Chapter 9
Alkynes
Organic Chemistry, 6th Edition
L. G. Wade, Jr.

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.
=>

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.
=>

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
5-bromopent-2-yne
=>
2,6-dimethyl-3-heptyne
2,6-dimethylpept-3-yne

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.
=>

Chapter 9 6
Examples
CH2 CH CH2 CH
CH3
C CH
4-methyl-1-hexen-5-yne
4-methylhex-1-en-5-yne
CH3 C C CH2 CH
OH
CH3
4-hexyn-2-ol
hex-4-yn-2-ol
=>

Chapter 9 7
Common Names
Named as substituted acetylene.
CH3 C CH
methylacetylene
(terminal alkyne)
CH3 CH
CH3
CH2 C C CH
CH3
CH3
isobutylisopropylacetylene
(internal alkyne) =>

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.
=>

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.
=>

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)2
CaC2 + 2 H2O +
C CaO
3 + +
CaC2 CO
coke lime
*This reaction was used to produce light
for miners’ lamps and for the stage. =>
*

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.
=>

Chapter 9 12
Bond Lengths
• More s character, so shorter length.
• Three bonding overlaps, so shorter.
Bond angle is 180, so linear geometry. =>

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.
=>

Chapter 9 14
Acidity Table
=>

Chapter 9 15
Forming Acetylide Ions
• H+ can be removed from a terminal
alkyne by sodium amide, NaNH2.
• NaNH2 is produced by the reaction
of ammonia with sodium metal.
=>

Chapter 9 16
Alkynes from
Acetylides
• Acetylide ions are good nucleophiles.
• SN2 reaction with 1 alkyl halides
lengthens the alkyne chain.
=>

Chapter 9 17
Must be 1
• Acetylide ions can also remove H+
• If back-side approach is hindered,
elimination reaction happens via E2.
=>

Chapter 9 18
Addition to Carbonyl
Acetylide ion + carbonyl group yields an
alkynol (alcohol on carbon adjacent to
triple bond).
+
H2O
O
H
H
H
R C C C O H
=>
C O
+
R C C R C C C O

Chapter 9 19
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

Chapter 9 20
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

Chapter 9 21
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

Chapter 9 22
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.
=>

Chapter 9 23
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
H2O
2)
NaNH2
1)
CH3 CH2 CH2 CHCl2

Chapter 9 24
Migration of Triple Bond
=>

Chapter 9 25
Addition Reactions
• Similar to addition to alkenes.
• Pi bond becomes two sigma bonds.
• Usually exothermic.
• One or two molecules may add.
=>

Chapter 9 26
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.
=>

Chapter 9 27
Lindlar’s Catalyst
• Powdered BaSO4 coated with Pd,
poisoned with quinoline.
• H2 adds syn, so cis-alkene is formed.
=>

Chapter 9 28
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
-
=>

Chapter 9 29
Mechanism
Step 1: An electron adds to the alkyne, forming a radical anion
Step 2: The radical anion is protonated to give a radical
Step 3: An electron adds to the alkyne, forming an anion
=>
Step 4: Protonation of the anion gives an alkene

Chapter 9 30
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
=>

Chapter 9 31
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
=>

Chapter 9 32
HBr with Peroxides
Anti-Markovnikov product is formed with a
terminal alkyne.
HBr
CH3 C C
H
H
H
Br
Br
ROOR
=>
CH3 C C H CH3 C C
H H
Br
HBr
ROOR
mixture of E and Z isomers

Chapter 9 33
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.
=>

Chapter 9 34
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
H2O
CH3 C
H
Hg
+
C
OH
H3O
+
CH3 C
H
H
C
OH
an enol
=>

Chapter 9 35
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
=>

Chapter 9 36
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

Chapter 9 37
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
=>

Chapter 9 38
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
=>

Chapter 9 39
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.
=>

Chapter 9 40
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 CH3
CH3 C
O
O +
H2O, warm
, KOH
KMnO4
CH3 C C CH2 CH3
=>

Chapter 9 41
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 CH3
CH3 C
O
OH
H2O
(2)
O3
(1)
CH3 C C CH2 CH3 +

Chapter 9 42
End of Chapter 9