Organic chemistry structure and function 6th edition by peter 2

THIRTEEN

C H A P T E R

Alkynes
The Carbon – Carbon Triple Bond

S

 N

O

 O

A

lkynes are hydrocarbons that contain carbon – carbon triple bonds. It should not come
as a surprise that their characteristics resemble the properties and behavior of alkenes,
their double-bonded cousins. In this chapter we shall see that, like alkenes, alkynes
find numerous uses in a variety of modern settings. For example, the polymer derived from
the parent compound, ethyne (HC q CH), can be fashioned into electrically conductive
sheets usable in lightweight, all-polymer batteries. Ethyne is also a substance with a relatively high energy content, a property that is exploited in oxyacetylene torches. A variety
of alkynes, both naturally occurring and synthetic, have found use in medicine for their
antibacterial, antiparasitic, and antifungal activities.
OCq CO
Alkyne triple bond

Because the O C q C O functional group contains two p linkages (which are mutually
perpendicular; recall Figure 1-21), its reactivity is much like that of the double bond. For
example, like alkenes, alkynes are electron rich and subject to attack by electrophiles. Many

of the alkenes that serve as monomers for the production of polymeric fabrics, elastics, and
plastics are prepared by electrophilic addition reactions to ethyne and other alkynes. Alkynes
can be prepared by elimination reactions similar to those used to generate alkenes, and they
are likewise most stable when the multiple bond is internal rather than terminal. A further,
and useful, feature is that the alkynyl hydrogen is much more acidic than its alkenyl or
alkyl counterpart, a property that permits easy deprotonation by strong bases. The resulting
alkynyl anions are valuable nucleophilic reagents in synthesis.
We begin with discussions of the naming, structural characteristics, and spectroscopy
of the alkynes. Subsequent sections introduce methods for the synthesis of compounds in
this class and the typical reactions they undergo. We end with an overview of the extensive
industrial uses and physiological characteristics of alkynes.

Scanning tunneling microscopy
(STM) is an indirect method for
imaging individual atoms and
molecules on a solid surface.
The dialkyne whose structure is
shown above glows brightly in
the top STM image because
it is in a conformation that is
highly electrically conductive.
In contrast, in the bottom
image a change in molecular
shape has “turned off” its
conductivity, and it goes “dark”
as a result. Such molecules are
prototypes for “molecular
switches,” which promise to
revolutionize the fields of
electronic components and

computers in the 21st century.


Chapter 13

Common Names
for Alkynes
HC q CH

Alkynes

13-1 Naming the Alkynes
A carbon–carbon triple bond is the functional group characteristic of the alkynes. The general
formula for the alkynes is CnH2n22, the same as that for the cycloalkenes. The common names
for many alkynes are still in use, including acetylene, the common name of the smallest
alkyne, C2H2. Other alkynes are treated as its derivatives—for example, the alkylacetylenes.
The IUPAC rules for naming alkenes (Section 11-1) also apply to alkynes, the ending
-yne replacing -ene. A number indicates the position of the triple bond in the main chain.

HC q CH

CH3C q CCH3

CH3C q CCHCH2CH3

CH3
3A 2
1
CH3CC q CH

Ethyne

2-Butyne

4-Bromo-2-hexyne

3,3-Dimethyl-1-butyne

(An internal alkyne)

(A terminal alkyne)

Br

Acetylene

CH3C q CCH3

Dimethylacetylene

1

3 A4

2

5

6

4

A
CH3

Alkynes having the general structure RC q CH are terminal, whereas those with the structure of RC q CR9 are internal.
Substituents bearing a triple bond are alkynyl groups. Thus, the substituent – C q CH is
named ethynyl; its homolog – CH2C q CH is 2-propynyl (propargyl). Like alkanes and
alkenes, alkynes can be depicted in straight-line notation.
[
ð

CH3CH2CH2C q CH

1

trans-1,2-Diethynylcyclohexane

Propylacetylene

2

3

CH2C q CH
2-Propynylcyclobutane
(Propargylcyclobutane)

3

2 1

HCq CCH2OH
2-Propyn-1-ol
(Propargyl alcohol)

In IUPAC nomenclature, a hydrocarbon containing both double and triple bonds is called
an alkenyne. The chain is numbered starting from the end closest to either of the functional
groups. When a double bond and a triple bond are at equidistant positions from either terminus,
the double bond is given the lower number. Alkynes incorporating the hydroxy function are
named alkynols. Note the omission of the final e of -ene in -enyne and of -yne in -ynol. The
OH group takes precedence over both double and triple bonds in the numbering of a chain.
OH
2

4
6

5

4

3

2

1

CH3CH2CH P CHC q CH

1

2

3

4

5

5

CH2 P CHCH2C q CH

3

1

6

3-Hexen-1-yne

1-Penten-4-yne

5-Hexyn-2-ol

(Not 3-hexen-5-yne)

(Not 4-penten-1-yne)

(Not 1-hexyn-5-ol)

Exercise 13-1
Give the IUPAC names for (a) all the alkynes of composition C6H10;
H3C
(b)

C
G

568

CG C q CH
H (
CH PCH2

(c) all butynols. Remember to include and designate stereoisomers.

13-2 Properties and Bonding in the Alkynes
The nature of the triple bond helps explain the physical and chemical properties of the
alkynes. In molecular-orbital terms, we shall see that the carbons are sp hybridized, and
the four singly filled p orbitals form two perpendicular p bonds.


13-2 Properties and Bonding in the Alkynes

Chapter 13

569

Alkynes are relatively nonpolar
Alkynes have boiling points very similar to those of the corresponding alkenes and alkanes.
Ethyne is unusual in that it has no boiling point at atmospheric pressure; rather, it sublimes
at 2848C. Propyne (b.p. 223.28C) and 1-butyne (b.p. 8.18C) are gases, whereas 2-butyne
is barely a liquid (b.p. 278C) at room temperature. The medium-sized alkynes are distillable
liquids. Care must be taken in the handling of alkynes: They polymerize very easily —
frequently with violence. Ethyne explodes under pressure but can be shipped in pressurized
gas cylinders that contain acetone and a porous filler such as pumice as stabilizers.

Dissociation Energies
of C–C Bonds
HC q CH

Ethyne is linear and has strong, short bonds

DHЊ ϭ 229 kcal molϪ1
(958 kJ molϪ1)

In ethyne, the two carbons are sp hybridized (Figure 13-1A). One of the hybrid orbitals on
each carbon overlaps with hydrogen, and a s bond between the two carbon atoms results
from mutual overlap of the remaining sp hybrids. The two perpendicular p orbitals on each
carbon contain one electron each. These two sets overlap to form two perpendicular p bonds
(Figure 13-1B). Because p bonds are diffuse, the distribution of electrons in the triple bond
resembles a cylindrical cloud (Figure 13-1C). As a consequence of hybridization and the
two p interactions, the strength of the triple bond is about 229 kcal mol21, considerably
stronger than either the carbon – carbon double or single bonds (margin). As with alkenes,
however, the alkyne p bonds are much weaker than the s component of the triple bond, a
feature that gives rise to much of its chemical reactivity. The C – H bond-dissociation energy
of terminal alkynes is also substantial: 131 kcal mol21 (548 kJ mol21).
π bond

p orbital

H2C P CH2

DHЊ ϭ 173 kcal molϪ1
(724 kJ molϪ1)
H3C O CH3

DHЊ ϭ 90 kcal molϪ1
(377 kJ molϪ1)

π electron
cloud

sp orbital

H

H

C

C

C

H

C

C

H

C

sp orbital
π bond

p orbital
A

σ bond

B

Figure 13-1 (A) Orbital picture of sp-hybridized carbon, showing the two perpendicular p orbitals.
(B) The triple bond in ethyne: The orbitals of two sp-hybridized CH fragments overlap to create a
s bond and two p bonds. (C) The two p bonds produce a cylindrical electron cloud around the
molecular axis of ethyne. (D) The electrostatic potential map reveals the (red) belt of high electron
density around the central part of the molecular axis.

