1. Chapter 9 Alkynes
Dr. Wolf's CHM 201 & 202 1

2. Sources of Alkynes
Dr. Wolf's CHM 201 & 202 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
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4. Nomenclature
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5. Acetylene and ethyne are both acceptable IUPAC names for
Nomenclature HC CH
Acetylene and ethyne are both acceptable IUPAC names for
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
(CH3)3CC
CCH3
4,4-Dimethyl-2-pentyne
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6. Physical Properties of Alkynes
The physical properties of alkynes are similar to those of alkanes and alkenes.
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7. Structure and Bonding in Alkynes: sp Hybridization
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8. linear geometry for acetylene
Structure
linear geometry for acetylene C H
120 pm
106 pm
121 pm
CH3 C C H
146 pm
106 pm
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9. Cyclooctyne polymerizes on standing.
Cyclononyne is the smallest cycloalkyne stable enough to be stored at room temperature for a reasonable length of time.
Cyclooctyne polymerizes on standing.
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10. Bonding in acetylene is based on sp-hybridization for each carbon
Mix together (hybridize) the 2s orbital and one of the three 2p orbitals 2p 2p
2sp 2s
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11. Bonding in acetylene is based on sp-hybridization for each carbon
Mix together (hybridize) the 2s orbital and one of the three 2p orbitals 2p
Each carbon has two half-filled sp orbitals available to form s bonds.
2sp
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12. s Bonds in Acetylene
Each carbon is connected to a hydrogen by a s bond. The two carbons are connected to each other by a s bond and two p bonds.
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13. p Bonds in Acetylene
One of the two p bonds in acetylene is shown here. The second p bond is at right angles to the first.
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14. p Bonds in Acetylene
This is the second of the two p bonds in acetylene.
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15. The region of highest negative charge lies above and below the molecular plane in ethylene.
The region of highest negative charge encircles the molecule around its center in acetylene.
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16. Table 9.1 Comparison 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
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17. Acidity of Acetylene and Terminal AlkynesH C
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18. In general, hydrocarbons are exceedingly weak acids
Compound pKa
HF 3.2
H2O 16
NH3 36 45
CH4 60
H2C
CH2
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19. 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
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20. Electronegativity of carbon increases with its s character
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.
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21. Objective:
Prepare a solution containing sodium acetylide Will treatment of acetylene with NaOH be effective?
NaC CH
H2O
NaOH + HC CH
NaC
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22. No. Hydroxide is not a strong enough base to deprotonate acetylene.
H2O
NaOH + HC CH
NaC HO .. : HO H .. – – + + H C CH : C CH
weaker acid pKa = 26
stronger acid pKa = 16
In acid-base reactions, the equilibrium lies to the side of the weaker acid.
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23. Solution: Use a stronger base
Solution: Use a stronger base. Sodium amide is a stronger base than sodium hydroxide.
NH3
NaNH2 + HC CH
NaC .. .. – – : +
H2N +
H2N H C CH 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.
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24. Preparation of Alkynes by Alkylation of Acetylene and Terminal Alkynes
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25. 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
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26. Alkylation of acetylene and terminal alkynes
H—C
C—H
R—C
C—H
R—C
C—R
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27. Alkylation of acetylene and terminal alkynes– :
H—C
SN2 + R X 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.
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28. Example: Alkylation of acetylene
NaNH2 HC CH HC
CNa
NH3
CH3CH2CH2CH2Br HC C
CH2CH2CH2CH3
(70-77%)
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29. Example: Alkylation of a terminal alkyneCH
(CH3)2CHCH2C
NaNH2, NH3
CNa
(CH3)2CHCH2C
CH3Br
(81%)
C—CH3
(CH3)2CHCH2C
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30. 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%)
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31. Effective only with primary alkyl halides
Limitation
Effective only with primary alkyl halides
Secondary and tertiary alkyl halides undergo elimination
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32. E2 predominates over SN2 when alkyl halide is secondary or tertiary– :
H—C H C
C— X E2 + C
H—C —H X– :
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33. Preparation of Alkynes by Elimination Reactions
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34. 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.
