1. Chapter 9 Alkynes Sources of Alkynes and Nomenclature
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1

2. Introduction Alkynes contain a triple bond.
General formula is CnH2n–2.
Two elements of unsaturation for each triple bond.
Some reactions resemble the reactions of alkenes, like addition and oxidation.
Some reactions are specific to alkynes.

3. Nomenclature: IUPAC Find the longest chain containing the triple bond.
Change -ane ending to -yne.
Number the chain, starting at the end closest to the triple bond.
Give branches or other substituents a number to locate their position.

4. Examples of Nomenclature
All other functional groups, except ethers and halides, have a higher priority than alkynes.

5. Physical Properties Nonpolar, insoluble in water.
Soluble in most organic solvents.
Boiling points are similar to alkane of same size.
Less dense than water.
Up to four carbons, gas at room temperature.

6. Ethyne Commonly called acetylene. It is used in welding torches.
The oxyacetylene flame reaches temperatures as high as 2800 °C.
Thermodynamically unstable. When it decomposes to its elements, it releases 234 kJ (56 kcal) of energy per mole.

7. Synthesis of Acetylene from Coal
CaC2 + 2 H2O H—C≡C—H + Ca(OH)2
Heat coke with lime in an electric furnace to form calcium carbide.
Then drip water on the calcium carbide.

8. 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

9. Structure and Bonding in Alkynes: sp Hybridization5

10. Molecular Structure of Acetylene
Triple-bonded carbons have sp hybrid orbitals.
A sigma bond is formed between the carbons by overlap of the sp orbitals.
Sigma bonds to the hydrogens are formed by using the second sp orbital.
Since the sp orbitals are linear, acetylene will be a linear molecule.

11. Overlap of the p Orbitals of Acetylene
Each carbon in acetylene has two unhybridized p orbitals with one nonbonded electron. It is the overlap of the parallel p orbitals that forms the triple bond (two pi orbitals).

12. Bond Lengths
Triple bonds are shorter than double or single bonds because of the two pi overlapping orbitals.

13. 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

14. Acidity of Acetylene and Terminal AlkynesH C
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17

15. 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

16. 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

17. Acidity of Alkynes
Terminal alkynes are more acidic than other hydrocarbons due to the higher s character of the sp hybridized carbon.
Terminal alkynes can be deprotonated quantitatively with strong bases such as sodium amide (–NH2).
Hydroxide (HO–) and alkoxide (RO–) bases are not strong enough to deprotonate the alkyne quantitatively.

18. Sodium Acetylide Objective:
Prepare a solution containing sodium acetylide Will treatment of acetylene with NaOH be effective?
NaC CH
H2O
NaOH + HC CH
NaC 22

19. No. Hydroxide is not a strong enough base to deprotonate acetylene.
Sodium Acetylide
No. 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

20. 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

21. Preparation of Alkynes by Alkylation of Acetylene and Terminal Alkynes26

22. 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

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

24. Acetylide Ions in SN2 Reactions
One of the best methods for synthesizing substituted alkynes is a nucleophilic attack by the acetylide ion on an unhindered alkyl halide.
SN2 reaction with 1 alkyl halides lengthens the alkyne chain.
Unhindered alkyl halides work better in an SN2 reaction: CH3X > 1°.

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

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

27. 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

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

29. 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

30. Preparation of Alkynes by Elimination Reactions34

31. 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

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

33. 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

34. Vicinal dihalide  AlkyneBr
CH3(CH2)7CH—CH2Br
1. 3NaNH2, NH3
2. H2O
(54%)
CH3(CH2)7C CH
KOBut in DMSO can be used in place of amide. 38

35. Reactions of Alkynes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1

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

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

38. 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

39. Partial HydrogenationRC
CR'
cat H2
RCH
CHR'
cat H2
RCH2CH2R'
Alkynes 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

40. syn-Hydrogenation occurs; cis alkenes are formed.
Lindlar Catalyst 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

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

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

43. trans-Alkenes are formed.
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

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

45. Metal (Li, Na, K) is reducing agent; H2 is not involved
Mechanism
Metal (Li, Na, K) is reducing agent; H2 is not involved
Four steps
(1) electron transfer
(2) proton transfer
(3) electron transfer
(4) proton transfer 13

46. Mechanism
Step (1): Transfer of an electron from the metal to the alkyne to give an anion radical. M . + R R' C .. – M+ 14

47. Mechanism
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

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

49. Mechanism
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

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

51. Strategy 19

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

53. Addition of Hydrogen Halides to Alkynes21

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

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

56. Figure 9.4

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

58. Anti-Markovnikov Addition of Hydrogen Bromide to Alkynes
By using peroxides, hydrogen bromide can be added to a terminal alkyne anti-Markovnikov.
The bromide will attach to the least substituted carbon, giving a mixture of cis and trans isomers.

59. Hydration of Alkynes 26

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. Keto–Enol Tautomerism
Enols are not stable, and they isomerize to the corresponding aldehyde or ketone in a process known as keto–enol tautomerism.
Keto-enol equilibration is rapid in acidic media

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

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

64. Mechanism of Conversion of Enol to Ketone: + .. 30

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

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

67. 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

68. 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

69. Addition of Halogens to Alkynes26

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

71. Addition is anti CH3CH2 Br Br2 C CH3CH2C CCH2CH3 Br CH2CH3 (90%)
When alkyne and the halogen are in equal ratio, a dihaloalkene can be isolated. 37

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

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