Because both carbon atoms in ethyne are sp hybridized, its structure is linear (Figure 13-2).
The carbon – carbon bond length is 1.20 Å, shorter than that of a double bond (1.33 Å, Figure 11-1). The carbon–hydrogen bond also is short, again because of the relatively large
degree of s character in the sp hybrids used for bonding to hydrogen. The electrons in these
orbitals (and in the bonds that they form by overlapping with other orbitals) reside relatively
close to the nucleus and produce shorter (and stronger) bonds.

Alkynes are high-energy compounds

The alkyne triple bond is characterized by a concentration of four p electrons in a relatively
small volume of space. The resulting electron – electron repulsion contributes to the relative
weakness of the two p bonds and to a very high energy content of the alkyne molecule
itself. Because of this property, alkynes often react with the release of considerable amounts
of energy. In addition to being prone to explosive decomposition, ethyne has a heat of
combustion of 311 kcal mol21. As shown in the equation for ethyne combustion on the next
page, this energy is distributed among only three product molecules, one of water and two

D

1.203 A؇

HO C q C O H
1.061 A؇

180؇

Linear ethyne

Figure 13-2 Molecular structure
of ethyne.


570

Alkynes

Chapter 13

of CO2, causing each to be heated to extremely high temperatures (.25008C), sufficient for

use in welding torches.
Combustion of Ethyne
HC q CH

ϩ

2 CO2

2.5 O2

ϩ

ΔH° ϭ Ϫ311 kcal molϪ1
(Ϫ1301 kJ molϪ1)

H2O

As we found in our discussion of alkene stabilities (Section 11-5), heats of hydrogenation
also provide convenient measures of the relative stabilities of alkyne isomers. In the presence
of catalytic amounts of platinum or palladium on charcoal, the two isomers of butyne hydrogenate by addition of two molar equivalents of H2 to produce butane. Just as we discovered
in the case of alkenes, hydrogenation of the internal alkyne isomer releases less energy,
allowing us to conclude that 2-butyne is the more stable of the two. Hyperconjugation is the
reason for the greater relative stability of internal compared with terminal alkynes.

The high temperatures required for
welding are attained by combustion of ethyne (acetylene).

CH3CH2C q CH

ϩ

2 H2

CH3C q CCH3

ϩ

2 H2

Catalyst

Catalyst

CH3CH2CH2CH3

ΔH° ϭ Ϫ69.9 kcal molϪ1
(Ϫ292.5 kJ molϪ1)

CH3CH2CH2CH3

ΔH° ϭ Ϫ65.1 kcal molϪ1
(Ϫ272.4 kJ molϪ1)

Exercise 13-2
Are the heats of hydrogenation of the butynes consistent with the notion that alkynes are highenergy compounds? Explain. (Hint: Compare these values with the heats of hydrogenation of
alkene double bonds.)

Terminal alkynes are remarkably acidic
Relative Stabilities of
the Alkynes

In Section 2-2 you learned that the strength of an acid, H – A, increases with increasing
electronegativity, or electron-attracting capability, of atom A. Is the electronegativity of an
atom the same in all structural environments? The answer is no: Electronegativity varies
with hybridization. Electrons in s orbitals are more strongly attracted to an atomic nucleus
than are electrons in p orbitals. As a consequence, an atom with hybrid orbitals high in
s character (e.g., sp, with 50% s and 50% p character) will be slightly more electronegative
than the same atom with hybrid orbitals with less s character (sp3, 25% s and 75% p character). This effect is indicated below in the electrostatic potential maps of ethane, ethene,
and ethyne. The increasingly positive polarization of the hydrogen atoms is reflected in their
increasingly blue shadings, whereas the carbon atoms become more electron rich (red) along
the series. The relatively high s character in the carbon hybrid orbitals of terminal alkynes
makes them more acidic than alkanes and alkenes. The pKa of ethyne, for example, is 25,
remarkably low compared with that of ethene and ethane.

RC q CH , RC q CR9
More stable

Deprotonation
of 1-Alkynes
RC q C O H

Ϫ

ϩ ðB

RC q CðϪ ϩ

HB

Relative Acidities of Alkanes, Alkenes, and Alkynes

H3C
Hybridization:
pKa:

CH3

Ͻ

H2C

CH2

sp3

sp2

50

44

Increasing acidity

Ͻ

HC

CH
sp
25

13-3 Spectroscopy of the Alkynes

Chapter 13

This property is useful, because strong bases such as sodium amide in liquid ammonia,
alkyllithiums, and Grignard reagents can deprotonate terminal alkynes to the corresponding
alkynyl anions. These species react as bases and nucleophiles, much like other carbanions
(Section 13-5).
Deprotonation of a Terminal Alkyne
pKa Ϸ 50

pKa Ϸ 25

CH3CH2C q CH
(Stronger acid)

ϩ

CH3CH2CH2CH2Li

(CH3CH2)2O

H
A
CH3CH2C q CLi ϩ CH3CH2CH2CH2

(Stronger base)

(Weaker base)

(Weaker acid)

Exercise 13-3

Working with the Concepts: Deprotonation of Alkynes
What is the equilibrium constant, Keq, for the acid-base reaction shown above? Does its value explain
why the reaction is written with only a forward arrow, suggesting that it is “irreversible”?
Strategy
Recall how pKa values relate to acid dissociation constants. Use this information to determine the
value for Keq.
Solution
• The pKa is the negative logarithm of the acid dissociation constant. Dissociation of the alkyne
therefore has a Ka < 10225, very unfavorable, at least in comparison with the more familiar acids.
However, butyllithium is the conjugate base of butane, which has a Ka < 10250. As an acid, butane
is 25 orders of magnitude weaker than is the terminal alkyne. Thus, butyllithium is that much
stronger a base compared with the alkynyl anion.
• The Keq for the reaction is found by dividing the Ka for the acid on the left by the Ka for the
acid on the right: 10225y10250 5 1025. The reaction is very favorable in the forward direction, so
much so that for all practical purposes it may be considered to be irreversible. (Caution: Use common
sense to avoid major errors in solving acid-base problems, such as deciding that the equilibrium
lies the wrong direction. Use this hint: The favored direction for an acid-base reaction converts
the stronger acid/stronger base pair into the weaker acid/weaker base pair.)

Exercise 13-4

Try It Yourself
Strong bases other than those mentioned here for the deprotonation of alkynes were introduced
earlier. Two examples are potassium tert-butoxide and lithium diisopropylamide (LDA). Would
either (or both) of these compounds be suitable for making ethynyl anion from ethyne? Explain,

in terms of their pKa values.

In Summary The characteristic hybridization scheme for the triple bond of an alkyne
controls its physical and electronic features. It is responsible for strong bonds, the linear
structure, and the relatively acidic alkynyl hydrogen. In addition, alkynes are highly energetic compounds. Internal isomers are more stable than terminal ones, as shown by the
relative heats of hydrogenation.

13-3 Spectroscopy of the Alkynes
Alkenyl hydrogens (and carbons) are deshielded and give rise to relatively low-field NMR
signals compared with those in saturated alkanes (Section 11-4). In contrast, alkynyl hydrogens have chemical shifts at relatively high field, much closer to those in alkanes. Similarly,
the sp-hybridized carbons absorb in a range between that recorded for alkenes and alkanes.
Alkynes, especially terminal ones, are also readily identified by IR spectroscopy. Finally, mass
spectrometry can be a useful tool for identification and structure elucidation of alkynes.