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35. Geminal dihalide ® Alkyne
(CH3)3CCH2—CHCl2
1. 3NaNH2, NH3
2. H2O
(56-60%)
(CH3)3CC CH
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36. Geminal dihalide ® Alkyne
(CH3)3CCH2—CHCl2
NaNH2, NH3
(slow)
(CH3)3CCH
CHCl
(slow)
NaNH2, NH3
(CH3)3CC CH
H2O
NaNH2, NH3
(fast)
(CH3)3CC
CNa
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37. Vicinal dihalide ® Alkyne
CH3(CH2)7CH—CH2Br Br
1. 3NaNH2, NH3
2. H2O
(54%)
CH3(CH2)7C CH
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38. Reactions of Alkynes
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39. 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)
Addition of Halogens (Section 9.13)
Ozonolysis (Section 9.14)
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40. Hydrogenation of Alkynes
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41. alkene is an intermediate
Hydrogenation of Alkynes
cat RC
CR' +
2H2
RCH2CH2R'
catalyst = Pt, Pd, Ni, or Rh
alkene is an intermediate
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42. 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.
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43. 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.
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44. syn-Hydrogenation occurs; cis alkenes are formed.
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.
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45. Example CH3(CH2)3C C(CH2)3CH3 + H2 Lindlar Pd CH3(CH2)3 (CH2)3CH3 C H
(87%)
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46. Metal-Ammonia Reduction of Alkynes
Alkynes ® trans-Alkenes
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47. 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.
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48. Example CH3CH2C CCH2CH3 Na, NH3 CH3CH2 H C CH2CH3 H (82%) 10
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49. 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
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50. Mechanism
Step (1): Transfer of an electron from the metal to the alkyne to give an anion radical. M . + R R' C .. – M+
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51. Mechanism
Step (2) Transfer of a proton from the solvent (liquid ammonia) to the anion radical. . R' R C H H
NH2 .. R R' C . –
NH2 .. – :
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52. 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
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53. Mechanism
Step (4) Transfer of a proton from the solvent (liquid ammonia) to the carbanion . H
NH2 .. R' R C – H C R
NH2 .. – : R'
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54. Suggest efficient syntheses of (E)- and (Z)-2- heptene from propyne and any necessary organic or inorganic reagents.
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55. 1. NaNH2 2. CH3CH2CH2CH2Br Na, NH3 H2, Lindlar Pd 19
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56. Addition of Hydrogen Halides to Alkynes
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57. Follows Markovnikov's Rule
HBr
CH3(CH2)3C CH
CH3(CH2)3C
CH2 Br
(60%)
Alkynes are slightly less reactive than alkenes
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58. Termolecular transition state.. Br H : RC CH .. Br H :
Observed rate law: rate = k[alkyne][HX]2
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59. Reaction with two moles of a hydrogen halide yields a geminal dihalide
CH3CH2C
CCH2CH3
2 HF F C H
CH3CH2
CH2CH3
(76%)
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60. Free-radical addition of HBr occurs when peroxides are present
CH3(CH2)3C CH
CH3(CH2)3CH
CHBr
peroxides
(79%)
regioselectivity opposite to Markovnikov's rule
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61. Hydration of Alkynes
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62. expected reaction: Hydration of Alkynes H+ RC CR' H2O + OH RCH CR'
observed reaction: H+ + RC
CR'
H2O
RCH2CR' O
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63. 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
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64. Mechanism of conversion of enol to ketone.. : O H C H + : O H H
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65. Mechanism of conversion of enol to ketone.. : O H C H + : O H H
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66. Mechanism of conversion of enol to ketone.. : O H H H C C + : O : H
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67. Mechanism of conversion of enol to ketone.. O : : O H H H C C +
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68. Mechanism of conversion of enol to ketone.. O : : O H H H C C +
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69. Mechanism of conversion of enol to ketone.. : H O + : O H H C C
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70. Key carbocation intermediate is stabilized by electron delocalization (resonance)H C + .. : O C H .. +
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71. Example CH3(CH2)2C C(CH2)2CH3 via Hg2+ H2O, H+ OH CH3(CH2)2CH
CH3(CH2)2CH2C(CH2)2CH3
(89%)
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72. Markovnikov's rule followed in formation of enol
H2O, H2SO4
CH3(CH2)5C CH
CH3(CH2)5CCH3
HgSO4
(91%)
via
CH3(CH2)5C
CH2 OH
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73. Addition of Halogens to Alkynes
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74. Example Cl (63%) C Cl2CH CH3 HC CCH3 + 2 Cl2 36
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75. Addition is anti CH3CH2 Br Br2 C CH3CH2C CCH2CH3 Br CH2CH3 (90%) 37
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76. gives two carboxylic acids by cleavage of triple bond
Ozonolysis of Alkynes
gives two carboxylic acids by cleavage of triple bond
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77. Example CH3(CH2)3C CH 1. O3 2. H2O CH3(CH2)3COH (51%) O HOCOH O + 39
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78. End of Chapter 9
Dr. Wolf's CHM 201 & 202