63%) C
Cl2CH
CH3 HC
CCH3
+ 2Cl2
Both pi bonds react with excess X2. 36

69. Addition is anti as with an alkene.
CH3CH2 Br
Br2 C
CH3CH2C
CCH2CH3 Br
CH2CH3
(90%)
One addition 1mol of X2.
Addition is anti as with an alkene. 37

70. 9.14 Ozonolysis of Alkynes
Gives two carboxylic acids by cleavage of triple bond 5

71. Reduction is not needed in step 2. 1. O3
Example
CH3(CH2)3C CH
Reduction is not needed in step 2.
1. O3
2. H2O +
CH3(CH2)3COH
(51%) O
HOCOH 39

72. Hydroboration-Oxidation of Terminal Alkynes
Gives the anti-Markovnikov enol 5

73. Disecondaryisoamyl borane, Sia2BH
This is similar to hydroboration-oxidation of alkenes. Disiaborane (a bulky borane) is used to prevent subsequent reaction with the alkene.
Addition of the borane is syn as before.
Sia2BH RC CH
RCH
CH-BSia2
THF
H2O2
RCH
CH-BSia2
RCH
CH-OH
HOˉ
an enol which rearranges to an aldehyde. 8

74. End of Chapter 9 Alkynes 5