571


572

Alkynes

Chapter 13

Figure 13-3 300-MHz 1H NMR
spectrum of 3,3-dimethyl-1-butyne
showing the high-field position
(d 5 2.06 ppm) of the signal due
to the alkynyl hydrogen.

1H NMR

9H
CH3
CH3CC

CH

(CH3)4Si

CH3

1H

9

8

7

6

5

4

3

2

1

0

ppm (δ )

The NMR absorptions of alkyne hydrogens show a
characteristic shielding
Unlike alkenyl hydrogens, which are deshielded and give 1H NMR signals at d 5 4.6–5.7 ppm,
protons bound to sp-hybridized carbon atoms are found at d 5 1.7 – 3.1 ppm (Table 10-2).
For example, in the NMR spectrum of 3,3-dimethyl-1-butyne, the alkynyl hydrogen resonates at d 5 2.06 ppm (Figure 13-3).
Why is the terminal alkyne hydrogen so shielded? Like the p electrons of an alkene,
those in the triple bond enter into a circular motion when an alkyne is subjected to an
external magnetic field (Figure 13-4). However, the cylindrical distribution of these electrons (Figure 13-1C) now allows the major direction of this motion to be perpendicular to

Local magnetic
field

hlocal

Opposes H0 in this
region of space

hlocal

Local magnetic
field

R

H

H
hlocal

C

hlocal

C

H

Strengthens H0
in this region
of space

C

hlocal

hlocal
C

H

H
π electron
movement

A

hlocal

External field, H0

Opposes H0 in this
region of space

B

hlocal

π electron
movement

External field, H0

Figure 13-4 Electron circulation in the presence of an external magnetic field generates local
magnetic fields that cause the characteristic chemical shifts of alkenyl and alkynyl hydrogens.
(A) Alkenyl hydrogens are located in a region of space where hlocal reinforces H0. Therefore, these
protons are relatively deshielded. (B) Electron circulation in an alkyne generates a local field that
opposes H0 in the vicinity of the alkynyl hydrogen, thus causing shielding.


13-3 Spectroscopy of the Alkynes

Chapter 13

573

that in alkenes and to generate a local magnetic field that opposes H0 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 and gives rise to a relatively high-field
chemical shift.

The triple bond transmits spin – spin coupling
The alkyne functional group transmits coupling so well that the terminal hydrogen is split
by the hydrogens across the triple bond, even though it is separated from them by three
carbons. This result is an example of long-range coupling. The coupling constants are small
and range from about 2 to 4 Hz. Figure 13-5 shows the NMR spectrum of 1-pentyne. The
alkynyl hydrogen signal at d 5 1.94 ppm is a triplet (J 5 2.5 Hz) because of coupling to
the two equivalent hydrogens at C3, which appear at d 5 2.16 ppm. The latter, in turn, give
rise to a doublet of triplets, representing coupling to the two hydrogens at C4 (J 5 6 Hz)
as well as that at C1 (J 5 2.5 Hz).

CH
1.1 1.0 0.9
ppm

1.6 1.5
ppm

3H
2.0 1.9
ppm

2H

2.2 2.1
ppm

9

8

7

6

J ‫ ؍‬2–4 Hz
H
A
O C O C q COH
A

Figure 13-5 300-MHz 1H NMR
spectrum of 1-pentyne showing
coupling between the alkynyl
(green) and propargylic (blue)
hydrogens.

1H NMR

CH3CH2CH2C

Long-Range Coupling
in Alkynes

5

4

3

1H
2H

2

(CH3)4Si

1

0

ppm (δ )

Exercise 13-5

Working with the Concepts: Predicting an NMR Spectrum
Predict the first-order splitting pattern in the 1H NMR spectrum of 3-methyl-1-butyne.
Strategy
First, write out the structure. Then identify groups of hydrogens within coupling distance of each
other, both neighboring and long range. Finally, use information regarding approximate values of
coupling constants (and the N 1 1 rule) to generate expected splitting patterns.
Solution
• The structure of the molecule is
CH3
A
CH3 O CHO C q CH

3

2

1

• The two methyl groups are equivalent and give one signal that is split into a doublet by the
single hydrogen atom at C3 (N 1 1 5 2 lines). The coupling constant (J value) for this splitting
is the typical 6 – 8 Hz found in saturated systems (Section 10-7).


574

Chapter 13

Alkynes

• The alkynyl O CqCH hydrogen at C1 experiences long-range coupling to the same H at C3,
appearing also as a doublet, but J is smaller, about 3 Hz.
• Finally, the signal for the hydrogen at C3 displays a more complex pattern. The 6 – 8-Hz splitting
by the six hydrogens of the methyl groups gives a septet (N 1 1 5 7 lines). Each line of this
septet is further split by the additional 3-Hz coupling to the alkynyl H. As the actual spectrum
below shows, the outermost lines of this signal, a doublet of septets, are so small that they are
barely visible (see Tables 10-4 and 10-5). (Caution: When interpreting 1H NMR spectra, be aware
of the very low intensity of the outer lines in highly split signals. In fact, it is prudent to assume
that such signals may consist of more lines than are readily visible.)

1H NMR

6H

H
C

H3C

C

CH
1.2 1.1

CH3

ppm

2.1 2.0 1.9

ppm

1H
2.6 2.5
ppm

9

8

7

6

(CH3)4Si

1H

5

4

3

2

1

0

ppm (δ )

Exercise 13-6

Try It Yourself
Predict the first-order splitting pattern in the 1H NMR spectrum of 2-pentyne.

The 13C NMR chemical shifts of alkyne carbons are distinct from
those of the alkanes and alkenes
Carbon-13 NMR spectroscopy also is useful in deducing the structure of alkynes. For example,
the triple-bonded carbons in alkyl-substituted alkynes resonate in the range of d 5 65–95 ppm,
quite separate from the chemical shifts of analogous alkane (d 5 5 – 45 ppm) and alkene
(d 5 100 – 150 ppm) carbon atoms (Table 10-6).
Typical Alkyne 13C NMR Chemical Shifts

HC q CH
␦ ‫ ؍‬71.9

HC q CCH2CH2CH2CH3
68.6

84.0

18.6

31.1

22.4

14.1

CH3CH2C q CCH2CH3
81.1 15.6

13.2 ppm

Terminal alkynes give rise to two characteristic infrared absorptions
Infrared spectroscopy is helpful in identifying terminal alkynes. Characteristic stretching
bands appear for the alkynyl hydrogen at 3260 – 3330 cm21 and for the CqC triple bond
at 2100 – 2260 cm21. There is also a diagnostic n|Csp–H bending absorption at 640 cm21


13-3 Spectroscopy of the Alkynes

Chapter 13

575

Figure 13-6 IR spectrum
of 1,7-octadiyne:
n| Csp2H stretch 5 3300 cm21;
n| CqC stretch 5 2120 cm21;
n| Csp2H bend 5 640 cm21.

Transmittance (%)

100

2120
H
H

C

C

H

C

C

C

H

C
H

3300

C
C

H
H

IR
0
4000
3500

H

640

H
H

3000

2500

2000

1500

1000

600 cm−1

Wavenumber

(Figure 13-6). Such data are especially useful when 1H NMR spectra are complex and difficult to interpret. However, the band for the CqC stretching vibration in internal alkynes
is often weak, like that for internal alkenes (Section 11-8), thus reducing the value of IR
spectroscopy for characterizing these systems.

