Chapter 9
Alkynes
Jo Blackburn
Richland College,Dallas,TX
Dallas County Community College District
2003,Prentice Hall
Organic Chemistry,5th 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:
C H 3 C H
C H 3
C H 2 C C C H
C H 3
C H 3
C H 3 C C C H 2 C H 2 B r
C H 3 C C H
propyne
5-bromo-2-pentyne
2,6 -d i m e t h y l - 3 -h e p t y n e=>
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
C H 2 C H C H 2 C H
C H 3
C C H
4-methyl-1-hexen-5-yne
C H 3 C C C H 2 C H
O H
C H 3
4-hexyn-2-ol
=>
Chapter 9 7
Common Names
Named as substituted acetylene.
C H 3 C C H
methylacetylene
C H 3 C H
C H 3
C H 2 C C C H
C H 3
C H 3
isobutylisopropylacetylene
=>
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 C a( O H ) 2C aC 2 + 2 H 2 O +
C C aO3 + +C aC 2 C O
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.
C H 3 C C H + N aN H 2 C H 3 C C,- N a + + N H 3
NaNH2 is produced by the reaction
of ammonia with sodium metal.
N H
H
H + N a
F e
+ 3
+ 1 / 2 H 2N
H
H N a
+
=>
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,
=>
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,
=>
C H 3 C C H + C u + C H 3 C C C u + H +
Chapter 9 18
Separation of Mixtures
C H 3 C C C H 3
+
C H 3 C H 2 C C H
C u
+
C u
+
N o r e a c t i o n
C H 3 C H 2 C C C u
red-brown precipitate
Filter the solid to separate,then regenerate the terminal
alkyne by adding dilute acid.
+ +C H 3 C H 2 C C C u H C l C H 3 C H 2 C C H C u C l
=>
Chapter 9 19
Alkynes from
Acetylides
Acetylide ions are good nucleophiles.
SN2 reaction with 1? alkyl halides
lengthens the alkyne chain.
++C H 3 C C,
-
N a
+
C H
3
C H
2
B r C H
3
C C C H
2
C H
3
N aB r
=>
Chapter 9 20
Must be 1?
Acetylide ions can also remove H+
If back-side approach is hindered,
elimination reaction happens via E2.
C H 3 C C,- N a + + C H 3 C H
B r
C H 3 C H 3 C C H H 3 C C H C H 2+
=>
Chapter 9 21
Addition to Carbonyl
Acetylide ion + carbonyl group yields an
alkynol (alcohol on carbon adjacent to
triple bond).
+H 2 O
O
H
H
HR C C C O H
=>
C O+R C C R C C C O
Chapter 9 22
Add to Formaldehyde
Product is a primary alcohol with one
more carbon than the acetylide.
+ C O
H
H
C H 3 C C C H 3 C C C
H
H
O
=>
+H 2 O O
H
H
H
C H 3 C C C O H
H
H
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
C H 3
H
C H 3 C C C H 3 C C C
C H 3
H
O
=>
+H 2 O O
H
H
H
C H 3 C C C O H
C H 3
H
Chapter 9 24
Add to Ketone
Product is a tertiary alcohol.
+ C O
C H 3
C H 3
C H 3 C C C H 3 C C C
C H 3
C H 3
O
=>
+H 2 O O
H
H
H
C H 3 C C C O H
C H 3
C H 3
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,
=>
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,
C H 3 C C C H 2 C H 32 0 0 ° CK O H ( f u s e d )C H 3 C H C H C H 2 C H 3
B r B r
=>
,1 5 0 ° C C H
3 C H 2 C C HH
2 O2)
N aN H 21)C H
3 C H 2 C H 2 C H C l 2
Chapter 9 27
Migration of Triple Bond
=>
Chapter 9 28
Addition Reactions
Similar to addition to alkenes
Pi bond becomes two sigma bonds.
Usually exothermic
One or two molecules may add.
=>
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,
=>
Chapter 9 30
Lindlar’s Catalyst
Powdered BaSO4 coated with Pd,
poisoned with quinoline.
H2 adds syn,so cis-alkene is formed.
=>
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.
N H 3 + N a + N a +N H 3 e -=>
Chapter 9 32
Mechanism
=>
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.
C H 3 C C C H 3
B r 2 C H 3
C
B r
C
B r
C H 3
+
C H 3 C
B r
C
C H 3
B r
B r 2
C H 3 C
B r
B r
C
B r
B r
C H 3
=>
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.
C H 3 C C H C H 3 C C H 2
B rH B rH B r
C H 3 C C H 3
B r
B r=>
Chapter 9 35
HBr with Peroxides
Anti-Markovnikov product is formed with a
terminal alkyne.
C H 3 C C H C H 3 C C
H H
B r
H B r
R O O R
H B r
C H 3 C C
H
H
H
B r
B r
R O O R
=>
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,
=>
Chapter 9 37
Mechanism for
Mercuration
Mercuric ion (Hg2+) is electrophile.
Vinyl carbocation forms on most-sub,C.
Water is the nucleophile.
C H 3 C C H C H 3 C
+
C
H g
+
H
H g
+ 2
H 2 O
C H 3 C
H
H g +
C
O +
H H
H 2 OC H 3 C
H
H g +
C
O H
H 3 O +C H
3 C H
H
C
O H
an enol =>
Chapter 9 38
Enol to Keto (in Acid)
Add H+ to the C=C double bond.
Remove H+ from OH of the enol.
C H 3 C C
O H
H
H
H
H 2 O
C H 3 C C
O
H
H
H
C H 3 C
H
H
C
O H
H 3 O +
C H 3 C C
O H
H
H
H
A methyl ketone
=>
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
C H
C H
H
C H 3
C H
C H 3
H 3 C
H 3 C
H C
C H 3
H 3 C
Chapter 9 40
Hydroboration -
Oxidation
B and H add across the triple bond.
Oxidation with basic H2O2 gives the enol.
C H 3 C C H C H
3 C
H
C
H B S i a 2
S i a 2 B H
C H 3 C
O H
H
C
H
H 2 O 2
N aO H =>
Chapter 9 41
Enol to Keto (in Base)
H+ is removed from OH of the enol.
Then water gives H+ to the adjacent
carbon,
C H 3 C
O
H
C
H H O H
C H 3 C
O
H
C
H
H
O H
C H 3 C
O H
H
C
H
C H 3 C
O
H
C
H
An aldehyde
=>
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,
=>
Chapter 9 43
Reaction with KMnO4
Mild conditions,dilute,neutral
Harsher conditions,warm,basic
C H 3 C
O
C
O
C H 2 C H 3
H 2 O,n e u t r al
K M n O 4
C H 3 C C C H 2 C H 3
O C
O
C H 2 C H 3C H 3 C
O
O +
H 2 O,w ar m
,K O HK M n O 4C H
3 C C C H 2 C H 3
=>
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,
=>
H O C
O
C H 2 C H 3C H 3 C
O
O H
H 2 O( 2 )
O 3( 1 )C H
3 C C C H 2 C H 3 +
Chapter 9 45
End of Chapter 9