This chapter discusses alkynes, carbon-carbon triple bonds. Alkynes contain two pi bonds and have the general formula CnH2n-2. They can be named using IUPAC nomenclature by changing the -ane ending of the parent alkane to -yne. Alkynes undergo addition reactions like alkenes but also have unique reactions like forming acetylide ions. They can be synthesized through elimination and by reactions of acetylide ions. Oxidation and ozonolysis reactions of alkynes cleave the triple bond.

1. Chapter 9
Alkynes
Organic Chemistry

2. Chapter 9 2
Introduction
• Alkynes contain a triple bond.
• General formula is CnH2n-2
• Two elements of unsaturation for each
triple bond.
• Some reactions are like alkenes:
addition and oxidation.
• Some reactions are specific to alkynes.
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3. Chapter 9 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.
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4. Chapter 9 4
Name these:
CH3 CH
CH3
CH2 C C CH
CH3
CH3
CH3 C C CH2 CH2 Br
CH3 C CH
propyne
5-bromo-2-pentyne
2,6-dimethyl-3-heptyne =>

5. Chapter 9 5
Additional Functional
Groups
• All other functional groups, except
ethers and halides have a higher priority
than alkynes.
• For a complete list of naming priorities,
look inside the back cover of your text.
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6. Chapter 9 6
Examples
CH2 CH CH2 CH
CH3
C CH
4-methyl-1-hexen-5-yne
CH3 C C CH2 CH
OH
CH3
4-hexyn-2-ol
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7. Chapter 9 7
Common Names
Named as substituted acetylene.
CH3 C CH
methylacetylene
CH3 CH
CH3
CH2 C C CH
CH3
CH3
isobutylisopropylacetylene
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8. Chapter 9 8
Physical Properties
• Nonpolar, insoluble in water.
• Soluble in most organic solvents.
• Boiling points similar to alkane of same
size.
• Less dense than water.
• Up to 4 carbons, gas at room temperature.
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9. Chapter 9 9
Acetylene
• Acetylene is used in welding torches.
• In pure oxygen, temperature of flame
reaches 2800°C.
• It would violently decompose to its
elements, but the cylinder on the torch
contains crushed firebrick wet with
acetone to moderate it.
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10. Chapter 9 10
Synthesis of Acetylene
• Heat coke with lime in an electric
furnace to form calcium carbide.
• Then drip water on the calcium carbide.
H C C H Ca(OH)2CaC2 + 2 H2O +
C CaO3 + +CaC2 CO
coke lime
*This reaction was used to produce light
for miners’ lamps and for the stage. =>
*

11. Chapter 9 11
Electronic Structure
• The sigma bond is sp-sp overlap.
• The two pi bonds are unhybridized p
overlaps at 90°, which blend into a
cylindrical shape.
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12. Chapter 9 12
Bond Lengths
• More s character, so shorter length.
• Three bonding overlaps, so shorter.
Bond angle is 180°, so linear geometry. =>

13. Chapter 9 13
Acidity of Alkynes
• Terminal alkynes, R-C≡C-H, are more
acidic than other hydrocarbons.
• Acetylene → acetylide by NH2
-
, but not
by OH-
or RO-
.
• More s character, so pair of electrons in
anion is held more closely to the
nucleus. Less charge separation, so
more stable.
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14. Chapter 9 14
Acidity Table
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15. Chapter 9 15
Forming Acetylide Ions
• H+
can be removed from a terminal
alkyne by sodium amide, NaNH2.
CH3 C C H + NaNH2 CH3 C C:
-
Na
+
+ NH3
• NaNH2 is produced by the reaction
of ammonia with sodium metal.
N H
H
H + Na
Fe
+3
+ 1/2 H2N
H
H Na
+
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16. Chapter 9 16
Heavy Metal Acetylides
• Terminal alkynes form a precipitate with
Ag(I) or Cu(I) salts.
• Internal alkynes do not react.
• Two uses:
Qualitative test for terminal alkyne
Separation of a mixture of terminal and
internal alkynes.
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17. Chapter 9 17
Qualitative Test
• Reagent is AgNO3 or CuNO3 in alcohol, or
ammonia is added to form the complex ion.
• The solid is explosive when dry.
• Copper tubing is not used with acetylene.
=>
CH3 C C H + Cu
+
CH3 C C Cu + H
+