Mass spectral fragmentation of alkynes gives
resonance-stabilized cations
The mass spectra of alkynes, like those of alkenes, frequently show prominent molecular
ions. Thus high-resolution measurements can reveal the molecular formula and therefore
the presence of two degrees of unsaturation derived from the presence of the triple bond.
In addition, fragmentation at the carbon once removed from the triple bond is
observed, giving resonance-stabilized cations. For example, the mass spectrum of 3-heptyne
(Figure 13-7) shows an intense molecular ion peak at myz 5 96 and loss of both methyl

CH3CH2C

Relative abundance

67 (M − CH2CH3)
81 (M − CH3)

MS

100

CCH2CH2CH3

M •
96

50

0
0

20

40

60

m/z

80

100

Figure 13-7 Mass spectrum of
.
3-heptyne, showing M1 at myz 5
96 and important fragments at
myz 5 67 and 81 arising from
cleavage of the C1 – C2 and

C5 – C6 bonds.


576

Chapter 13

Alkynes

(cleavage a) and ethyl (cleavage b) fragments to give two different stabilized cations,
with myz 5 81 and 67 (base peak), respectively:
Fragmentation of an Alkyne in the Mass Spectrometer
ϩ

CH2 O C O
O C O CH2 O CH2CH3

a
a

b

CH3 O CH2 O C q C O CH2 O CH2CH3

ϩj

ϩ

ϪCH3j

m/z ‫ ؍‬96
ϪC2H5j

CH2 O C O C O CH2 O CH2CH3
m/z ‫ ؍‬81

b

ϩ

CH3 O CH2 O C O
O C O CH2

ϩ

CH3 O CH2 O C O C O CH2
m/z ‫ ؍‬67

Unfortunately, under the high energy conditions of the mass spectrometry experiment,
migration of the triple bond can occur. Thus this fragmentation is not typically very useful
for identifying the location of the triple bond in a longer-chain alkyne.

In Summary The cylindrical p cloud around the carbon – carbon triple bond induces local
magnetic fields that lead to NMR chemical shifts for alkynyl hydrogens at higher fields than
those of alkenyl protons. Long-range coupling is observed through the C q C linkage. Infrared spectroscopy provides a useful complement to NMR data, displaying characteristic
bands for the C q C and qC – H bonds of terminal alkynes. In the mass spectrometer,
alkynes fragment to give resonance-stabilized cations.

13-4 Preparation of Alkynes by Double Elimination
The two basic methods used to prepare alkynes are double elimination from 1,2-dihaloalkanes

and alkylation of alkynyl anions. This section deals with the first method, which provides a
synthetic route to alkynes from alkenes; Section 13-5 addresses the second, which converts
terminal alkynes into more complex, internal ones.

Alkynes are prepared from dihaloalkanes by elimination
As discussed in Section 11-6, alkenes can be prepared by E2 reactions of haloalkanes. Application of this principle to alkyne synthesis suggests that treatment of vicinal dihaloalkanes with
two equivalents of strong base should result in double elimination to furnish a triple bond.
Double Elimination from Dihaloalkanes to Give Alkynes
X

X

C

C

H

H

Base (2 equivalents)
Ϫ2 HX

Cq C

Vicinal
dihaloalkane

Indeed, addition of 1,2-dibromohexane (prepared by bromination of 1-hexene, Section 12-5)
to sodium amide in liquid ammonia followed by evaporation of solvent and aqueous work-up

gives 1-hexyne.


13-5 Preparation of Alkynes from Alkynyl Anions

577

Chapter 13

Example of Double Dehydrohalogenation to Give an Alkyne

A

CH3CH2CH2CH2CH
A
Br

CH2Br

1. 3 NaNH2, liquid NH3
2. H2O
Ϫ2 HBr

CH3CH2CH2CH2C q CH

Three equivalents of NaNH2 are necessary in the preparation of a terminal alkyne because,
as this alkyne forms, its acidic terminal hydrogen (Section 13-2) immediately protonates an
equivalent amount of base. Eliminations in liquid ammonia are usually carried out at its
boiling point, 2338C.
Because vicinal dihaloalkanes are readily available from alkenes by halogenation, this

sequence, called halogenation – double dehydrohalogenation, is a ready means of converting alkenes into the corresponding alkynes.
A Halogenation–Double Dehydrohalogenation
Used in Alkyne Synthesis
1. Br2, CCl4
2. NaNH2, liquid NH3
3. H2O

53%
1,5-Hexadiene

1,5-Hexadiyne

Exercise 13-7
Illustrate the use of halogenation – double dehydrohalogenation in the synthesis of the alkynes
(a) 2-pentyne; (b) 1-octyne; (c) 2-methyl-3-hexyne.

Haloalkenes are intermediates in alkyne synthesis by elimination
Dehydrohalogenation of dihaloalkanes proceeds through the intermediacy of haloalkenes,
also called alkenyl halides. Although mixtures of E- and Z-haloalkenes are in principle
possible, with diastereomerically pure vicinal dihaloalkanes only one product is formed
because elimination proceeds stereospecifically anti (Section 11-6).

X

X

X

C

C

H

H

X

or
H

H

Ϫ

Exercise 13-8
Give the structure of the bromoalkene intermediate in the bromination – dehydrobromination of
cis-2-butene to 2-butyne. Do the same for the trans isomer. (Caution: There is stereochemistry
involved in both steps. Hint: Refer to Section 12-5 for useful information, and use models.)

The stereochemistry of the intermediate haloalkene is of no consequence when the
sequence is used for alkyne synthesis. Both E- and Z-haloalkenes eliminate with base to
give the same alkyne.

In Summary Alkynes are made from vicinal dihaloalkanes by double elimination. Alkenyl
halides are intermediates, being formed stereospecifically in the first elimination.

13-5 Preparation of Alkynes from Alkynyl Anions
Alkynes can also be prepared from other alkynes. The reaction of terminal alkynyl anions
with alkylating agents, such as primary haloalkanes, oxacyclopropanes, aldehydes, or

ketones, results in carbon – carbon bond formation. As we know (Section 13-2), such anions
are readily prepared from terminal alkynes by deprotonation with strong bases (mostly
alkyllithium reagents, sodium amide in liquid ammonia, or Grignard reagents). Alkylation

Bð

BðϪ
Newman projection
Anti
elimination

X
G
CP C

G
H

An alkenyl
halide

1 equivalent
NaOCH3,
CH3OH

ϩ BH ϩ XϪ


578

Alkynes

Chapter 13

with methyl or primary haloalkanes is typically done in liquid ammonia or in ether solvents.
The process is unusual, because ordinary alkyl organometallic compounds are unreactive in
the presence of haloalkanes. Alkynyl anions are an exception, however.
Alkylation of an Alkynyl Anion
C q CH

C q CðϪϩLi

CH3CH2CH2CH2Li,
THF

C q CCH2CH2CH3
CH3CH2CH2I, 65ЊC
ϪLiI
Alkylation by
SN2 reaction

Deprotonation
by strong base

85%
1-Pentynylcyclohexane

Attempted alkylation of alkynyl anions with secondary and tertiary halides leads to E2
products because of the strongly basic character of the nucleophile (recall Section 7-8).
Ethyne itself may be alkylated in a series of steps through the selective formation of the

monoanion to give mono- and dialkyl derivatives.
Alkynyl anions react with other carbon electrophiles such as oxacyclopropanes and carbonyl
compounds in the same manner as do other organometallic reagents (Sections 8-8 and 9-9).
Reactions of Alkynyl Anions

LiNH2 (1 equivalent), liquid NH3

HC q CH

ϪNH2H
Deprotonation

HC q CLi

O
f i
1. H2C O CH2
2. HOH
ϪLiOH
Nucleophilic
ring opening

OH
A
HC q CCH2CH2
92%
3-Butyn-1-ol

O

CH3C q CH

CH3CH2MgBr, (CH3CH2)2O, 20°C
ϪCH3CH2H
Deprotonation

CH3C q CMgBr

1.
2. H2O
Nucleophilic
addition

OH
Cq

CH

CH3

66%
1-(1-Propynyl)cyclopentanol

Exercise 13-9
Suggest efficient and short syntheses of these two compounds. (Hint: Review Section 8-9.)
from

(a)

OH

OH

(b)

from ethyne

Exercise 13-10
3-Butyn-2-ol is an important raw material in the pharmaceutical industry. It is the starting
point for the synthesis of a variety of medicinally valuable alkaloids (Section 25-8), steroids
(Section 4-7), and prostaglandins (Chemical Highlight 11-1 and Section 19-13), as well as
vitamins E (Section 22-9) and K. Propose a short synthesis of 3-butyn-2-ol by using the
techniques outlined in this section.


