Organic
Chemistry
William H. Brown
Christopher S. Foote
Brent L. Iverson
7-1
Alkynes
Chapter 7
7-2
Nomenclature
 IUPAC:
use the infix -yn- to show the presence of
a carbon-carbon triple bond
4
3
2
1
2
3
4
6
7
5
1
2
3
4
6
7
5
1
3-Methyl-1-butyne
6,6-Dimethyl-3-heptyne
1,6-Heptadiyne
 Common
names: prefix the substituents on the
triple bond to the word “acetylene”
IUPAC name:
2-Butyne
1-Buten-3-yne
Common name: Dimethylacetylene Vinylacetylene
7-3
Cycloalkynes
 Cyclononyne
is the smallest cycloalkyne isolated
• it is quite unstable and polymerizes at room temp
• the C-C-C bond angle about the triple bond is
approximately 155°, indicating high angle strain
Cyclononyne
7-4
Physical Properties
 Similar
to alkanes and alkenes of comparable
molecular weight and carbon skeleton
Density
at 20°C
(g/mL)
(a gas)
(a gas)
(a gas)
2-Butyne
1-Pentyne
Melting Boiling
Point
Point
Formula
(°C)
(°C)
-81
-84
HC CH
CH3 C CH
-102
-23
CH 3 CH 2 C CH
-126
8
-32
CH3 C CCH 3
27
CH 3 ( CH2 ) 2 C CH
-90
40
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
Name
Ethyne
Propyne
1-Butyne
0.691
0.690
7-5
Acidity
 The
pKa of acetylene and terminal alkynes is
approximately 25, which makes them stronger
acids than ammonia but weaker acids than
alcohols (Section 4.1)
• terminal alkynes react with sodium amide to form
alkyne anions
H-C C-H +
pKa 25
(Stronger acid)
NH2
H-C C:- + NH3
pKa 38
(Weaker acid)
7-6
Acidity
• terminal alkynes can also be converted to alkyne
anions by reaction with sodium hydride or lithium
diisopropylamide (LDA)
+
–
Na H
Sodium hydride
–
+
[( CH3 ) 2 CH] 2 N Li
Lithium diisopropylamide
(LDA)
• because water is a stronger acid than terminal
alkynes, hydroxide ion is not a strong enough base to
convert a terminal alkyne to an alkyne anion
HC CH + OH pKa 25
W(eaker acid)
HC
C- + H2 O
Keq = 10-9. 3
pKa 15.7
(Stronger acid)
7-7
Alkylation of Alkyne Anions
 Alkyne
anions are both strong bases and good
nucleophiles
 They participate in nucleophilic substitution
reactions with alkyl halides to form new C-C
bonds to alkyl groups; they undergo alkylation
• because alkyne anions are also strong bases,
alkylation is practical only with methyl and 1° halides
• with 2° and 3° halides, elimination is the major reaction
Br
HC C- Na+ +
Sodium
acetylide
H
Bromocyclohexane
elimination
(Ch 9)
HC CH +
+ Na+ Br
Acetylene Cyclohexene
7-8
-
Alkylation of Alkyne Anions
• alkylation of alkyne anions is the most convenient
method for the synthesis of terminal alkynes
-
HC C Na
+
+ Na+ Br-
+
Br
Sodium
acetylide
1-Bromobutane
1-Hexyne
• alkylation can be repeated and a terminal alkyne can
be converted to an internal alkyne
CH3 CH2 C C- Na+ + CH3 CH2 -Br
Sodium butynide
Bromoethane
CH3 CH2 C CCH2 CH3
3-Hexyne
+ Na+ Br-
7-9
Preparation from Alkenes
 Treatment
of a vicinal dibromoalkane with two
moles of base, most commonly sodium amide,
results in two successive dehydrohalogenation
reactions (removal of H and X from adjacent
carbons) and formation of an alkyne
CH3 CH=CHCH3 + Br2
2-Butene
CH2 Cl2
