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Alkynes
State University of New York at Albany
Alkynes:
Are hydrocarbons that contain carbon-carbon
triple bonds.
 Are also known as acetylenes.
 Have the general structural formula CnH2n-2.
 Are similar in physical properties to alkanes
and alkenes. They are nonpolar and water
insoluble. The triple bond is shorter than C=C
and C-C bonds with a length of 1.20 Å.
 The most important commercial alkyne is
acetylene, which is used as a fuel.

Nomenclature of Alkynes


Similar to that of alkenes, except the ending of the
root name corresponding to the longest continuous
chain containing the triple bond is changed from
“ene” to “yne”. E.g. ethene becomes ethyne,
propene becomes propyne. The chain is numbered
from the end closest to the triple bond.
When a double and a triple bond is present, the
compound is named as an “alkenyne”. When a
triple bond and an OH are present, the compound is
named as an alkynol.
Examples:
H
C
C
H
H
C
ethyne
(acetylene)
H2C
C
C
C
CH3
H3C
propyne
C
C
CH3
H3C
CH3
2-butyne
CH3
CH3
C
CH
CH3
C
C
CH2 CH
CH3
6-bromo-2-ethyl-3-pentyne
4-methyl-1-penten-2-yne
Common names:
H
C
C
acetylene
H
Ph
C
C
Ph
diphenylacetylene
Ph
C
C
Ph
ethylmethylacetylene
Alkynes can be described as internal or
terminal. If the triple bond is between two
carbons, the alkyne is internal. If it is flanked
by a hydrogen and a carbon, it is a terminal
alkyne. The terminal hydrogen of a terminal
alkyne is called the acetylenic hydrogen. In
general, internal alkynes are more stable
than terminal alkynes.
H3C
C
C
CH3
2-butyne
an internal alkyne
CH3
CH2 C
C
1-butyne
a terminal alkyne
H
acetylenic
hydrogen
Acidity of Alkynes
Recall that the amount of s character of sp, sp2,
and sp3 hybrid orbitals is ,  and 
respectively. As the s character increases, so
does the acidity. Therefore, acetylenic protons are
relatively acidic, with a pKa of ~ 25. Acetylenic
protons are more acidic than vinyl (pKa = 44) or
alkane (pka = 50) protons. Strong bases such as
-NH (but not alkoxide and hydroxide ions) can
2
deprotonate alkynes.
 Internal alkynes do not have acetylenic protons,
so they cannot undergo this type of deprotonation.

Acidity of Alkynes—Deprotonation.
When terminal alkynes are deprotonated by very
strong base, the resulting anion is called an
acetylide ion.
 Deprotonation of terminal alkynes is commonly
done with NH2, in the form of NaNH2 (sodium
amide or “sodamide”).
 Acetylide ions are strong nucleophiles.

from NaNH2
CH3
CH2 C
C
H
NH2
CH3
CH2 C
C
Na
sodium acetylide
(sodium butynide)
Acidity of Alkynes—Heavy Metal Acetylides.
Silver (I) and copper (I) salts react with terminal
alkynes to form silver and copper acetylides.
 Silver and copper acetylides are much less basic
and less nucleophilic than their sodium counterpart. Thus, they don’t have much synthetic utility.
 Silver and copper acetylides are not very soluble,
and so they form characteristic precipitates. This
forms the basis for a simple chemical test for
terminal alkynes. Internal alkynes do not react
and form a precipitate because they don’t have
an acidic proton.

Examples:
CH3
CH2 C
C
H
Ag
CH3
CH2 C
C
Ag
a light colored precipitate
CH3
CH2 C
C
H
Cu
CH3
CH2 C
C
Ag
a brick red precipitate
H3C
C
C
CH3
Ag or Cu
No reaction
Treatment of the silver or copper acetylide
with acid (e.g. HCl) yields the original
alkyne.
Synthesis of Alkynes
1. Alkylation of Acetylide Ions: Acetylide ions react with
primary or methyl halides via an SN2 mechanism to
produce elongated alkynes. If 2o or 3o alkyl halides are
used, the acetylide ion may act as a base, yielding an
alkene by an E2 mechanism.
H
1. NaNH2
CH2CH3
2. CH3CH2Br
Br
CH3
CH2 C
C
+
E2
CH3
CH2 C
C
H +
Synthesis of Alkynes
2. Addition of Acetylide Ions to carbonyl groups: A
carbonyl is a C=O group. Because the C=O is polarized,
there is a + charge on carbon, and a - charge on oxygen.
Therefore, the carbonyl carbon iselectrophilic. Acetylide
ions can attack carbonyl carbonsto give alkoxide ions
which when protonated, yield alcohols.
CH3
CH3
CH3 C
C
C

