ORGANIC CHEMISTRY 171
Section 201
Alkynes
Alkynes
• Hydrocarbons that contain carbon-carbon triple
bonds
• Acetylene is the simplest alkyne.
• Our study of alkynes provides nomenclature,physical
properties,structure and SPhybridization,preparation and reactions.
3
Alkynes.
General Formula
CnH2n-2
C2H2
H—C  C—H
acetylene
ethyne
C3H4
CH3CCH
methylacetylene
propyne
sp => linear, 180o
NOMENCLATURE
Naming Alkynes
• General hydrocarbon rules apply with “-yne” as a
suffix indicating an alkyne.
• Numbering of chain with triple bond is set so that
the smallest number possible include the triple
bond.
H3C H2C C C CH2 CH2 CH2 CH2 CH3
3-Nonyne
5
nomenclature:
common names: “alkylacetylene”
IUPAC: parent chain = longest continuous carbon chain that
contains the triple bond.
alkane
drop –ane
add -yne
prefix locant for the triple bond, etc.
CH3CH2CCCH3
2-pentyne
ethylmethylacetylene
“terminal” alkynes have the triple bond at the end of the chain:
CH3CH2CCH
1-butyne
ethylacetylene
CH3
HCCCHCH2CH3
3-methyl-1-pentyne
sec-butylacetylene
Substitutive nomenclature: Similar to alkenes, but with the following
differences:
1. The ending "-ene" is replaced with "-yne"
H3C
C
H3C
C
CH3
4-Methyl-2-pentyne
2. The double bond has a priority over the triple bond
when numbered
5
HC
4
C
3
CH 2
2
CH
1
CH2
1-Pentene-4-yne
3. There are no cis-trans-isomers, because the triple bond is linear
Enynes
• An enyne has a double bond and triple bond.
• Number for an Enynes starts at the multiple bond
closest to the end (it does not matter wheather it is a
double or triple bond)
H3C CH2 C C CH2 CH CH2
1-Hepten-4-yne
9
Diyines and Triynes
• A compound with two triple bonds is a diyine.
– A triyne has three triple bonds.
• Number from chain that ends nearest of triple bond.
H3C CH2 C C CH2 C
CH
1,4-Heptdiyne
10
Alkynes as Substituents
Alkynes as substituents are called “alkynyl”.
H3C CH2 C C
1-butynyl
11
Cycloalkynes
• The smallest cycloalkyne isolated is
cyclononyne
– the C-C-C bond angle about the triple bond is
approximately 156°
Physical Properties
Similar to alkanes and alkenes of
comparable molecular weight and carbon
skeletone.
Name
Ethyne
Propyne
1-Butyne
Formula
HC CH
CH3 C CH
CH 3 CH 2 C CH
Boiling
Point
(°C)
-84
-23
8
Density
at 20°C
(g/mL)
(a gas)
(a gas)
(a gas)
-90
27
40
0.691
0.690
Melting
Point
(°C)
-81
-102
-126
-32
2-Butyne
1-Pentyne
CH3 C CCH 3
CH 3 ( CH2 ) 2 C CH
1-Hexyne
CH 3 ( CH2 ) 3 C CH
-132
71
0.716
1-Octyne
CH 3 ( CH2 ) 5 C CH
-79
125
0.746
1-Decyne
CH 3 ( CH2 ) 7 C CH
-36
174
0.766
physical properties:
1-Weakly or non-polar and have no Hbonding.
2-Relatively low m.p. or b.p.but they increase
as the molecular weight increase.
3-Water insoluble but soluble in organic
solvents.
4-Relatively acidic (have relative acidic
character).
Acidity
• A major difference between the chemistry of
alkynes and that of alkenes and alkanes is the
acidity of the hydrogen bonded to a triply
bonded carbon.