Br Br
CH3 CH-CHCH3 + 2 NaNH2
Sodium
amide
NH3 ( l)
-33o C
CH3 C CCH3
2-Butyne
+
2 NaBr
+
2 NH3
7-10
Preparation from Alkenes
• for a terminal alkene to a terminal alkyne, 3 moles of
base are required
CH3 ( CH2 ) 3 CH = CH 2
Br Br
Br2
CH3 ( CH2 ) 3 CH -CH2
1,2-Dibromohexane
1-Hexene
-
CH3 ( CH2 ) 3 C C N a
+
Sodium salt of 1-hexyne
H2 O
3 N aN H2
- 2 HBr
CH3 ( CH2 ) 3 C CH
1-Hexyne
7-11
Preparation from Alkenes
• a side product may be an allene, a compound
containing adjacent carbon-carbon double bonds,
C=C=C
R
H
C C C
H X H
RC–C= CR
R
NaNH 2
- HBr
A haloalkene
(a vinylic halide)
R
R
An allene
H
RCC CR
R
An alkyne
7-12
Allene
 Allene:
a compound containing a C=C=C group
• the simplest allene is 1,2-propadiene, commonly
named allene
7-13
Allenes
• most allenes are less stable than their isomeric
alkynes, and are generally only minor products in
alkyne-forming dehydrohalogenation reactions
CH2 =C=CH2
CH2 =C=CHCH3
CH3 C CH
H0 = -6.7 kJ (-1.6 kcal)/mol
CH3 C CCH3
H0 = -16.7 kJ (-4.0 kcal)/mol
7-14
Addition of X2
 Alkynes
add one mole of bromine to give a
dibromoalkene
• addition shows anti stereoselectivity
CH3 C
CCH3 + Br2
2-Butyne
CH3 COOH, LiBr
anti addition
Br
H3 C
C
C
Br
CH3
(E)-2,3-Dibromo-2-butene
7-15
Addition of X2
• the intermediate in bromination of an alkyne is a
bridged bromonium ion
Br
Br
H3 C C
C CH3
Br
C
H3 C
H3 C
Br
C
CH3
C
H3 C
Br
C
C
CH3
Br
C
CH3
Br
7-16
Addition of HX
 Alkynes
undergo regioselective addition of either
1 or 2 moles of HX, depending on the ratios in
which the alkyne and halogen acid are mixed
Br
CH3 C CH
Propyne
HBr
CH3 C= CH 2
2-Bromopropene
Br
HBr
CH3 CCH 3
Br
2,2-Dibromopropane
7-17
Addition of HX
• the intermediate in addition of HX is a 2° vinylic
carbocation
CH3 C CH + H-Br
+
CH3 C=CH2 + Br
A 2° vinylic
carbocation
• reaction of the vinylic cation (an electrophile) with
halide ion (a nucleophile) gives the product
+
CH3 C=CH2 +
Br
Br
CH3 C=CH2
2-Bromopropene
7-18
Addition of HX
• in the addition of the second mole of HX, Step 1 is
reaction of the electron pair of the remaining pi bond
with HBr to form a carbocation
• of the two possible carbocations, the favored one is
the resonance-stabilized 2° carbocation
Br
H
CH3 C CH2
Br
H
+
slower
CH3 C CH2
Br
1° Carbocation
Br
faster
H
+
CH3 C CH2
H
CH3 C CH2
+ Br
Resonance-stabilized 2° carbocation
Br
Br
CH3 CCH3
Br
7-19
Hydroboration
 Addition
of borane to an internal alkyne gives a
trialkenylborane
• addition is syn stereoselective
+ BH3
3-Hexyne
THF
H
B
R
R
A trialkenylborane
(R = cis-3-hexenyl group)
7-20
Hydroboration
• to prevent dihydroboration with terminal alkynes, it is
necessary to use a sterically hindered dialkylborane,
such as (sia)2BH
B-H
Di-sec-isoamylborane
[(sia)2 BH]
• treatment of a terminal alkyne with (sia)2BH results in
stereoselective and regioselective hydroboration
H
+ ( sia ) 2 BH
1-Octyne
B( sia ) 2
H
An alkenylborane
7-21
Hydroboration
 Treating
an alkenylborane with H2O2 in aqueous
NaOH gives an enol
1 . BH3
2-Butyne
2 . H2 O2 , NaOH
H
OH
