CHAPTER 13 Alkynes: The CarbonCarbon Triple Bond

1
CHAPTER 13Alkynes The Carbon-Carbon Triple
Bond
2
Naming the Alkynes
13-1
The general formula for the alkynes is CnH2n-2,
the same as for cycloalkenes. The common name for
the smallest alkyne, C2H2, is acetylene. Common
names of other alkynes are treated as its
derivatives for instance, the alkylacetylenes.
3
The IUPAC rules for naming alkenes also apply to
alkynes with the ending yne replacing ene. A
number indicates the position of the triple bond
in the main chain.
Alkynes having the structure, RC?CH are terminal,
those with the structure RC?CR are
internal. Substituents bearing a triple bond are
alkynl groups -C?CH is named ethynyl -CH2C?CH
is 2-propynyl (propargyl).
4
In IUPAC nomenclature, a hydrocarbon containing
both a double and a triple bond is called an
alkenyne. The chain is numbered starting at the
end closest to either functional group. In the
case of a tie, the double bond is given the lower
number. Alkynes containing a hydroxyl group are
named alkynols. In this case, the OH takes
precedence over both double and triple bonds in
numbering the chain.
5
Properties and Bonding in the Alkynes
13-2
Alkynes are relatively nonpolar. Corresponding
alkynes, alkenes and alkanes have very similar
boiling points. Ethyne sublimes at
-84oC Propyne b.p. -23.2oC 1-Butyne b.p.
8.1oC 2-Butyne b.p. 27oC Medium sized
alkanes distillable liquids Alkynes polymerize
easily, frequently with violence.
6
Ethyne is linear and has strong, short bonds. The
two carbons in ethyne are sp2 hybridized. The ?
bonds are diffuse and resemble a cylindrical
cloud
7
The strength of a C-C triple bond is about 229
kcal mol-1. As with alkenes, the strength of
the ? bonds is much weaker than that of the d
bond which gives rise to much of the chemical
reactivity of the alkyknes.
The C?C bond length is 1.20 Å (CC is 1.33
Å). The C-H bond lengths are also shorter than in
ethene due to the larger degree of s character in
the sp hybrid bonds (as compared to sp2).
8
Alkynes are high-energy compounds. Alkynes often
react with the release of considerable amounts of
energy (prone to explosive decomposition). Ethyne
has a heat of combustion of 311 kcal mol-1 which
is capable of generating a flame temperature
gt2500o C, sufficient for use in welding torches.
9
The heats of hydrogenation of alkyne isomers can
be used to determine their relative stabilities
The greater relative stabililty of internal
alkynes is due to hyperconjugation.
10
Terminal alkynes are remarkably acidic. The
electronegativity of a carbon atom depend upon
its hybridization. The more s character in its
hybrid orbitals, the greater the
electronegativity. The acidity of a C-H bond is
directly related to the electronegativity of the
carbon atom
11
Strong bases such as sodium amide in liquid
ammonia, alkyllithiums and Grignard reagents can
deprotonate terminal alkynes to the corresponding
alkynyl anions.
Alkynyl anions can react as bases and
nucleophiles, much like other carbanions.
12
Spectroscopy of the Alkynes
13-3
The NMR absorptions of alkyne hydrogens show a
characteristic shielding. The 1H NMR absorptions
of alkynyl hydrogens occur at d 1.7 3.1 ppm,
much more shielded than alkenyl hydrogens at d
4.6 5.7 ppm.
13
The shielding of an alkynyl hydrogen is due to
the circular motions of the ? electrons. Unlike
in an alkene, the ? electrons in an alkyne are
found in a cylindrical distribution about the C-C
bond. The motions of these electrons generate a
local magnetic field which opposes Ho in the
vicinity of the alkyne hydrogen.
The result is a strong shielding effect that
cancels the deshielding tendency of the
electron-withdrawing sp-hybridized carbon.
14
The triple bond transmits spin-spin coupling. The
triple bond in an alkyne transmits long range
coupling between the alkynyl hydrogen and the
hydrogens located 3 carbon atoms away (J 2-4
Hz).
15
The signal for the C1 proton at 1.94 ppm is split
into a triplet by the protons on C3. The signal
for the C3 protons at 2.16 ppm are split into a
doublet of triplets by the protons on C1 and C4.
16
The 13C NMR chemical shifts of alkyne carbons are
distinct from those of the alkanes and
alkenes. The 13C absorbances of alkyne, alkene,
and alkane carbons are all sufficiently different
to unambiguous assignments in a 13C NMR
spectrum. Alkyne C d 65-95 ppm Alkene C d
100-150 ppm Alkane C d 5-45 ppm
17
Terminal alkynes give rise to two characteristic
infrared absorptions. Terminal alkynes exhibit
characteristic stretching bands for the alkynyl
hydrogen at 3260-3330 cm-1 and for the C?C at
2100-2260 cm-1 (often weak for internal alkynes).
18
Preparation of Alkynes by Double Elimination
13-4
Alkynes are prepared from dihaloalkanes by
elimination. Treatment of vicinal dihaloalkanes
with two equivalents of strong base results in a
double elimination reaction to form a triple bond.
19
In the case of a terminal alkyne, three
equivalents of base are required due to the
immediate deprotonation of each alkyne formed by
an equivalent of base. A synthetic sequence
called halogenation-double dehydrohalogneation
can be used to convert alkenes into the
corresponding alkynes.
20
Haloalkenes are intermediates in alkyne synthesis
by elimination. An intermediate product in the