Acidity
Acetylene reacts with sodium amide to form
sodium acetylide
–
HC CH +
pK a 25
Stronger
acid
NH2
Stronger
base
HC C- +
Weaker
base
NH3
pK a 38
Weaker
acid
it can also be converted to its metal salt by reaction with
sodium hydride or lithium diisopropylamide (LDA)
+
Na H–
Sodium hydride
[ ( CH3 ) 2 CH] 2 N – Li +
Lithium diisopropylamide
(LDA)
Acidity
• Water is a stronger acid than acetylene;
hydroxide ion is not a strong enough base to
convert acetylene to its anion
HC CH + OH pK a 25
Weaker
Weaker
acid
base
HC
C- +
Stronger
base
H2 O
pK a 15.7
Stronger
acid
Keq = 10 -9.3
Electronic Structure of Alkynes
• Carbon-carbon triple bond result from
• sp hybridized orbital on each C forming a sigma
bond at 180º
• unhybridized 2pX and 2py orbitals forming a 2p
bond
• The formed C-C triple bond is shorter and stronger
than single or double
• Breaking a p bond in acetylene (HC=CH) requires
318 kJ/mole (in ethylene it is 268 kJ/mole)
H C C H
18
In other words, in alkynes:
1-One s- and two p- bonds form a triple bond.
2-The valence shell of the atom of carbon has one s-orbital
and three p-orbitals.
3- The carbon is bonded by two p-bonds, each of two p-orbitals
participates in the formation of this p- bond
4-The remaining one s-orbital and one p-orbital are equally
involved in the formation of s-bonds, giving rise to the sphybridization state.
So,the presence of sp-hybridized carbons is characteristic for
alkynes.
s- and 2p-bonds in alkynes
p
p-bond
H
C
C
H
s
sp
p-bond
CH
HC
s-bond
Preparation
Preparation of Alkynes
1.Elimination Reactions of Dihalides or
Dihydrohaloalkenes
• Treatment of a 1,2 dihydrohaloalkene with KOH or
NaOH (strong Base) produces a two-fold elimination
of HX
H
C C
CH2 CH3
Cl
H3C
1) 2 NaNH2
2) H3O+
H3C C CH2 CH3
23
dehydrohalogenation of vicinal dihalides
H
H
|
|
—C—C—
|
|
X
X
H
|
+ KOH  — C = C —
|
X
H
|
—C=C—
|
X
+ NaNH2  — C  C — + NaX + NH3
+ KX + H2O
Preparation of Alkynes: Vicinal Dihalides
• Vicinal dihalides are available from addition of
bromine or chlorine to an alkene.
• Intermediate is a vinyl halide.
H
C C
H
Br2
CH2Cl 2
H
Br C C Br
H
2 KOH
+ 2 H2O + 2 KBr
Ethanol
A vicinal dibromide
25
2.Alkylation of Acetylene and Terminal
Alkynes
–
H—C
C :
SN 2
+
R
X
H—C
C—R
+
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.
: X–
Example: Alkylation of Acetylene
NaNH2
HC
HC
CH
CNa
NH3
CH3CH2CH2CH2Br
HC
C
CH2CH2CH2CH3
(70-77%)
Example:
Alkylation of a Terminal Alkyne
(CH3)2CHCH2C
CH
NaNH2, NH3
(CH3)2CHCH2C
CNa
CH3Br
(CH3)2CHCH2C
(81%)
C—CH3
Example: Dialkylation of Acetylene
H—C
C—H
1. NaNH2, NH3
2. CH3CH2Br
CH3CH2—C
C—H
1. NaNH2, NH3
2. CH3Br
CH3CH2—C
(81%)
C—CH3
Reactions of Alkynes
1. Alkyne Adition Reactions
a.Reactions of Alkynes: Addition of HX and X2
• Addition reactions of alkynes are similar to those of
alkenes
• Intermediate alkene reacts further with excess reagent
• Regiospecificity according to Markovnikov
HBr
HC C CH2 CH3
CH2Cl 2
Br
C C
CH2 CH3
H
H
32
b.Addition of Bromine and Chlorine
• Initial addition gives trans intermediate.
• Product with excess reagent is tetrahalide.
