Alkynes
Chapter 7
1
The Carbon is sp hybridized:
sp
sp
2
There are 2“left over” p orbitals for each C:
3
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-bu tyn e
6,6-D imethyl-3-hep tyn e
1,6-Hep tadiyne
• Common names: prefix the substituents on the
triple bond to the word “acetylene”.
IUPAC name:
Common name:
2-Butyne
D imethylacetylene
1-Buten-3-yne
Vinylacetylene
4
Physical Properties
• Similar to alkanes and alkenes of comparable
molecular weight and carbon skeleton.
Name
Ethyne
Propyne
1-Butyne
Melting
Point
(°C)
-81
Formula
HC CH
CH 3 C CH
CH 3 CH2 C CH
Boiling
Point
(°C)
-84
Density
at 20°C
(g/mL)
-102
-126
-23
8
(a gas)
(a gas)
(a gas)
0.691
0.690
2-Butyne
1-Pentyne
CH 3 C CCH 3
CH 3 ( CH 2 ) 2 C CH
-32
-90
27
40
1-Hexyne
CH 3 ( CH 2 ) 3 C CH
-132
71
0.716
1-Octyne
CH 3 ( CH 2 ) 5 C CH
-79
125
0.746
1-Decyne
CH 3 ( CH 2 ) 7 C CH
-36
174
0.766
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 +
p Ka 25
(S tronger acid)
NH2
H-C C:- + NH3
pK a 38
(Weaker acid)
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
Lith ium diisoprop ylamide
(LD A )
– 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 -
pK a 25
(Weaker acid)
HC
C- +
H2 O
Keq = 10-9.3
pK a 15.7
(Stronger acid)
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°& 3° halides, elimination is the major
reaction.
8
Alkylation of Alkyne Anions
– Alkylation of alkyne anions is the most
convenient method for the synthesis of
terminal alkynes.
-
HC C Na
+
Sodium
acetylid e
+ Na+ Br-
+
Br
1-Bromob utane
1-Hexyne
– Alkylation can be repeated and a terminal
alkyne can be converted to an internal alkyne.
CH3 CH2 C C- Na+
Sodiu m bu tyn ide
+ CH3 CH2 -Br
Bromoethan e
CH3 CH2 C CCH2 CH3
+ Na+ Br9
3-Hexyne
Alkyne Anions as Strong Bases
Because alkyne anions are also strong bases,
alkylation is practical only with methyl and
1°halides, however-
With 2°
° & 3°
° halides, elimination is
the major reaction:
Br
-
+
HC C Na +
S od ium
acetylide
H
Bromocyclohexane
elimination
(Ch 9)
HC CH +
+
+ Na Br
-
Acetylene Cycloh exene
10
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-Buten e
CH2 Cl2
Br Br
CH3 CH-CHCH3 + 2 NaNH2
Sodiu m
amid e
NH3 ( l)
-33o C
CH3 C CCH3
2-Bu tyn e
+
2 NaBr
+
2 NH3
11
Preparation from Alkenes
– For a terminal alkene to a terminal alkyne, 3
moles of base are required:
CH3 ( CH2 ) 3 CH = CH 2
1-Hexene
Br 2
Br Br
CH3 ( CH2 ) 3 CH -CH2
1,2-Dibromohexane
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
12
Addition of X2
• Alkynes add one mole of bromine to give a
dibromoalkene.
– Addition shows anti stereoselectivity.
CH3 C
CCH3 + Br2
Br
H3 C
CH3 COOH, LiBr
an ti ad dition
C
C
Br
CH3
(E)-2,3-D ibromo-2-b utene
2-Bu tyne
13
Addition of X2
– The intermediate in bromination of an alkyne
is a bridged bromonium ion.
Br
Br
H3 C C
C CH 3
Br
C
H3 C
H3 C
Br
C
CH 3
C
H3 C
Br
C
C
CH 3
Br
C
CH 3
Br
14
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= CH2
2-Bromopropene
Br
HBr
CH3 CCH3
Br
2,2-Dibromopropane
Follows Markovnikov’s rule, but why?
15
Addition of HX
– The intermediate in addition of HX is a 2°
vinylic carbocation in preference to a 1°vinylic
carbocation.
+
CH3 C=CH2 + Br
A 2° vin ylic
carbocation
CH3 C CH + H-Br
– Reaction of the vinylic cation (an electrophile)
with halide ion (a nucleophile) gives the
product.