dehydrohalogenation of a dihaloalkane is a
haloalkene or alkenyl halide. With
diastereomerically pure vicinal dihaloalkones a
single haloalkene product is formed due to the
anti elimination reaction mechanism.
21
Preparation of Alkynes from Alkynyl Anions
13-5
Terminal alkynyl anions will react with
alkylating agents such as primary haloalkanes,
oxacyclopropanes, and aldehydes or
ketones. Unlike ordinary alkyl organometallic
compounds, the reaction alkynyl anions with
primary haloalkanes results in C-C bond formation.
Reaction with secondary and tertiary halides
leads to E2 products.
22
Other reactions of alkynyl anions
23
Reduction of Alkynes The Relative Reactivity of
the Two Pi Bonds
13-6
Alkynes can undergo addition reactions, such as
hydrogenation and electrophilic attack.
24
Cis alkenes can be synthesized by catalytic
hydrogenation. Catalytic hydrogenation of alkynes
using hydrogen and a platinum or palladium on
charcoal catalyst results in complete saturation.
25
Catalytic hydrogenation using a Lindlar catalyst
(palladium precipitated on CaCO3, and treated
with lead acetate and quinoline) adds only one
equivalent of hydrogen in a syn process
This method affords a stereoselective synthesis
of cis alkenes from alkynes.
26
Sequential one-electron reductions of alkynes
produce trans alkenes. Reduction of alkynes using
metallic sodium dissolved in liquid ammonia
(dissolving-metal reduction) produces trans
alkenes.
27
The second electron transfer takes place faster
than any cis/trans equilibrium of the alkenyl
radical.
The final alkene is stable to further reduction
by this reagent.
28
Electrophilic Addition Reactions of Alkynes
13-7
Addition of hydrogen halides forms haloalkenes
and geminal dihaloalkanes. In an analogous
reaction with alkenes, hydrogen halides add
across alkyne triple bonds.
The stereochemistry of this reaction is typically
anti, particularly when excess halide ion is used.
29
A second molecule of hydrogen halide may also
add, following Markovnikovs rule, producing a
geminal dihaloalkane.
Terminal alkynes also react with hydrogen halide,
again following Mrakovnikovs rule, although it
is difficult to limit the reaction to a single
molecule of hydrogen halide.
30
Halogenation also takes place once or
twice. Halogenation of alkynes proceeds through
an isolatable intermediate vicinal dihaloalkene,
to the tetrahaloalkane. The two additions are
anti.
31
Mercuric ion-catalyzed hydration of alkynes
furnishes ketones. In a reaction catalyzed by
mercuric ion, water can be added to alkynes to
give enols, which then tautomerize to give
ketones.
32
Hydration follows Markovnikovs rule Terminal
alkynes give methyl ketones
33
Symmetrical internal alkynes give a single
carbonyl compound unsymmetrical systems give a
mixture of products
34
Anti-Markovnikov Additions to Triple Bonds
13-8
Radical addition of HBr gives 1-bromoalkenes. In
the presence of light or other radical
initiators, HBr can add to an alkyne by a radical
mechanism in an anti-Markovnikov fashion. Both
syn and anti additions are observed.
35
Aldehydes result from hydroboration-oxidation of
terminal alkynes. Hydroboration of terminal
alkynes occurs in an anti-Markovnikov fashion
the less hindered carbon is attacked by the
boron. In the case of borane, BH3, both ? bonds
react. To stop at the alkenyl-borane stage, a
bulky borane reagent must be used
36
Chemistry of Alkenyl Halides
13-9
Alkenyl halides do not undergo SN2 or SN1
reactions. Alkenyl halides are relatively
unreactive towards nucleophiles. They undergo
elimination reactions with strong bases to give
alkynes, however do not react with weak bases and
relatively nonbasic nucleophiles, such as
iodide. In addition, SN1 reactions of alkenes do
not generally take place because the intermediate
alkenyl cations are of too high an energy.
37
Organometallic derivatives of alkenyl halides can
react to produce a variety of specifically
substituted alkenes
38
Ethyne as in Industrial Starting Material
13-10
Production of ethyne from coal requires high
temperatures. Several thousand degrees Celsius
are required to convert coal and hydrogen gas
into ethyne
In an alternate reaction, coke and limestone are
heated to form calcium carbide, which is then
treated with water to produce ethyne.
39
Ethyne is a source of valuable monomers for
industry. Ethyne is involved in a wide variety of
industrial reactions.
Propenoic acid can be polymerized into
polyacrylates which have replaced natural rubber
in many applications.
40
2-Butyne-1,4-diol is the precursor for the
production of oxacyclopentane (tetrahydrofuran)
41
Polymers of acrylonitrile (acrylic fibers)
include clothing (Orlon), carpets, and
insulation. Copolymers of acrylonitrile and
10-15 vinyl chloride are used in childrens
sleepwear due to their fire resistant properties.
42
Naturally Occurring and Physiologically Active
Alkynes
13-11
Chamomile flower
Chrysanthemum
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Lower Amazon Indian arrowhead poison.
Poison arrow frog toxin
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Nonprescription hypnotic
Naturally occurring antibiotic-antitumor agents.
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Important Concepts
13