HC C CH 2 CH 3
Br2
CH 2Cl 2
Br
H
C C
Br
CH 2 CH 3
Br2
CH 2Cl 2
Br Br H H
H C C C C H
Br Br H H
33
c.Addition of HX to Alkynes Involves Vinylic
Carbocations
• Addition of H-X to alkyne should produce a vinylic carbocation
intermediate
– Secondary vinyl carbocations form less readily than
primary alkyl carbocations
– Primary vinyl carbocations probably do not form at all
C C
+ H Br
H
C C
+ Br-
H
C C
H
C C
Br
Br-
HC CH + H Br
H
C C
-
+ Br
H
C C
Br-
H
C C
Br
34
d.Hydration of Alkynes
• Addition of H-OH as in
alkenes
– Mercury (II) catalyzes
Markovinikov HC C CH2 CH3
oriented addition
– Hydroborationoxidation gives the
non-Markovnikov
product
HC C CH 2 CH 3
OH
C C
CH2 CH3
H
H
HgSO 4, H2SO4
H2O
BH3 THF
H2O2 OH
H
C C
CH 2 CH 3
HO
H
35
Anti- Markinov
e.Mercury(II)-Catalyzed Hydration of Alkynes
• Mercuric ion (as the sulfate) is a Lewis acid catalyst that
promotes addition of water in Markovnikov orientation
• The immediate product is a vinylic alcohol, or enol, which
spontaneously transforms to a ketone
36
Mechanism of Mercury(II)-Catalyzed Hydration of Alkynes
• Addition of Hg(II) to alkyne gives a vinylic cation
• Water adds and loses a proton
• A proton from aqueous acid replaces Hg(II)
37
Keto-enol Tautomerism
• Isomeric compounds that can rapidily interconvert by the
movement of a proton are called tautomers and the
phenomenon is called tautomerism
• Enols rearrange to the isomeric ketone by the rapid
transfer of a proton from the hydroxyl to the alkene
carbon
• The keto form is usually so stable compared to the enol
that only the keto form can be observed
O
OH
H
Rapid
H
H
H
H
Enol Tautomer
Enol Tautomer
(Less favored)
(More favored)
38
f.Hydration of Unsymmetrical Alkynes
• If the alkyl groups at either end of the C-C triple bond are not the
same, both products can form and this is not normally useful
• If the triple bond is at the first carbon of the chain (then H is what is
attached to one side) this is called a terminal alkyne
• Hydration of a terminal always gives the methyl ketone, which is
useful
39
g.Hydroboration/Oxidation of Alkynes
• BH3 (borane) adds to alkynes to give a vinylic borane
• Oxidation with H2O2 produces an enol that converts to the ketone or
aldehyde
• Process converts alkyne to ketone or aldehyde with orientation
opposite to mercuric ion catalyzed hydration
40
Comparison of Hydration of Terminal Alkynes
• Hydroboration/oxidation converts terminal alkynes to
aldehydes because addition of water is non-Markovnikov
• The product from the mercury(II) catalyzed hydration converts
terminal alkynes to methyl ketones
41
2.Reduction of Alkynes
a.Addition of H2 over a metal catalyst (such as palladium on
carbon, Pd/C) converts alkynes to alkanes (complete
reduction)
b.The addition of the first equivalent of H2 produces an
alkene, which is more reactive than the alkyne so the
alkene is not observed
42
c.Conversion of Alkynes to cis-Alkenes
• Addition of H2 using chemically deactivated palladium on calcium
carbonate as a catalyst (the Lindlar catalyst) produces a cis alkene
• The two hydrogens add syn (from the same side of the triple
bond)
43
d.Conversion of Alkynes to trans-Alkenes
• Anhydrous ammonia (NH3) is a liquid below -33 ºC
– Alkali metals dissolve in liquid ammonia and function as
reducing agents
• Alkynes are reduced to trans alkenes with sodium or lithium in
liquid ammonia
• The reaction involves a radical anion intermediate (see Figure
8-4)
44
3.Oxidative Cleavage of Alkynes
• Strong oxidizing reagents (O3 or KMnO4) cleave internal
alkynes, producing two carboxylic acids
• Terminal alkynes are oxidized to a carboxylic acid and carbon
dioxide
• Neither process is useful in modern synthesis – were used to
elucidate structures because the products indicate the
structure of the alkyne precursor
45
4.Alkyne Acidity: Formation of Acetylide Anions
• Terminal alkynes are weak Brønsted acids (alkenes and
alkanes are much less acidic (pKa ~ 25. See Table 8.1 for
comparisons))
• Reaction of strong anhydrous bases with a terminal acetylene
produces an acetylide ion
• The sp-hydbridization at carbon holds negative charge
relatively close to the positive nucleus (see figure 8-5)
46
Alkylation of Acetylide Anions
• Acetylide ions can react as nucleophiles as well as bases
(see Figure 8-6 for mechanism)
• Reaction with a primary alkyl halide produces a
hydrocarbon that contains carbons from both partners,
providing a general route to larger alkynes
47