Chapter 9: Alkynes
9.1: Sources of Alkynes (please read)
9.2: Nomenclature
Systematic Nomenclature: Prefix-Parent-Suffix
Naming Alkynes: Suffix: -yne
Many of the same rules for alkenes apply to alkynes
1. Number the carbon chain from the end of the carbon nearest
the triple bond
2. The alkyne position is indicated by the number of the alkyne
carbon in the chain
3. Compounds with two triple bonds are referred to as diynes,
three triple bonds as triynes, etc
9.3: Physical Properties of Alkynes (please read)
205
9.4: Structure and Bonding in Alkynes: sp Hybridization
H
C
C
H
acetylene
(ethyne)
bond angles:
H-C-C = 180° (linear geometry
bond distances:
C-H = 106 pm
CC = 120 pm
Each carbon is sp hybridized – linear geometry
CC bond consists of one σ–bond (sp hybridized orbitals) and
two π–bond (unhybridized p-orbitals) (see ch. 2 notes)
Bond dissociation energies (ΔH°C-C)
H° C=C = 611 KJ/mol H° CC = 820 KJ/mol
H° C–C = 368 KJ/mol H° C–C = 368 KJ/mol
 -bond = 243 KJ/mol
-bonds = 452 KJ/mol
226 KJ/mol per π -bond
The -bond of an alkene is ~17 KJ/mol more stable
than the -bond of an alkyne.
206
H
H
H
~111°
111 pm
H
H
106 pm
C
C
H
H
134 pm
H
153 pm
H
180°
110 pm
C
C
~121°
C
C
H
120 pm
H
H
hybridization
of C
geometry
sp3
sp2
sp
tetrahedral
trigonal planar
linear
ΔH°C
368 kJ/mol
611 kJ/mol
820 kJ/mol
-C
ΔH°
410 kJ/mol
452 kJ/mol
536 kJ/mol
25%
33%
50%
62
45
26
C-H
% s character
pKa
207
9.5: Acidity of Acetylene and Terminal Alkynes
In general, the C-H bond of hydrocarbons are very weak acids.
The R-CC-H bond of a terminal acetylene is weakly acid,
pKa ~26.
H
H
H C H
H C
H
H
+ H+
pKa = 60
+ H+
pKa = 45
+ H+
pKa = 26
H
H
_
H
C C
H
_
C C
H
H
H
R C C
R C C H
_
A strong base can deprotonate terminal acetylenes to generate
an acetylide anion.
R C C H
pKa = 26
+ B:
_
Na +
R C C
Na +
_
+
B:H
pKa =
208
9.6: Preparation of Alkynes by Alkylation of Acetylene and
Terminal Alkynes - Acetylide anions are strong nucleophiles
and will undergo nucleophilic substitution reactions with primary
alkyl halides, resulting in the formation of a C-C bond.
R C C
R
_
+
H
Na +
R
THF
C Br
SN2
H
+
R C C C
H
NaBr
H
new C-C
bond formed
Alkylation of acetylide anions is a general method of making
higher alkynes from simpler alkynes.
+ H2N: Na+
H C C H
H C C
+
H3CH2CH2C-Br
H C C
H3CH2CH2C C C H
pentyne
H3CH2CH2C C C H
pentyne
H3CH2CH2C C C
+ H2N: Na+
+
H3C-I
H3CH2CH2C C C
H3CH2CH2C C C CH3
2-hexyne
209
Alkylation of acetylide anions is generally limited to primary alkyl
halides. Acetylide anions act as a base with secondary and
tertiary alkyl halides resulting in E2 elimination.
9.7: Preparation of Alkynes by Elimination Reactions
Double dehydrohalogenation reaction
R
R
X2
H X
R
R
NaNH2
H X
H
vicinal
dihalide
R
R
NaNH2
R
R
X
H H
R
R
NaNH2
X X
geminal
dihalide
3 equivalents of NaNH2 are required for preparing terminal
alkynes from from 1,2- or 1,1-dihaloalkane.
210
9.8: Reactions of Alkynes - alkynes undergo addition reactions
similar to alkenes.
Increasing oxidation state
C C
C C
C C
Cl
C Cl
Cl
C Cl
C Cl
Cl
O
C OH
C O
C
OR
9.9: Hydrogenation of Alkynes - The hydrogenation of an
alkyne cannot be stopped at the alkene stage under normal
hydrogenation conditions (H2, metal catalyst)
R1 C C R2
H2, Pd/C
H
H
C C
R1
R2
cis alkene
intermediate
H2, Pd/C
R1
H H
C C R2
H H
211
The π-bonds of alkenes are slightly more reactive toward
hydrogenation than the π-bond of an alkene.