13-6 Reduction of Alkynes: Reactivity of Two Pi Bonds

Chapter 13

In Summary Alkynes can be prepared from other alkynes by alkylation with primary
haloalkanes, oxacyclopropanes, or carbonyl compounds. Ethyne itself can be alkylated in a
series of steps.

13-6 Reduction of Alkynes: The Relative Reactivity of
the Two Pi Bonds
Now we turn from the preparation of alkynes to the characteristic reactions of the triple
bond. In many respects, alkynes are like alkenes, except for the availability of two p bonds.
Thus, alkynes can undergo additions, such as hydrogenation and electrophilic attacks.
Addition of Reagents A–B to Alkynes
G
D

R O C q C OR

B
R
R
R
G
G
or
C PC
C PC
D
D
A
B
A
R
G
D

A–B

A–B

R R
R R
A A
A A
A O C O C OB or A O C O C OB

A A
A A
A B
B A

In this section we introduce two new hydrogen addition reactions: step-by-step hydrogenation and dissolving-metal reduction by sodium to give cis and trans alkenes, respectively.

Cis alkenes can be synthesized by catalytic hydrogenation
Alkynes can be hydrogenated under the same conditions used to hydrogenate alkenes. Typically, platinum or palladium on charcoal is suspended in a solution containing the alkyne
and the mixture is exposed to a hydrogen atmosphere. Under these conditions, the triple
bond is saturated completely.
Complete Hydrogenation of Alkynes
CH3CH2CH2C q CCH2CH3

H2, Pt

CH3CH2CH2CH2CH2CH2CH3
100%
Heptane

3-Heptyne

Hydrogenation is a stepwise process that may be stopped at the intermediate alkene
stage by the use of modified catalysts, such as the Lindlar* catalyst. This catalyst is
palladium that has been precipitated on calcium carbonate and treated with lead acetate
and quinoline. The surface of the metal rearranges to a less active configuration than
that of palladium on carbon so that only the first p bond of the alkyne is hydrogenated.
As with catalytic hydrogenation of alkenes (Section 12-2), the addition of H2 is a syn
process. As a result, this method affords a stereoselective synthesis of cis alkenes from
alkynes.

Hydrogenation with Lindlar Catalyst
H

H

Lindlar Catalyst
5% Pd–CaCO3,
O
B
Pb(OCCH3)2,

H2, Lindlar catalyst, 25°C
syn Addition of H2

100%
3-Heptyne

*Dr. Herbert H. M. Lindlar (b. 1909), Hoffman – La Roche Ltd., Basel.

cis-3-Heptene

N
Quinoline

579


580

Chapter 13

Alkynes

Exercise 13-11
Write the structure of the product expected from the following reaction.

CH3
O
O

H2, Lindlar catalyst, 25°C

O

Exercise 13-12

Some perfumes have star quality
(from left to right): Jean Paul
Gaultier MaDame Perfume, Paris
Hilton Fairy Dust, Armani Prive
Oranger Alhambra, and Jeanne
Lanvin.

The perfume industry makes considerable use of naturally occurring substances such as those
obtained from rose and jasmine extracts. In many cases, the quantities of fragrant oils available
by natural product isolation are so small that it is necessary to synthesize them. Examples are the
olfactory components of violets, which include trans-2-cis-6-nonadien-1-ol and the corresponding
aldehyde. An intermediate in their large-scale synthesis is cis-3-hexen-1-ol, whose industrial preparation is described as “a closely guarded secret.” Using the methods in this and the preceding
sections, propose a synthesis from 1-butyne.

With a method for the construction of cis alkenes at our disposal, we might ask: Can
we modify the reduction of alkynes to give only trans alkenes? The answer is yes, with a
different reducing agent and through a different mechanism.

Sequential one-electron reductions of alkynes
produce trans alkenes
When we use sodium metal dissolved in liquid ammonia (dissolving-metal reduction) as
the reagent for the reduction of alkynes, we obtain trans alkenes as the products. For
example, 3-heptyne is reduced to trans-3-heptene in this way. Unlike sodium amide in
liquid ammonia, which functions as a strong base, elemental sodium in liquid ammonia acts
as a powerful electron donor (i.e., a reducing agent).
REACTION

Dissolving-Metal Reduction of an Alkyne
1. Na, liquid NH3
2. H2O

H

H
86%
3-Heptyne

trans-3-Heptene

In the first step of the mechanism of this reduction, the p framework of the triple bond
accepts one electron to give a radical anion. This anion is protonated by the ammonia solvent
(step 2) to give an alkenyl radical, which is further reduced (step 3) by accepting another
electron to give an alkenyl anion. This species is again protonated (step 4) to give the product alkene, which is stable to further reduction. The trans stereochemistry of the final alkene
is set in the first two steps of the mechanism, which give rise preferentially to the less sterically hindered trans alkenyl radical. Under the reaction conditions (liquid NH3, 2338C), the

second one-electron transfer takes place faster than cis-trans equilibration of the radical. This
type of reduction typically provides .98% stereochemically pure trans alkene.


13-6 Reduction of Alkynes: Reactivity of Two Pi Bonds

Chapter 13

581

Mechanism of the Reduction of Alkynes by Sodium in Liquid Ammonia

MECHANISM
Step 1. One-electron transfer

R groups adopt trans-like geometry
to minimize steric repulsion

−
R
RC

Na

CR

C

Na

C

R
Alkyne radical anion

A

Step 2. First protonation
−
R
C

R

H
H−NH2

C

R

C

C
−

R

NH2

Alkenyl radical

B

Step 3. Second one-electron transfer
R

H
C

R

H
Na

C

Na

C

R

C

R

−
Alkenyl anion

C

Step 4. Second protonation
R

H
C

C

R

R

H
H−NH2

−

C

−

C

R

NH2

H

Trans alkene

D

The equation below illustrates the application of dissolving-metal reduction in the synthesis of the sex pheromone of the spruce budworm, which is the most destructive pest to
the spruce and fir forests of North America. The pheromone “lure” is employed at hundreds
of sites in the United States and Canada as part of an integrated pest-management strategy
(Section 12-17). The key reaction is reduction of 11-tetradecyn-1-ol to the corresponding
trans alkenol. Subsequent oxidation to the aldehyde completes the synthesis.
Primary alcohol

HO(CH2)10C q CCH2CH3
11-Tetradecyn-1-ol

Na, liquid NH3
Reduction

HO(CH2)10

H
G
G
C PC
D
D
CH2CH3
H

trans-11-Tetradecen-1-ol

The spruce budworm, a serious
pest.
Oxidized by PCC

PCC, CH2Cl2
Oxidation
(Section 8-6)

to aldehyde
O
B
H
HC(CH2)9
G
G
C PC
D
D
H
CH2CH3

Sex pheromone of the
spruce budworm


582

Chapter 13

Alkynes

Exercise 13-13

Working with the Concepts: Selectivity in Reduction
When 1,7-undecadiyne (11 carbons) was treated with a mixture of sodium and sodium amide in liquid
ammonia, only the internal bond was reduced to give trans-7-undecen-1-yne. Explain. (Hint: What
reaction takes place between sodium amide and a terminal alkyne? Note that the pKa of NH3 is 35.)
Strategy
First, write out the equation for the reaction. Then consider the substrate’s functionality in the
context of the reaction conditions.
Solution
• The equation is
Na, NaNH2, NH3

• The conditions are strongly reducing (Na), but also strongly basic (NaNH2). We learned earlier
in the chapter that the pKa of a terminal alkynyl hydrogen is about 25. Sodium amide, which is
the conjugate base of the exceedingly weak acid ammonia, readily deprotonates the terminal
alkyne, giving an alkynyl anion, RC q C:2.
• The dissolving-metal reduction process requires electron transfer to the triple bond. However,
the negative charge on the deprotonated terminal alkyne repels any attempt to introduce additional
electrons, rendering that particular triple bond immune to reduction. Therefore, only the internal
triple bond is reduced, producing a trans alkene.