+
CH3 C=CH2 +
Br
Br
CH3 C=CH2
2-Bromopropen e
16
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
1° Carb ocation
s low er
H
CH3 C CH2
Br
Br
faster
H
+
CH 3 C CH2
H
CH3 C CH 2
+ Br
Resonance-stabilized 2° carbocation
Br
Br
CH3 CCH3
Br
17
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
Pd , Pt, or N i
3 atm
2-Butyne
CH3 CH 2 CH2 CH3
Butane
No selectivity-complete reduction!!
18
Reduction
• With the Lindlar catalyst, reduction stops
at addition of one mole of H2.
This reduction shows syn stereoselectivity.
Lin dlar
catalyst
CH3 C CCH3 + H2
2-Butyne
Pt on CaCO3
H3 C
CH3
C
C
H
H
cis -2-Buten e
19
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
NH 3 ( l)
R
H
H
R
+ 2 NaNH 2
H
2 Na
NH 3 ( l)
4-Octyne
H
trans- 4-Octene
20
Hydroboration
• Addition of borane to an internal alkyne
gives a trialkenylborane.
– Addition is syn stereoselective.
+ BH 3
3
T HF
H
3-Hexyne
R
B
R
A trialkenylborane
(R = cis- 3-hexenyl group)
21
Hydroboration
• Treating an alkenylborane with H2O2 in
aqueous NaOH gives an enol.
– Enol: A compound containing an OH group on
one carbon of a carbon-carbon double bond.
1 . BH3
H
OH
2 . H2 O 2 , NaOH
2-Butyne
2-Buten-2-ol
(an enol)
O
Ke q = 6.7 x 106
2-Butanone
(a ketone)
(for keto-enol
tautomerism)
Combination of alkene and alcohol
22
Hydroboration
• An enol is in equilibrium with a keto form by
migration of a hydrogen from oxygen to carbon
and migratrion of 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.
H
O
OH
Ke q = 6.7 x 106
2-Buten-2-ol
(an enol)
2-Butanone
(a ketone)
(for keto-enol
tautomerism)
23
Hydroboration
– To prevent dihydroboration with terminal alkynes, it is
necessary to use a sterically hindered dialkylborane,
such as (sia)2BH.
B-H
D i-sec-isoamylboran e
[(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
24
B-H
D i-sec-isoamylboran e
[(sia)2 BH]
25
R
H
B
R
R
C
CH
NaOH
H 2O 2
R
H
OH
C
CH
26
Hydroboration
– Hydroboration/oxidation of an internal alkyne
gives a ketone:
1 . BH 3
2 . H2 O 2 , Na OH
3-Hexyne
O
3-Hexanone
– Hydroboration/oxidation of a terminal alkyne
gives an aldehyde:
1 . ( sia) 2 BH
2 . H2 O2 , NaOH
1-Octyne
O
OH
H
H
H
A n enol
Octanal
27
Addition of H2O: hydration
• In the presence of sulfuric acid and Hg(II)
salts, alkynes undergo addition of water.
H2 SO 4
CH3 C CH + H2 O
Hg SO 4
Propyne
OH
O
CH3 C= CH2
CH3 CCH3
1-Propen-2-ol
(an enol)
Propanone
(Acetone)
(Omit mechanism)
28
Organic Synthesis
We use a method called a
retrosynthesis and 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.
29
Organic Synthesis
Target molecule: cis-3-hexene.
d isconnect
here
cis-3-Hexene
3-Hexyne
-
:C C:
-
Acetylide
dianion
+
2 CH3 CH2 Br
Bromoethane
30
Organic Synthesis
Starting materials are acetylene and
bromoethane.
HC CH
Acetylene
1 . NaNH2
2 . CH3 CH2 Br
3 . NaNH2
1-Butyne
3-Hexyne
4 . CH3 CH2 Br
5 . H2
Lin dlar
catalyst
cis-3-Hexene
31
Organic Synthesis
Target molecule: 2-heptanone
An acid-catalyzed h yd ration
of th is alkyne gives a
mixture of 2-heptan on e
and 3-heptanone
O
2-Heptyne
2-Hep tanone
-
An acid-catalyzed
hydration of this
alkyne gives 2-heptan on e
1-Heptyne
HC C
+ Br
Acetylide 1-Bromopentane
anion
32
Organic Synthesis
Starting materials are acetylene and
1-bromopentane.
HC CH
1 . NaNH2
2 . Br
1-Hep tyne
O
3 . H2 O
H2 SO4 , HgSO4
2-Heptanone
33