Lindlar’s catalyst: “poisoned” palladium catalyst
Pd on CaCO3 + Pb(OAc)4 + quinoline (amine)
“poisons” reduce the catalysts activity so only the most
reactive functional groups are hydrogenated
H2,
Lindlar's Catalyst
H
R1 C C R2
H
C C
R1
R2
cis -addition
of H2
Hydrognation can be stopped at the cis-alkene stage with
Lindlar’s catalyst
9.10: Metal-Ammonia Reduction of Alkynes - Dissolving
Metal Reduction: Li(0) metal in liquid ammonia (NH3)
e•
Li(0) in NH3
R1 C C R2
Li, NH3, (CH3)3COH
H
(solvated electron)
R2
C C
R1
H
trans-alkene
212
9.11: Addition of Hydrogen Halides to Alkynes
Addition of HX to acetylenes: Markovnikov addition
X
HX
R C C H
only useful for
terminal or
symmetrical
acetylenes
(R1 = R2)
C
R
C
H
H
vinyl halide
X
HX
R1 C C R2
R1
C
C
R2
H
trans addition
of HX
Mechanism ?
HX
R C C H
R
+
C
C
H
H
vinyl
carbocation
rate = k [alkyne][HX]2
213
Carbocation Stability
R
R
R
R
H
H
H
+
R C C
H
3°
alkyl
2°
alkyl
1°
alkyl
2°
vinyl
R C+
>
R C+
>
H C+
~
H
>
H C+
H
+
H C C
H
~
H
1°
vinyl
methyl
carbocation
gem-dihaloalkanes are obtained with excess HX - addition
follows Markovnikov’s rule
X
HX
R1 C C R2
excess
R1
C
X X
R2
C
R1
C
H H
HX
C
R2
H
R2=H or R2=R1
gem-dihaloalkane
Anti-Markovnikov addition of HBr to terminal alkynes in the
Presence of peroxides (radical mechanism)
R C C H
HBr
peroxides
H
HX
R1
C
C
H
Br
R1
C
C
H
Br
214
9.13: Addition of Halogens to Alkynes
X
X2
R1 C C R2
R1
C
C
X X
R2
C
R1
C
X X
X2
R2
excess
X
anti addition
of X2
9.12: Hydration of Alkynes
Alkenes
Alkynes
hydration
Alcohols
hydration
Ketones or Aldehydes
Hg (II) catalyzed hydration of alkynes: similar to oxymercuration
- Markovnikov Addition
R C C H
OH
HgSO4, H2SO4,
H2O
R1
C
C
H
enol
H
O
tautomerization
R
CH3
methyl ketone
215
Tautomerization - equilibrium between two isomers (tautomer),
which differ by the migration of a proton and a -bond.
Keto-enol tautomerization - normally favors the keto form (C=O)
C=C
C-O
O-H
H° = 611 KJ/mol
380
426
O
C
H
O
C
C
enol
C
H
C=O
C-C
C-H
H° = 735 KJ/mol
368
420
keto
Keto-enol tautomerization is acid-catalyzed
216
Hydroboration of Alkynes - anti-Markovnikov hydration of an
alkyne (complementary to the Hg(II) catalyzed method)
R C C H
1) BH3, THF
2) H2O2, NaOH
O
R
H
Aldehyde
product
Hg(II)-catalyzed hydration and hydroboration are equivalent
for internal alkynes
1) BH3, THF
2) H2O2, NaOH
R1 C C R2
R1
HgSO4, H2SO4,
H2O
O
O
R2
+
R1
R2
R1 C C R2
Ketone products
Hydration of an internal alkyne is only useful
for symmetrical acetylenes (R1 = R2)
217
9.14: Ozonolysis of Alkynes
Ozonolysis of alkenes (sect. XX):
R1
R2
R3
1) O3
2) Zn
R1
R3
O
R4
+
ketones and
aldehydes
O
R2
R4
Ozonolysis of Alkynes
1) O3
2) Zn
R C C H
O
+
R C
H2CO3
OH
terminal alkyne
Carboxylic acid
R1 C C R2
1) O3
2) Zn
+
R1 C
OH
internal alkyne
O
O
C R2
HO
Carboxylic acids
Alkynes are less reactive toward ozonolysis than alkenes. An
alkenes can be oxidatively cleaved by ozone in the presence
218
of an alkyne.
C-C bond forming reactions are important for organic synthesis
Prepare octane from 1-pentyne
H3CH2CH2C C CH
CH3CH2CH2CH2CH2CH2CH2CH3
Prepare (Z)-2-hexene from 1-pentyne
H3CH2CH2C C CH
Z-2-hexene
Prepare 4-octanone from acetylene
O
HC CH
219