O

CH3 C
C
C
H
H
CH3
CH3 C
C
C
H
CH3
O
H
CH3 C
C
C
OH
H
acetylenic alcohol
O
Synthesis of Alkynes
3. Synthesis of Alkynes by Elimination Reactions:
Elimination of two molecules of HX from a geminal or
vicinal dihalide yields an alkyne. When the base used is
KOH, the product formed is the internal alkyne. When the
base used is NaNH2, the terminal alkyne is formed.
Cl
KOH, 200 oC
CH3CH2 C
C CH3
Cl
Cl
NaNH2, 150 oC
Cl
CH3CH2CH2 C
C H
Addition Reactions of Alkynes
Reagents add to the triple bond of alkynes just as they
add across the double bond of alkenes. Since alkynes
have two triple bonds, up to two molecules can add
across the triple bond, depending upon the reagents
and conditions.
1a. Reduction to Alkanes: Hydrogen adds to alkynes in
the presence of a catalyst to form alkanes.
H
CH3CH2 C
C CH3
Pt, Pd, or Ni
CH3CH2 C
H
H
C CH3
H
Addition Reactions of Alkynes
1b. Hydrogenation to cis Alkenes: Cis alkenes can be
formed from alkynes by using Lindlars catalyst;
internal alkynes yield the cis-product.
Lindlar's catalyst
CH3CH2 C
C CH3
H2, Pd/BaSO4
quinoline, CH3OH
Addition Reactions of Alkynes
1c. Metal-Ammonia Reduction to trans Alkenes:
Treatment of alkynes with sodium in liquid ammonia
generates alkynes with a trans orientation.
CH3CH2 C
C CH3
Na/NH3
2. Addition of Halogens: Br2 and Cl2 add to alkynes
just as they add to alkenes. If the alkene forms, there
is often a mixture of cis and trans isomers. Often the
reaction proceeds all the way to the tetrahalide.
Br
CH3CH2 C
C CH3
Br2
+
Br 72%
Br
Br
28%
Addition of HX to Alkynes
HX adds to alkynes just as it does to alkenes. When HX
is added to terminal alkynes, Markovnikov orientation
is observed. When two moles of HX are added, a
geminal dihalide is formed. In the addition of HBr in the
presence of peroxides, anti-Markovnikov orientation is
observed.
Examples:
CH3 C
C H
HCl
CH3 C
CH2
Cl
CH3 C
C H
2 HCl
Cl
CH3 C
Cl
CH3
Mechanism
1. CH3 C
C H
H
CH3 C
CH2
vinyl cation
intermediate
2. CH3 C
CH2
Cl
CH3 C
Cl
CH2
Hydration of Alkynes to Aldehydes
and Ketones
1. Mercuric Ion Catalyzed Hydration: In the presence
of H3O+ and mercuric ion catalyst, water adds across
the triple bond with Markovnikov orientation.
Aldehydes and ketones are produced in the process.
H
CH3 C
C H
HgSO4
H
H2SO4
H
OH
a vinyl alcohol (enol)
O
Mechanism
H
Hg
1. CH3 C
C H
CH3 C
C
Hg
H
H
2. CH3 C
H O
H
OH2
C
C
Hg
H3C
C
Hg
Mechanism (continued):
OH2
H
3. H O
H
H
C
O
C
H3C
H
C
Hg
C
H3C
+ H3O
Hg
vinyl mercurial alcohol
H
4.
O
H
C
H3C
HO
H3O
C
Hg
H
C
H3C
C
H
enol
Mechanism (continued):
Vinyl alcohols (enols) are very unstable, and isomerize
to the ketone form. This type of rapid equilibrium is
called tautomerism. Tautomerism is NOT resonance!
HO
H
C
H3C
HO
H
C
H
enol
H
C
C
H3C
H
H
keto-enol
tautomerism
H
O
H
C
H3C
C
O
H
H
OH2
C
H3C
CH3
Hydroboration-Oxidation of Alkynes
Hydroboration of alkynes with diisoamyl borane results
in the formation of a vinyl alcohol with anti-Markovnikov
orientation. This vinyl alcohol then tautomerizes to
an aldehyde.
H
CH3 C
C H
1. Sia2BH
2. H2O2/OH
CH3 CH2 C
O
Steps:
Sia2BH
1. CH3 C
CH3
C H
H
C
C
H
BSia2
vinyl borane
CH3
C
2.
C
H
C
NaOH
C
H
BSia2
CH3
H
H2O2
C
H
3.
CH3
H
OH
enol
H
H
OH
C
CH3 CH2 C
OH
O
aldehyde
Permanganate Oxidation of Alkynes
Treatment of alkynes with permanganate under
neutral conditions results in the formation of
-diketones.
CH3CH2 C
C CH3
KMnO4
H2O, neutral
CH3CH2 C
C CH3
O
O
2,3-pentanedione
Permanganate Oxidation of Alkynes
If the reaction mixture becomes too warm or too
basic, the diketone undergoes oxidative cleavage
to form salts of carboxylic acids
CH3CH2 C
C CH3
KMnO4/KOH
H2O, heat
CH3CH2 C O
+
O
C CH3
O
O
HCl/H2O
CH3CH2 C OH + HO
O
C CH3
O
Permanganate Oxidation of Alkynes
Terminal alkynes are cleaved similarly to give a
carboxylic acid and CO2.
CH3CH2 C
C H
1. KMnO4/KOH
H2O, heat
CH3CH2 C OH + CO2
2. H
O
Ozonolysis of Alkynes
Ozonolysis of an alkyne gives products similar
to those obtained from oxidative cleavage by
permanganate.
CH3CH2 C
C CH3
1. O 3
2. H2O
CH3CH2 C OH + HO C
O
O
CH3