• Naming Alkynes Same as for alkenes.
• If both double and triple bonds present, the name
  alkenyne is used with the double bond receiving
  the lower number if both are at equivalent
  positions.
• Hydroxyl groups are given precedence when
  numbering alkynl alcohols (alkynols).
• Triple Bond Electronic Structure Two
  perpendicular ? bonds and a d bond (formed from
  overlapping sp orbitals).
• Linear Structure
• C?C bond energy 229 kcal/mol
• ?C-H bond energy 131 kcal/mol
• C?C bond length 1.20 Å
• ?C-H bond length 1.06 Å

46
Important Concepts
13

• C1 Bound Hydrogen Is Relatively Acidic pKa
  25.
• NMR
• Alkynl hydrogen shift (d 1.7-3.1 ppm) low
  compared to alkenyl hydrogen.
• Triple bond allows for long-range coupling.
• IR terminal alkynes
• C?C 2100-2260 cm-1
• C-H 3260-3330 cm-1
• Vicinal Dihaloalkanes Elimination reaction
  proceeds regioselectively and stereospecifically
  to give alkenyl halides.

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Important Concepts
13

• Selective Dihydrogenation of Alkynes -
• Syn dihydrogenation using a Lindlar catalyst
  (does not hydrogenate alkenes).
• Anti hydrogenation using sodium metal dissolved
  in liquid ammonia (simple alkenes cannot be
  reduced by one electron transfer).
• Addition Reactions Alkynes undergo the same
  reactions as alkenes. Reactions may occur twice
  in succession.
• Hydration of alkynes is unusual. It requires a
  Hg(II) catalyst and the enol formed tautomerizes
  to a ketone.
• Hydroboration Dialkylboranes
  (dicyclohexylborane) are used to stop the
  hydroboration of terminal alkynes at the
  alkenylboron stage. Oxidation of the resulting
  alkenylboranes produces enols which tautomerize
  to aldehydes.