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ALKYNES
F04
Alkynes
Doubly unsaturated C≡C functional group
Terminal alkynes, R-C≡C-H
1)
2)
3)
4)
Nomenclature:
Functional group suffix =
Disubstituted alkynes, R-C≡C-R'
Monosubstituted alkynes, R-C≡C-H
Stability:
Substituted alkynes
The "extra" π bond in an alkyne
than the alkene π
bond by kJ/mol or
kcal/mol
kJ/mol or
kcal/mol
∆Hh 1-hexyne =
∆Hh 1-hexene =
kJ/mol or
kcal/mol
C≡C - C=C =
kJ/mol or
kcal/mol
C=C - C-C =
kJ/mol or
kcal/mol
171
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ALKYNES
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Rank 1-hexyne (terminal) and 3-hexyne(internal) for
each of the following properties:
Heat of
Heat of
hydrogenation
formation
Stability
Heat of
combustion
Structure:
Hybridization:
Shape:
Y C
C X Y C
Physical Properties:
Polarity:
Volatililty:
Melting points:
Boiling points:
Aqueous solubility:
Organic solubility:
Acidity:
pKa =
172
C X
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ALKYNES
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Reactivity:
Alkynes act as nucleophiles.
Addition reactions:
Terminal alkynes: R-C≡C-H
R C
C H
B:
R C
Acetylide ion:
173
C
+ H-B
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ALKYNES
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Reactions:
X X
NaNH2
C C
Alkyne
H H
Preparation
H X
NaNH2
C C
C C
+ 2 HX
C C
+ 2 HX
H X
Hydrogenation
H2 / Catalyst
C C
Metal
Reduction
Na / NH3(l)
C C
H
+ 1 HX
H
H
C C
X
H X
xs HX
C C
C C
C C
H
C C
Hydrogen
Halides
H
C C
H X
Hydration
C C
Halogenation
C C
Ozonolysis
Deprotonation
Alkylation
C C
R
R
C
C
+ H 2O
H+
H
Hg2+
C C
X2
X
+ O3
C
C
Then H2O
H
B:
R
1. NaNH2
H
2. R'X
174
H C C
X
C C
HO
H O
OH
C
O
C
R
C
C
+
O
C
+ H-B
C R'
OH
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ALKYNES
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Preparation of Alkynes
X
X
C C
NaNH2
C C
+ 2 HX
C C
+ 2 HX
H H
H X
C C
NaNH2
H X
Reaction Type:
Overall reaction:
Leaving groups:
Requires:
Regioselectivity:
Stereospecificity:
Mechanism:
H
B:
+ LG
LG
Leaving Group
Base:
Selectivity:
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ALKYNES
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What is the alkyne product from the reactions of the
following with NaNH2 :
1,2-dibromopropane
2,3-dibromooctane
1,2-dibromohexane
2,2-dibromohexane
Synthesis:
Try it: 2-methyl-4-octene Æ 2-methyl-4-octyne
E2 mechanism (For both alkenes and alkynes)
Leaving Groups: -LG
Base: B
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Stereochemistry
E2 reactions
180º
0º
Synperiplanar
Regioselectivity:
Zaitsev's rule:
Zaitsev product:
Hoffman product:
Stereoselectivity:
177
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Addition Reactions of Alkynes
Two factors influence the relative reactivity of alkynes
compared to alkenes:
Hydrogenation of Alkynes
H2 / Catalyst
C C
H
C C
H
Reaction type:
Overall reaction:
Regioselectivity:
Stereoselectivity:
Requirements:
Alkanes:
Alkenes:
Lindlar's catalyst: Pd / CaCO3 / quinoline
Mechanism: CATALYTIC HYDROGENATION
Step 1:
Step 2:
Step 3:
Step 4:
H
H H
HH
HH
178
H
HH
H
H
H H
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Dissolving Metal Reduction of Alkynes
Na / NH3(l)
C C
C C
H
Reaction type:
Overall reaction:
H
Reactivity order:
Regioselectivity:
Stereoselectivity:
Requirements:
Mechanism:
Step 1:
Step 2:
Step 3:
Step 4:
CH3
CH3
C
C
CH3
Na
C
C
+
Na
NH2 H
CH3
H
C
C
CH3
+
Na
CH3
H
C
CH3
C
CH3
NH2 H
CH3
H
H
C
+
+
Na
NaNH2
Synthesis:
How would you prepare trans 2-pentene from cis 2-pentene?