2-Buten-2-ol
(an enol)
O
Ke q = 6.7 x 106
2-Butanone
(a ketone)
(for keto-enol
tautomerism)
• enol: a compound containing an OH group on one
carbon of a carbon-carbon double bond
• an enol is in equilibrium with a keto form by migration
of a hydrogen from oxygen to carbon and the double
bond from C=C to C=O
• keto forms generally predominate at equilibrium
• keto and enol forms are tautomers and their
interconversion is called tautomerism
7-22
Hydroboration
• hydroboration/oxidation of an internal alkyne gives a
ketone
1 . BH 3
2 . H 2 O 2 , NaOH
3-Hexyne
O
3-Hexanone
• hydroboration/oxidation of a terminal alkyne gives an
aldehyde
1 . ( sia) 2 BH
1-Octyne
2 . H2 O2 , NaOH
O
OH
H
H
H
An enol
Octanal
7-23
Addition of H2O: hydration
 In
the presence of sulfuric acid and Hg(II) salts,
alkynes undergo addition of water
H2 SO4
CH3 C CH + H2 O
HgSO 4
Propyne
OH
O
CH3 C= CH 2
1-Propen-2-ol
(an enol)
CH3 CCH 3
Propanone
(Acetone)
7-24
Addition of H2O: hydration
• Step 1: attack of Hg2+ (an electrophile) on the triple
bond (a nucleophile) gives a bridged mercurinium ion
• Step 2: attack of water (a nucleophile) on the bridged
mercurinium ion intermediate (an electrophile) opens
the three-membered ring
Hg
+
+
C
C
H3 C
C C
H
H
O
H
Hg+
H3 C
+
H
O
H
H
7-25
Addition of H2O: hydration
• Step 3: proton transfer to solvent gives an
organomercury enol
H
O
H
Hg +
H3 C
+
H3 C
C C
O+
H
H
Hg
C
H
O
+
+
C
H3 O+
H
H
• Step 4: tautomerism of the enol gives the keto form
+
H3 C
Hg
C C
HO
H
O
+
CH3 -C-CH2 -Hg
Keto form
Enol form
7-26
Addition of H2O: hydration
• Step 5: proton transfer to the carbonyl oxygen gives
an oxonium ion
+
H
O
+
CH3 -C-CH2 -Hg + H2 O:
O
+
H O H + CH3 -C-CH2 -Hg+
H
• Steps 6 and 7: loss of Hg2+ gives an enol; tautomerism
of the enol gives the ketone
H+
O
CH3 -C-CH2 -Hg+
H
Hg2+
O
+ CH3 -C=CH2
(enol form)
O
CH3 -C-CH3
(Keto form)
7-27
Reduction
 Treatment
of an alkyne with hydrogen in the
presence of a transition metal catalyst, most
commonly Pd, Pt, or Ni, converts the alkyne to an
alkane
CH3 C CCH3 + 2 H2
2-Butyne
Pd, Pt, or Ni
CH3 CH2 CH2 CH3
3 atm
Butane
7-28
Reduction
 With
the Lindlar catalyst, reduction stops at
addition of one mole of H2
• this reduction shows syn stereoselectivity
CH3 C CCH3 + H2
2-Butyne
Lindlar
catalyst
H3 C
CH3
C
C
H
H
cis-2-Butene
7-29
Hydroboration - Protonolysis
 Addition
of borane to an internal alkyne gives a
trialkenylborane
• addition is syn stereoselective
+ BH3
3-Hexyne
THF
H
B
R
R
A trialkenylborane
(R = cis-3-hexenyl group)
• treatment of a trialkenylborane with acetic acid results
in stereoselective replacement of B by H
O
+ 3 CH3 COH
H
B
R
R
A trialkenylborane
+ ( CH3 COO) 3 B
H
H
cis-3-Hexene
7-30
Dissolving Metal Reduction
 Reduction
of an alkyne with Na or Li in liquid
ammonia converts an alkyne to an alkene with
anti stereoselectivity
R
R' + 2 Na
R
H
NH3 (l) H
R'
+ 2 NaNH2
H
2 Na
NH3 ( l)
4-Octyne
H
trans-4-Octene
7-31
Dissolving Metal Reduction
• Step 1: a one-electron reduction of the alkyne gives a
radical anion
:
.