Exercise 13-14

Try It Yourself
What should be the result of the treatment of 2,7-undecadiyne with a mixture of excess sodium
and sodium amide in liquid ammonia? Explain any differences between this outcome and that in
Exercise 13-13.

In Summary Alkynes are very similar in reactivity to alkenes, except that they have two
p bonds, both of which may be saturated by addition reactions. Hydrogenation of the first
p bond, which gives cis alkenes, is best achieved by using the Lindlar catalyst. Alkynes are
converted into trans alkenes by treatment with sodium in liquid ammonia, a process that
includes two successive one-electron reductions.

13-7 Electrophilic Addition Reactions of Alkynes
As a center of high electron density, the triple bond is readily attacked by electrophiles.
This section describes the results of three such processes: addition of hydrogen halides, reaction with halogens, and hydration. The hydration is catalyzed by mercury(II) ions. As is the
case in electrophilic additions to unsymmetrical alkenes (Section 12-3), the Markovnikov
rule is followed in transformations of terminal alkynes: The electrophile adds to the terminal
(less substituted) carbon atom.

Addition of hydrogen halides forms haloalkenes and
geminal dihaloalkanes
The addition of hydrogen bromide to 2-butyne yields (Z)-2-bromobutene. The mechanism
is analogous to that of hydrogen halide addition to an alkene (Section 12-3).


13-7 Electrophilic Addition Reactions of Alkynes

Chapter 13

Addition of a Hydrogen Halide to an Internal Alkyne
HBr, BrϪ

CH3C q CCH3

CH
D 3

G
CP C
D
G
Br
H3 C
60%
H

(Z)-2-Bromobutene

The stereochemistry of this type of addition is typically anti, particularly when excess
halide ion is used. A second molecule of hydrogen bromide may also add, with regioselectivity that follows Markovnikov’s rule, giving the product with both bromine atoms bound
to the same carbon, a geminal dihaloalkane.
CH
D 3
G
CP C
G
D
Br
H3 C
H

HBr

H Br
A A
CH3CHCCH3
A

Br
90%

Both bromines
add to the
same carbon

2,2-Dibromobutane

The addition of hydrogen halides to terminal alkynes also proceeds in accord with the
Markovnikov rule.
Addition to a Terminal Alkyne

CH3C q CH

HI, Ϫ70ЊC

H
D
G
CP C
D
G
H
H3C
35%
I

ϩ

Both iodines add to
the same carbon

I H
A A
CH3C O C O H
A A
I H
65%

Both
hydrogens
add to the
same carbon

It is usually difficult to limit such reactions to addition of a single molecule of HX.
Exercise 13-15
Write a step-by-step mechanism for the addition of HBr twice to 2-butyne to give 2,2-dibromobutane.
Show clearly the structure of the intermediate formed in each step.

Halogenation also takes place once or twice
Electrophilic addition of halogen to alkynes proceeds through the intermediacy of isolable
vicinal dihaloalkenes, the products of a single anti addition. Reaction with additional
halogen gives tetrahaloalkanes. For example, halogenation of 3-hexyne gives the expected
(E)-dihaloalkene and the tetrahaloalkane.
Double Halogenation of an Alkyne
CH3CH2C q CCH2CH3

3-Hexyne

Br2, CH3COOH, LiBr

CH3CH2
Br
D
G
CP C
G
D
CH2CH3
Br
99%
(E)-3,4-Dibromo-3-hexene

Exercise 13-16
Give the products of addition of one and two molecules of Cl2 to 1-butyne.

Br2, CCl4

Br Br
A A
CH3CH2C O CCH2CH3
A A
Br Br
95%
3,3,4,4-Tetrabromohexane

583

584

Chapter 13

Alkynes

Mercuric ion – catalyzed hydration of alkynes furnishes ketones
In a process analogous to the hydration of alkenes, water can be added to alkynes in a
Markovnikov sense to give alcohols — in this case enols, in which the hydroxy group is
attached to a double-bond carbon. As mentioned in Section 12-16, enols spontaneously
rearrange to the isomeric carbonyl compounds. This process, called tautomerism, interconverts two isomers by simultaneous proton and double-bond shifts. The enol is said to
tautomerize to the carbonyl compound, and the two species are called tautomers (tauto,
Greek, the same; meros, Greek, part). We shall look at tautomerism more closely in Chapter 18 when we investigate the behavior of carbonyl compounds. Hydration followed by
tautomerism converts alkynes into ketones. The reaction is catalyzed by Hg(II) ions.
Hydration of Alkynes
RC q CR

HOH, Hϩ, HgSO4

OH
A
RCH P CR

H O
A B
RC O CR
A
H

Tautomerism

Enol

Ketone

Hydration follows Markovnikov’s rule: Terminal alkynes give methyl ketones.
Hydration of a Terminal Alkyne
OH

OH
O
B

H2SO4, H2O, HgSO4

91%
Exercise 13-17
Draw the structure of the enol intermediate in the reaction above.

Symmetric internal alkynes give a single carbonyl compound; unsymmetric systems lead to
a mixture of ketones.
Hydration of Internal Alkynes
H2SO4, H2O, HgSO4

O
80%
Only possible product

Example of Hydration of an Internal Alkyne That Gives a Mixture of Two Ketones
CH3CH2CH2C q CCH3

H2SO4, H2O, HgSO4

O
B
CH3CH2CH2CCH2CH3
50%

ϩ

O
B
CH3CH2CH2CH2CCH3
50%

Exercise 13-18
Give the products of mercuric ion – catalyzed hydration of (a) ethyne; (b) propyne; (c) 1-butyne;
(d) 2-butyne; (e) 2-methyl-3-hexyne.


13-8 Anti-Markovnikov Additions to Triple Bonds

585

Chapter 13

Exercise 13-19

Working with the Concepts: Using Alkynes in Synthesis
Propose a synthetic scheme that will convert compound A into B (see margin). [Hint: Consider

OH
A
a route that proceeds through the alkynyl alcohol (CH3)2CC q CH.]
Strategy
The hint reveals to us a possible retrosynthetic analysis of the problem:
HO

O

HO

O

Let us consider what we have learned so far in this chapter that can be helpful. This section has
shown how alkynes can be converted to ketones by mercury ion-catalyzed hydration. Section 13-5
introduced a new strategy for the formation of carbon – carbon bonds through the use of alkynyl
anions. Beginning with the three-carbon ketone A (acetone), our first task is to add a two-carbon
alkynyl unit. Referring to Section 13-5, we can use any of several methods to convert ethyne into
the corresponding anion.
Solution
• Adding the anion to acetone gives the necessary intermediate alcohol:
O

HO

HC q CH

LiNH2 (1 equivalent),
liquid NH3

HC q CLi

1.
2. H2O

Cq

CH

• Finally, hydration of the terminal alkyne function, as illustrated for the cyclohexyl derivative
shown on the previous page, completes the synthesis:
HO

HO
H2SO4, H2O, HgSO4

O

Exercise 13-20

Try It Yourself
Propose a synthesis of trans-3-hexene starting with 1-butyne.