179
C
CH3
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ALKYNES
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Reaction of Alkynes with Hydrogen Halides
C C
C
H
+ 1 HX
H X
xs HX
C
X
C C
C C
H X
Reaction type:
Overall reaction:
Reactivity order:
Regioselectivity:
H
CH3 C
C
H + 1 HX
C
C
CH3
X
X
H +
C
C
CH3
H
H
Stereoselectivity:
Requirements:
CH3 C C H
HX
CH3 C C H
X H
H X
CH3 C C H + CH3 C C H
H X
X H
+ 1 HX
H
C C
hυ CH
3
X
H
+
X
C C
CH3
H
H
Mechanism:
-
+
CH3
C C H
H
CH3
H
180
+
Br
Br
H
H
CH3
H
CH3
Br
H
H
H
Br
CH3
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ALKYNES
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Hydration of Alkynes
C C
H
+ H2O
+
H
Hg
2+
H O
OH
C C
H C C
Reaction type:
Overall reaction:
Regioselectivity:
Stereoselectivity:
Requirements:
Mechanism:
+
CH3
H
+
C C H
CH3
H +
O
H2O
H
H
CH3
H
H
H2O
O
H
H
CH3
H
+
H
+
O
H2O
H
CH3
H
H
Tautomerization:
enol and keto:
O
HN
N H
O
T
H NH
O H
N
N
N
N
NH
HN
O
A
T
181
O
N
NH
H N
NH
H O
G
O
H
CH3
H
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Halogenation of Alkynes
C
X2
C
X
Reaction type:
Overall reaction:
C C
X
Reactivity order:
Regioselectivity:
Stereoselectivity:
Requirements:
Mechanism:
Br
+
Br
Br
Br
_
+
Br
182
Br
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Ozonolysis of Alkynes
C C
+ O3
HO
Then H2O
C
O
+ O
Reaction type:
Overall reaction:
Requirements:
Mechanism:
What would be the products of the ozonolysis:
ethyne ?
1-butyne ?
2-butyne ?
cyclooctyne ?
183
C
OH
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Acidity of Terminal Alkynes
R
C
C
Terminal alkynes:
C C
H
B:
H
R
C
H
C C
C
+ H-B
C C
H
Stability of the conjugate base (i.e. the carbanion).
C
s character: (sp = %, sp2 =
Increased s =
=
Conjugate base of alkyne =
=
Mechanism:
R
C
C
C
C C
C
H
B:
C
% and sp3 =
R
C
C
%)
+ H-B
Could you use a NaOH or NaOEt for this reaction ?
pKa H2O =
pKa CH3CH2OH =
pKa NH3 =
184
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ALKYNES
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Alkylation of Alkynes:
Acetylide ion:
R
C
C
Reaction type:
Overall reaction:
H
1. NaNH2
2. R'X
R
C
C R'
Reactivity order:
Alkyne:
Alkyl halide:
Halide:
Regioselectivity:
Stereoselectivity:
Requirements:
Mechanism:
CH3
H+
_
RCH2Br
NH2
CH3
+ NH3
185
CH3
CH2R
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ALKYNES
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Examples:
Ethylene (acetylene):
Æ monosubstituted (R-C≡C-H):
Æ symmetrical (R = R'): R-C≡C-R':
Æ unsymmetrical (R ≠ R') R-C≡C-R':
Problems to try:
1. What is the product of the reactions of CH3-C≡C- with:
(a) 2-bromopropane
(b) ethanol
(c) 1-iodooctane
(d) ethyl tosylate
(e) (R)-2-bromohexane
(f) bromobenzene
2. What would be the product from the reaction of propene
with the following reagent sequence:
(i) Br2
(ii) NaNH2 , heat
(iii) NaNH2 then MeI
Final product:
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3. For the reaction 2-butyne with HBr:
(i) reaction with 1 equivalent gives 2-bromo-2-butene
(ii) reaction with excess gives 2,2-dibromobutane
(a) What conclusions can you reach about the reactivity
of 2-bromo-2-butene compared to 2-butyne?
(b) Consider the two possible carbocations that could
be formed in the conversion of 2-bromo-2-butene to
2,2-dibromobutane. Suggest a reason for the
observed regioselectivity.
5. Show the major products, with stereochemistry where
applicable, for the reactions of:
With:
1-pentyne
2-pentyne
(i) excess H2 / Pd
(ii) H2 / Lindlar Pd
(iii) Na / NH3
(iv) 1 equiv. HCl
(v) excess HCl
(vi) aq. H2SO4,
HgSO4
(vii) excess Br2
(viii) O3 then H2O
187