Alkynes
A. Structure
Suggested Reading: Chapter 8 in McMurry
Suggested Problems: 8.1, 8.3-8.14, 8.19-8.47
B. Nomenclature
C. Reactions
1. Addition of HX (geminal dihalide)
2. Addition of X2 (tetrahalide)
3. Hg2+ Catalyzed Hydration
4. Hydroboration/Oxidation
5. Reduction
a. alkane
b. cis-alkene
c. trans-alkene
6. Oxidative Cleavage
7. Alkylation Reactions
D. Retrosynthetic Analysis
ALKYNES!
H HO
O
H3C C C C C C C C
H3C C C C C C
Capillin
(fungicidal activity)
C
Ichthyothereol H
O
(convulsant used by Amazon Indians for
poisoned arrowheads)
H3C
OH
CH
C
Norquen Ovastol
(in oral contraceptives)
H3CO
1
Alkyne Nomenclature
1. Follow alkene rules, but use -yne as suffix.
2. If more than one triple bond is present, use -diyne, -triyne, tetrayne . . .
3. If both double and triple bonds are present,
use -enyne as suffix
Number from side with nearest multiple bond (either double or triple)
If a double and triple bond is equidistant, make the double bond the
lower number
H3C C C CH3
Br
CH3
H2
H3C C C C C C CH3
H
H
2-butyne
6-bromo-2-methyl-3-heptyne
CH3
HC C CH2CH2CH2CH2CH CH2
HC C CH2CHCH2CH2CH
1-octen-7-yne
CHCH3
4-methyl-7-nonen-1-yne
Alkynes: Common Names and Groups
R C C H
R C C R
terminal alkyne
internal alkyne
C
HC CH
HC C CH3
acetylene
methylacetylene
H2 H2
C C CH2CH3
butyl
C C CH2CH3
H H
butenyl
CH
phenylacetylene
C C CH2CH3
butynyl
H2
C C C H
propargyl
2
KetoKeto-Enol Tautomerism
Tautomers - constitutional isomers that differ in the location of a H and a double bond
Tautomerization - conversion between two tautomers
H
H O
H
H
H
R C CH3
acidic conditions
R
H
O
O H
O
O
R C CH3
R C CH3
H
H
O
H
O H
H O
R
H
H O
O
R C CH3
H
Enol
(alkene and alcohol)
H
R
Keto
(favored)
O
R C CH3
H
O H
basic conditions
H
O
H
Addition of Halogens to Alkynes
For Terminal Alkynes:
(internal alkynes result in a mixture of products)
R
H
H-X (1eq.)
X = Cl, Br, I
R
X
H
H-X (1eq.)
H
X H
geminal dihalide
vinylic halide
(E or Z)
Markovnikov
Carbocation intermediate
R
H
X-X (1eq.)
X = Cl, Br
R
X
X
H
X H
R C C H
Markovnikov
syn + anti addition
Carbocation intermediate
X-X (1eq.)
X
X
R C C H
X
X
trans-dihalide
tetrahalide
Markovnikov
Markovnikov
anti addition
halonium ion intermediate
3
Hydration of Terminal Alkynes
R
H
HO
O
H
R C CH3
R
Terminal alkynes
R
HgSO4, H2SO4, H2O
H
methyl ketone
enol
Markovnikov
carbocation intermediate
H
1. disiamylborane,
THF
2. H2O2, -OH, H2O
H
OH
R
H
enol
Anti-Markovnikov
Concerted
syn addition
B
O
H2
R C CH
aldehyde
Hydration of
internal alkynes
leads to a mixture
of ketones under
both conditions
H
Disiamylborane is
typically used instead
of BH 3 to sterically
block the addition of a
second borane to the
same alkyne
Reduction of Alkynes
R C C R
H2, Lindlar's catalyst
H
H2, Pd/C
H
Na or Li, NH3
H
R
R
H
R CH2CH2H
R
R
cis
trans
Lindlar’s catalyst
Deactivated (poisoned) catalyst
Pd, CaCO 3, lead acetate, quinoline
4
Oxidation of Alkynes
Internal Alkyne
R C C R'
1. O3, 2. H2O
or
KMnO4, H3O+/H2O
R
OH
O
R'
HO
O
Terminal Alkyne
R C C H
1. O3, 2. H2O
or
KMnO4, H3O+/H2O
R
OH
CO2
O
5