C-R + N a
.
R-C
+
R-C
C-R + N a
An alkenyl
radical anion
• Step 2: the alkenyl radical anion (a very strong base)
abstracts a proton from ammonia (a very weak acid)
R
••
R- C
C-R + H
N H
H
C
R
+
C
H
An alkenyl
radical
N H
H
Amide
ion
7-32
Dissolving Metal Reduction
• Step 3: a second one-electron reduction gives an
alkenyl anion
• this step establishes the configuration of the alkene
• a trans alkenyl anion is more stable than its cis isomer
+
Na + +
C C
R
R
:
Na
R
.
C C
R
H
An alkenyl anion
H
• Step 4: a second acid-base reaction gives the trans
alkene
R
H N H +
H
C
R
C
H N
H
R
H
H
Amide ion
+
C
R
C
H
A trans alkene7-33
Organic Synthesis
A
successful synthesis must
• provide the desired product in maximum yield
• have the maximum control of stereochemistry and
regiochemistry
• do minimum damage to the environment (it must be a
“green” synthesis)
 Our
strategy will be to work backwards from the
target molecule
7-34
Organic Synthesis
 We
analyze a target molecule in the following
ways
• the carbon skeleton: how can we put it together. Our
only method to date for forming new a C-C bond is the
alkylation of alkyne anions (Section 7.5)
• the functional groups: what are they, how can they be
used in forming the carbon-skeleton of the target
molecule, and how can they be changed to give the
functional groups of the target molecule
7-35
Organic Synthesis
 We
use a method called a retrosynthesis and use
an open arrow to symbolize a step in a
retrosynthesis
target
molecule
starting
materials
 Retrosynthesis:
a process of reasoning
backwards from a target molecule to a set of
suitable starting materials
7-36
Organic Synthesis
 Target
molecule: cis-3-hexene
disconnect
here
cis-3-Hexene
3-Hexyne
-
:C C:
-
Acetylide
dianion
+
2 CH3 CH2 Br
Bromoethane
7-37
Organic Synthesis
• starting materials are acetylene and bromoethane
HC CH
Acetylene
1 . NaNH2
2 . CH3 CH2 Br
1-Butyne
3-Hexyne
3 . NaNH2
4 . CH3 CH2 Br
5 . H2
Lindlar
catalyst
cis-3-Hexene
7-38
Organic Synthesis
 Target
molecule: 2-heptanone
An acid-catalyzed hydration
of this alkyne gives a
mixture of 2-heptanone
and 3-heptanone
O
2-Heptyne
2-Heptanone
HC C-
An acid-catalyzed
hydration of this
alkyne gives 2-heptanone
1-Heptyne
+ Br
Acetylide 1-Bromopentane
anion
7-39
Organic Synthesis
• starting materials are acetylene and 1-bromopentane
HC CH
1 . NaNH2
2 . Br
1-Heptyne
O
3 . H2 O
H2 SO4 , HgSO4
2-Heptanone
7-40
Alkynes
End Chapter 7
7-41