18. Chapter 9 18
Separation of Mixtures
CH3 C C CH3
+
CH3 CH2 C C H
Cu
+
Cu
+
No reaction
CH3 CH2 C C Cu
red-brown precipitate
Filter the solid to separate, then regenerate the terminal
alkyne by adding dilute acid.
+ +CH3 CH2 C C Cu HCl CH3 CH2 C C H CuCl
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19. Chapter 9 19
Alkynes from
Acetylides
• Acetylide ions are good nucleophiles.
• SN2 reaction with 1° alkyl halides
lengthens the alkyne chain.
++CH3 C C:
-
Na
+
CH3CH2 Br CH3 C C CH2 CH3 NaBr
=>

20. Chapter 9 20
Must be 1°
• Acetylide ions can also remove H+
• If back-side approach is hindered,
elimination reaction happens via E2.
CH3 C C:
-
Na
+
+ CH3 CH
Br
CH3 CH3 C C H H3C CH CH2+
=>

21. Chapter 9 21
Addition to Carbonyl
Acetylide ion + carbonyl group yields an
alkynol (alcohol on carbon adjacent to
triple bond).
+H2O
O
H
H
HR C C C O H
=>
C O+R C C R C C C O

22. Chapter 9 22
Add to Formaldehyde
Product is a primary alcohol with one
more carbon than the acetylide.
+ C O
H
H
CH3 C C CH3 C C C
H
H
O
=>
+H2O O
H
H
H
CH3 C C C O H
H
H

23. Chapter 9 23
Add to Aldehyde
Product is a secondary alcohol, one R
group from the acetylide ion, the other
R group from the aldehyde.
+ C O
CH3
H
CH3 C C CH3 C C C
CH3
H
O
=>
+H2O O
H
H
H
CH3 C C C O H
CH3
H

24. Chapter 9 24
Add to Ketone
Product is a tertiary alcohol.
+ C O
CH3
CH3
CH3 C C CH3 C C C
CH3
CH3
O
=>
+H2O O
H
H
H
CH3 C C C O H
CH3
CH3

25. Chapter 9 25
Synthesis by
Elimination
• Removal of two molecules of HX from a
vicinal or geminal dihalide produces an
alkyne.
• First step (-HX) is easy, forms vinyl
halide.
• Second step, removal of HX from the
vinyl halide requires very strong base
and high temperatures.
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26. Chapter 9 26
Reagents for
Elimination
• Molten KOH or alcoholic KOH at 200°C
favors an internal alkyne.
• Sodium amide, NaNH2, at 150°C, followed
by water, favors a terminal alkyne.
CH3 C C CH2 CH3
200°C
KOH (fused)
CH3 CH CH CH2 CH3
Br Br
=>
, 150°C
CH3 CH2 C CH
H2O2)
NaNH21)
CH3 CH2 CH2 CHCl2

27. Chapter 9 27
Migration of Triple Bond
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28. Chapter 9 28
Addition Reactions
• Similar to addition to alkenes
• Pi bond becomes two sigma bonds.
• Usually exothermic
• One or two molecules may add.
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29. Chapter 9 29
Addition of Hydrogen
• Three reactions:
• Add lots of H2 with metal catalyst (Pd,
Pt, or Ni) to reduce alkyne to alkane,
completely saturated.
• Use a special catalyst, Lindlar’s catalyst
to convert an alkyne to a cis-alkene.
• React the alkyne with sodium in liquid
ammonia to form a trans-alkene.
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30. Chapter 9 30
Lindlar’s Catalyst
• Powdered BaSO4 coated with Pd,
poisoned with quinoline.
• H2 adds syn, so cis-alkene is formed.
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31. Chapter 9 31
Na in Liquid Ammonia
• Use dry ice to keep ammonia liquid.
• As sodium metal dissolves in the
ammonia, it loses an electron.
• The electron is solvated by the
ammonia, creating a deep blue solution.
NH3 + Na + Na
+
NH3 e
-
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32. Chapter 9 32
Mechanism
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33. Chapter 9 33
Addition of Halogens
• Cl2 and Br2 add to alkynes to form vinyl
dihalides.
• May add syn or anti, so product is
mixture of cis and trans isomers.
• Difficult to stop the reaction at dihalide.
CH3 C C CH3
Br2 CH3
C
Br
C
Br
CH3
+
CH3
C
Br
C
CH3
Br
Br2
CH3 C
Br
Br
C
Br
Br
CH3
=>

