Organic Chemistry, 6th Edition
L. G. Wade, Jr.
Chapter 9
Alkynes
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
2
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
3
Name these:
CH3
C CH
propyne
CH3
C C CH2
CH2
Br
5-bromo-2-pentyne
5-bromopent-2-yne
CH3
CH3
CH CH2
CH3
C C CH CH3
2,6-dimethyl-3-heptyne
2,6-dimethylpept-3-yne
Chapter 9
=>
4
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
5
Examples
CH3
CH2
CH CH2
CH C CH
4-methyl-1-hexen-5-yne
4-methylhex-1-en-5-yne
OH
CH3
C C CH2
CH CH3
4-hexyn-2-ol
hex-4-yn-2-ol
Chapter 9
=>
6
Common Names
Named as substituted acetylene.
CH3
C CH
methylacetylene
(terminal alkyne)
CH3
CH3
CH CH2
CH3
C C CH CH3
isobutylisopropylacetylene
(internal alkyne)
Chapter 9
=>
7
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
8
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
9
Synthesis of Acetylene
• Heat coke with lime in an electric
furnace to form calcium carbide.
• Then drip water on the calcium carbide.
CaC2 +
3 C + CaO
coke
* CaC2 +
CO
lime
H C C H + Ca(OH)2
2 H2O
*This reaction was used to produce light
=>
for miners’ lamps and for the stage.
Chapter 9
10
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
11
Bond Lengths
• More s character, so shorter length.
• Three bonding overlaps, so shorter.
Bond angle is 180, so linear geometry.
Chapter 9
=>
12
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
13
Acidity Table
Chapter 9
=>
14
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
15
Alkynes from
Acetylides
• Acetylide ions are good nucleophiles.
• SN2 reaction with 1 alkyl halides
lengthens the alkyne chain.
=>
Chapter 9
16
Must be 1
• Acetylide ions can also remove H+
• If back-side approach is hindered,
elimination reaction happens via E2.
=>
Chapter 9
17
Addition to Carbonyl
Acetylide ion + carbonyl group yields an
alkynol (alcohol on carbon adjacent to
triple bond).
R C C
H2O +
+
R C C C O
C O
R C C C O H
H
O
H
H
=>
Chapter 9
18
Add to Formaldehyde
Product is a primary alcohol with one
more carbon than the acetylide.
H
H
CH3
C C
C O
+
CH3
H
C C C O
H
H
H2O + CH3 C C C O H
H
H
O
H
H
=>
Chapter 9
19
Add to Aldehyde
Product is a secondary alcohol, one R
group from the acetylide ion, the other R
group from the aldehyde.
CH3
C C
CH3
CH3
C O
+
H
CH3
C C C O
H
CH3
H2O + CH3 C C C O H
H
Chapter 9
H
O
H
H
=>
20
Add to Ketone
Product is a tertiary alcohol.
CH3
C C
CH3
CH3
C O
+
CH3
CH3
C C C O
CH3
CH3
H2O + CH3 C C C O H
CH3
H
O
H
H
=>
Chapter 9
21
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
22
Reagents for
Elimination
Br
CH3
Br
CH CH CH2 CH3
KOH (fused)
200°C
CH3
C C CH2
CH3
• 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
CH2
CH2
CHCl2
1) NaNH2 , 150°C
2) H2O
Chapter 9
CH3
CH2
C CH
=>
23
Migration of Triple Bond
=>
Chapter 9
24
Addition Reactions
•
•
•
•
Similar to addition to alkenes.
Pi bond becomes two sigma bonds.
Usually exothermic.
One or two molecules may add.
=>
Chapter 9
25
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
26
Lindlar’s Catalyst
• Powdered BaSO4 coated with Pd,
poisoned with quinoline.
• H2 adds syn, so cis-alkene is formed.
=>
Chapter 9
27
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
NH3 e
+ Na
Chapter 9
-
+
+ Na
=>
28
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
29
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
Br
C C
Br
+
CH3
CH3
CH3
C C
Br
Br2
Br
Br Br
CH3
C C CH3
Br Br
Chapter 9
30
=>
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
HBr
Br
Br
CH3
C CH2
HBr
CH3
C CH3
Br
Chapter 9
31
=>
HBr with Peroxides
Anti-Markovnikov product is formed with a
terminal alkyne.
H
CH3
C C H
HBr
ROOR
CH3
C C
H
HBr
Br ROOR
H Br
CH3
C C H
H Br
mixture of E and Z isomers
=>
Chapter 9
32
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
=>33
Mechanism for
Mercuration
• Mercuric ion (Hg2+) is electrophile.
• Vinyl carbocation forms on most-sub. C.
• Water is the nucleophile.
+
+2
CH3
Hg
C C H
+
CH3 C
+
Hg
C
Hg
CH3 C
H
+
O
H2O
H
CH3 C
OH
C
H
an enol
H
H
H
+
+
H3O
C
Hg
CH3 C
OH
Chapter 9
C
H
H2O
=>
34
Enol to Keto (in Acid)
• Add H+ to the C=C double bond.
• Remove H+ from OH of the enol.
H
CH3 C
OH
C
H
H
H
+
H3O
CH3 C
C H
CH3 C
OH H
OH H
H
CH3 C
A methyl ketone
=>
Chapter 9
C H
O
H2O
C H
H
35
Hydroboration Reagent
• Di(secondary
isoamyl)borane, called
disiamylborane.
• Bulky, branched reagent
adds to the least
hindered carbon.
• Only one mole can add.
H3C
HC CH3
CH
H3C
CH3
B CH
CH
H
CH3
H3C
=>
Chapter 9
36
Hydroboration Oxidation
• B and H add across the triple bond.
• Oxidation with basic H2O2 gives the enol.
CH3
C C H
Sia2 BH
H
CH3 C
H
C
H2O2
BSia2 NaOH
Chapter 9
H
CH3 C
H
C
OH
=>
37
Enol to Keto (in Base)
• H+ is removed from OH of the enol.
• Then water gives H+ to the adjacent
carbon.
H
CH3 C
H
C
OH
H
H
OH
CH3 C
C
CH3 C
O
H
H
An aldehyde
H
Chapter 9
O
HOH
H
CH3 C
C
H
C
O
=>
38
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
39
Reaction with KMnO4
• Mild conditions, dilute, neutral
CH3
C C CH2
CH3
O O
KMnO4
CH3
C C CH2
CH3
H2O, neutral
• Harsher conditions, warm, basic
CH3
C C CH2
CH3
KMnO4 , KOH
H2O, warm
O
CH3
C O
O
+ O C CH2
CH3
=>
Chapter 9
40
Ozonolysis
• Ozonolysis of alkynes produces carboxylic
acids (alkenes gave aldehydes and ketones).
CH3
C C CH2
CH3
(1) O3
(2) H2O
O
CH3
C OH
O
+ HO C CH2
CH3
• Used to find location of triple bond in an
unknown compound.
=>
Chapter 9
41
End of Chapter 9
Chapter 9
42