In Summary Alkynes can react with electrophiles such as hydrogen halides and halogens
either once or twice. Terminal alkynes transform in accord with the Markovnikov rule.
Mercuric ion – catalyzed hydration furnishes enols, which convert into ketones by a process
called tautomerism.

13-8 Anti-Markovnikov Additions to Triple Bonds
Just as methods exist to permit anti-Markovnikov additions to double bonds (Sections 12-8

and 12-13), similar techniques allow additions to terminal alkynes to be carried out in an
anti-Markovnikov manner.

Radical addition of HBr gives 1-bromoalkenes
As with alkenes, hydrogen bromide can add to triple bonds by a radical mechanism in an
anti-Markovnikov fashion if light or other radical initiators are present. Both syn and anti
additions are observed.

HO
O
O
A

B


586

Chapter 13

Alkynes

CH3(CH2)3C q CH

HBr, ROOR

CH3(CH2)3CH P CHBr
74%
cis- and trans-1-Bromo-1-hexene

1-Hexyne

Aldehydes result from hydroboration – oxidation of
terminal alkynes
Terminal alkynes are hydroborated in a regioselective, anti-Markovnikov fashion, the boron
attacking the less hindered carbon. However, with borane itself, this reaction leads ultimately to sequential hydroboration of both p bonds. To stop at the alkenylborane stage,
bulky borane reagents, such as dicyclohexylborane, are used.
Hydroboration of a Terminal Alkyne
CH3(CH2)5C q CH ϩ

BH

THF

Anti-Markovnikov
addition

H
D
G
CP C
G
D
B
H

CH3(CH2)5

2

2

1-Octyne

94%

Dicyclohexylborane

Exercise 13-21
Dicyclohexylborane is made by a hydroboration reaction. What are the starting materials for its
preparation?

Like alkylboranes (Section 12-8), alkenylboranes can be oxidized to the corresponding
alcohols — in this case, to terminal enols that spontaneously rearrange to aldehydes.
Hydroboration–Oxidation of a Terminal Alkyne
CH3(CH2)5C q CH

1. Dicyclohexylborane
2. H2O2, HOϪ
Anti-Markovnikov
hydroboration
followed by oxidation

CH3(CH2)5

H
D
G
CP C
G

D
OH
H
Enol

Tautomerism

OH on less
substituted
carbon

H O
A B
CH3(CH2)5CO CH
A
H
70%
Octanal

Exercise 13-22
Give the products of hydroboration – oxidation of (a) ethyne; (b) 1-propyne; (c) 1-butyne.

Exercise 13-23
Outline a synthesis of the following molecule from 3,3-dimethyl-1-butyne.
O
B
(CH3)3CCH2CH

In Summary HBr in the presence of peroxides undergoes anti-Markovnikov addition to
terminal alkynes to give 1-bromoalkenes. Hydroboration – oxidation with bulky boranes furnishes intermediate enols that tautomerize to the final product aldehydes.

13-9 Chemistry of Alkenyl Halides

13-9 Chemistry of Alkenyl Halides
We have encountered haloalkenes — alkenyl halides — as intermediates in both the preparation of alkynes by dehydrohalogenation and also the addition to alkynes of hydrogen halides.
Alkenyl halides have become increasingly important as synthetic intermediates in recent
years as a result of developments in organometallic chemistry. These systems do not, however, follow the mechanisms familiar to us from our survey of the haloalkanes (Chapters 6
and 7). This section discusses their reactivity.

Alkenyl halides do not undergo SN2 or SN1 reactions
Unlike haloalkanes, alkenyl halides are relatively unreactive toward nucleophiles. Although
we have seen that, with strong bases, alkenyl halides undergo elimination reactions to give
alkynes, they do not react with weak bases and relatively nonbasic nucleophiles, such as
iodide. Similarly, SN1 reactions do not normally take place, because the intermediate alkenyl
cations are species of high energy.
H
D
CH2 PC
G
Br

IϪ

H
D
CH2 PC
ϩ BrϪ
G
I

H
D
CH2 PC
G
Br

ϩ

CH2 PCO H ϩ BrϪ
Ethenyl (vinyl)
cation

Does not take place

Does not take place

Alkenyl halides, however, can react through the intermediate formation of alkenyl
organometallics (see Exercise 11-6). These species allow access to a variety of specifically
substituted alkenes.
Alkenyl Organometallics in Synthesis
Grignard addition to ketone gives tertiary alcohol

Br
D
CH2 PC
ϩ
G
H

Mg

THF

1-Bromoethene
(Vinyl bromide)

O
B
1. CH3CCH3
ϩ
2. H , H2O

MgBr
D
CH2 PC
G
H
90%

OH
A
C(CH3)2
D
CH2 PC
New
G
C–C
H
bond

65%
2-Methyl-3-buten-2-ol

Ethenylmagnesium
bromide
(A vinyl Grignard
reagent)

Metal catalysts couple alkenyl halides to alkenes in the
Heck reaction
In the presence of soluble complexes of metals such as Ni and Pd, alkenyl halides undergo
carbon – carbon bond formation with alkenes to produce dienes. In this process, called the
Heck* reaction, a molecule of hydrogen halide is liberated.
The Heck Reaction
HCl is lost

Cl
D
PC
H2C
G
H

H
ϩ

D
C P CH2
G
H

Ni or Pd
catalyst
Ϫ HCl

*Professor Richard F. Heck (b. 1931), University of Delaware.

A new C–C bond forms
H
A
KCH KCH2
H2C
C
A
H

Chapter 13

587


588

Alkynes

Chapter 13

CHE M I C AL H I G H L I G H T 1 3 - 1
Metal-Catalyzed Stille, Suzuki, and Sonogashira Coupling Reactions
Three additional processes, the Stille, Suzuki, and

Sonogashira* reactions, further broaden the scope of
transition metal – catalyzed bond-forming processes. All
utilize catalytic palladium or nickel; the differences lie

in the nature and functionality of the substrates commonly
employed.
In the Stille coupling, Pd catalyzes the direct linkage
between alkenyl halides and alkenyltin compounds:

Stille Coupling Reaction
O

O
I

O

ϩ

Pd catalyst,
CuI, R3As

(CH3)3Sn

O
O

O

93%

Copper(I) iodide and an arsenic-derived ligand, R3As,
facilitate this very efficient process. The product shown was
converted into a close relative of a microbially derived natural product that inhibits a factor associated with immune and

inflammation responses. This factor also affects HIV activation and cell-death processes that are disrupted in cancer.
The Suzuki reaction replaces tin with boron and provides a different spectrum of utility. In particular,

Suzuki Coupling Reaction
I

Ni catalyst,
base

ϩ

(HO)2B
63%

*Professor John K. Stille (1930–1990), Colorado State University;
Professor Akira Suzuki (b. 1930), Kurashiki University, Japan; Professor
Kenkichi Sonogashira (b. 1931), Osaka City University, Japan.