34. Chapter 9 34
Addition of HX
• HCl, HBr, and HI add to alkynes to form
vinyl halides.
• For terminal alkynes, Markovnikov
product is formed.
• If two moles of HX is added, product is
a geminal dihalide.
CH3 C C H CH3 C CH2
Br
HBr HBr
CH3 C CH3
Br
Br
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35. Chapter 9 35
HBr with Peroxides
Anti-Markovnikov product is formed with a
terminal alkyne.
CH3 C C H CH3 C C
H H
Br
HBr
ROOR
HBr
CH3 C C
H
H
H
Br
Br
ROOR
=>

36. Chapter 9 36
Hydration of Alkynes
• Mercuric sulfate in aqueous sulfuric
acid adds H-OH to one pi bond with a
Markovnikov orientation, forming a vinyl
alcohol (enol) that rearranges to a
ketone.
• Hydroboration-oxidation adds H-OH
with an anti-Markovnikov orientation,
and rearranges to an aldehyde.
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37. Chapter 9 37
Mechanism for
Mercuration
• Mercuric ion (Hg2+
) is electrophile.
• Vinyl carbocation forms on most-sub. C.
• Water is the nucleophile.
CH3 C C H CH3 C
+
C
Hg
+
H
Hg
+2
H2O
CH3 C
H
Hg
+
C
O
+
H H
H2OCH3 C
H
Hg
+
C
OH
H3O
+
CH3 C
H
H
C
OH
an enol
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38. Chapter 9 38
Enol to Keto (in Acid)
• Add H+
to the C=C double bond.
• Remove H+
from OH of the enol.
CH3 C C
OH
H
H
H
H2O
CH3 C C
O
H
H
H
CH3 C
H
H
C
OH
H3O
+
CH3 C C
OH
H
H
H
A methyl ketone
=>

39. Chapter 9 39
Hydroboration Reagent
• Di(secondary
isoamyl)borane, called
disiamylborane.
• Bulky, branched reagent
adds to the least
hindered carbon.
• Only one mole can add.
=>
B
CH
CH
H
CH3
CH
CH3
H3C
H3C
HC CH3
H3C

40. Chapter 9 40
Hydroboration -
Oxidation
• B and H add across the triple bond.
• Oxidation with basic H2O2 gives the enol.
CH3 C C H CH3 C
H
C
H BSia2
Sia2 BH CH3 C
OH
H
C
H
H2O2
NaOH
=>

41. Chapter 9 41
Enol to Keto (in Base)
• H+
is removed from OH of the enol.
• Then water gives H+
to the adjacent
carbon.
CH3 C
O
H
C
H
HOH
CH3 C
O
H
C
H
H
OH
CH3 C
OH
H
C
H
CH3 C
O
H
C
H
An aldehyde
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42. Chapter 9 42
Oxidation of Alkynes
• Similar to oxidation of alkenes.
• Dilute, neutral solution of KMnO4
oxidizes alkynes to a diketone.
• Warm, basic KMnO4 cleaves the triple
bond.
• Ozonolysis, followed by hydrolysis,
cleaves the triple bond.
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43. Chapter 9 43
Reaction with KMnO4
• Mild conditions, dilute, neutral
• Harsher conditions, warm, basic
CH3 C
O
C
O
CH2 CH3
H2O, neutral
KMnO4
CH3 C C CH2 CH3
O C
O
CH2 CH3CH3 C
O
O +
H2O, warm
, KOHKMnO4
CH3 C C CH2 CH3
=>

44. Chapter 9 44
Ozonolysis
• Ozonolysis of alkynes produces carboxylic
acids (Alkenes gave aldehydes and ketones)
• Used to find location of triple bond in an
unknown compound.
=>
HO C
O
CH2 CH3CH3 C
O
OH
H2O(2)
O3(1)
CH3 C C CH2 CH3 +

45. Chapter 9 45
End of Chapter 9