In common with other transition metal – catalyzed cross-couplings (see Chemical Highlight 8-3), assembly of the fragments around the catalyst precedes carbon – carbon bond
formation. A simplified mechanism for the Heck reaction begins with reaction between the
metal and the alkenyl halide to give an alkenylmetal halide (1). The alkene then complexes
with the metal (2), and inserts itself into the carbon – metal bond, forming the new carbon –
carbon linkage (3). Finally, elimination of HX in an E2-like manner gives the diene product
and frees the metal catalyst (4).
Mechanism of the Heck Reaction

(3)
Alkene
insertion

H2C

KCHH

H2C PCH O Pd O Cl

ECH2H ECl
Pd
CH
A
H

(4)
Elimination

(2)
ϩ H2CPCH2

H2C PCH O Pd O Cl
ł
H2C PCH2
—

H2C PCH O Cl

(1)
ϩ Pd

H
A
KC H KCH2
C
H2C
A
H

ϩ

HCl

ϩ

Pd

The growing popularity of the Heck reaction arises from both its versatility and its
efficiency. In particular, it requires only a very small amount of catalyst compared with the
quantity of the substrates; typically, 1% of palladium acetate in the presence of a phosphine
ligand (R3P) is sufficient.


13-9 Chemistry of Alkenyl Halides

Chapter 13

589

The boron-containing substrate (a boronic acid) is
efficiently prepared by hydroboration of a terminal alkyne
with a special reagent, catechol borane:

Suzuki coupling succeeds with primary and secondary
haloalkanes, which are poor Stille substrates. In the example
below, Ni gives better results than does Pd.

Preparation of an Alkenylboronic Acid
O

R

B OH

O

Hϩ, H2O

B
O

O

(HO)2B

R

R

H

Catechol borane

Alkenyl boronic acid

Boronic acids are prepared commercially in very large
quantities, and the Suzuki coupling has become a major
industrial process. Boronic acids are stable and easier to
handle than organotin compounds, which are toxic and must
be handled with great care.
Finally, the Sonogashira reaction has a niche of its
own as a preferred method for linking alkenyl and alkynyl

moieties. As in the Stille process, Pd, CuI, and ligands derived
from nitrogen-group elements are employed. However, there
is no need for tin; terminal alkynes react directly. The added
base removes the HI by-product.

Sonogashira Coupling Reaction
O

O
I

ϩ

H

Pd catalyst,

CuI, R3P, base
Ϫ HI

89%

Examples of Heck Reactions
Br
ϩ

O
B
COCH3

New
C–C bond

O
B
1% Pd(OCCH3)2, R3P, 100ЊC

72%
ON EOH
C
ϩ

O
B
COCH3

O

B
1% Pd(OCCH3)2, R3P, 100ЊC

ON EOH
C
New
C–C bond

Br
67%

COCH3
B
O

Exercise 13-24
Write out a detailed step-by-step mechanism for the first of the two examples of Heck reactions
above.

O
B
COCH3


590

Chapter 13

Alkynes

In Summary Alkenyl halides are unreactive in nucleophilic substitutions. However, they
can participate in carbon – carbon bond-forming reactions after conversion to alkenyllithium
or alkenyl Grignard reagents, or in the presence of transition-metal catalysts such as
Ni and Pd.

13-10 Ethyne as an Industrial Starting Material
Ethyne was once one of the four or five major starting materials in the chemical industry
for two reasons: Addition reactions to one of the p bonds produce useful alkene monomers
(Section 12-15), and it has a high heat content. Its industrial use has declined because of
the availability of cheap ethene, propene, butadiene, and other hydrocarbons through oilbased technology. However, in the 21st century, oil reserves are expected to dwindle to the
point that other sources of energy will have to be developed. One such source is coal. There
are currently no known processes for converting coal directly into the aforementioned
alkenes; ethyne, however, can be produced from coal and hydrogen or from coke (a coal
residue obtained after removal of volatiles) and limestone through the formation of calcium
carbide. Consequently, it may once again become an important industrial raw material.

Production of ethyne from coal requires high temperatures
The high energy content of ethyne requires the use of production methods that are costly
in energy. One process for making ethyne from coal uses hydrogen in an arc reactor at
temperatures as high as several thousand degrees Celsius.
Coal

ϩ

H2

Δ

HC q CH

ϩ

nonvolatile salts

33% conversion

The oldest large-scale preparation of ethyne proceeds through calcium carbide. Limestone (calcium oxide) and coke are heated to about 20008C, which results in the desired
product and carbon monoxide.
3C

ϩ

Coke

2000°C

CaO
Lime

ϩ

CaC2

CO

Calcium carbide

The calcium carbide is then treated with water at ambient temperatures to give ethyne and
calcium hydroxide.
Vivid demonstration of the

combustion of ethyne, generated
by the addition of water to calcium
carbide.

CaC2

ϩ

2 H2O

HC q CH

ϩ

Ca(OH)2

Ethyne is a source of valuable monomers for industry
Ethyne chemistry underwent important commercial development in the 1930s and 1940s in
the laboratories of Badische Anilin and Sodafabriken (BASF) in Ludwigshafen, Germany.
Ethyne under pressure was brought into reaction with carbon monoxide, carbonyl compounds, alcohols, and acids in the presence of catalysts to give a multitude of valuable raw
materials to be used in further transformations. For example, nickel carbonyl catalyzes the
addition of carbon monoxide and water to ethyne to give propenoic (acrylic) acid. Similar
exposure to alcohols or amines instead of water results in the corresponding acid derivatives.
All of these products are valuable monomers (see Section 12-15).
Industrial Chemistry of Ethyne
H
HC q CH

ϩ

CO

ϩ

H2O

Ni(CO)4, 100 atm, >250°C

G
C PCHCOOH
D
H
Propenoic acid
(Acrylic acid)


13-10 Ethyne as an Industrial Starting Material

Chapter 13

591

Polymerization of propenoic (acrylic) acid and its derivatives produces materials
of considerable utility. The polymeric esters (polyacrylates) are tough, resilient, and
flexible polymers that have replaced natural rubber (see Section 14-10) in many applications. Poly(ethyl acrylate) is used for O-rings, valve seals, and related purposes in
automobiles. Other polyacrylates are found in biomedical and dental appliances, such
as dentures.
The addition of formaldehyde to ethyne is achieved with high efficiency by using copper
acetylide as a catalyst.
HC q CH

ϩ

CH2 P O

Cu2C2–SiO2, 125°C, 5 atm

HC q CCH2OH

or

HOCH2C q CCH2OH

2-Propyn-1-ol
(Propargyl alcohol)

2-Butyne-1,4-diol

The resulting alcohols are useful synthetic intermediates. For example, 2-butyne-1,4-diol is
a precursor for the production of oxacyclopentane (tetrahydrofuran, one of the solvents most
frequently employed for Grignard and organolithium reagents) by hydrogenation, followed
by acid-catalyzed dehydration.
Oxacyclopentane (Tetrahydrofuran) Synthesis

HOCH2C q CCH2OH

Catalyst, H2

HO(CH2)4OH

H3PO4, pH 2,
260–280°C, 90–100 atm

O
99%

ϪH2O

Oxacyclopentane
(Tetrahydrofuran, THF)

Several technical processes have been developed in which reagents d1A – Bd2 in the
presence of a catalyst add to the triple bond. For example, the catalyzed addition of hydrogen chloride gives chloroethene (vinyl chloride), and addition of hydrogen cyanide produces
propenenitrile (acrylonitrile).
Addition Reactions of Ethyne
H
HC q CH

ϩ

HCl

Hg2ϩ, 100–200°C

G
C PCHCl
D
H
Chloroethene
(Vinyl chloride)

HC q CH

ϩ

HCN

Cuϩ, NH4Cl, 70–90°C, 1.3 atm

H

G
C PCHCN
D
H
80–90%
Propenenitrile
(Acrylonitrile)

In 2007, the world produced 2.5 million tons of acrylic fibers, polymers containing at least
85% propenenitrile (acrylonitrile). Their applications include clothing (Orlon), carpets, and
insulation. Copolymers of acrylonitrile and 10 – 15% vinyl chloride have fire-retardant properties and are used in children’s sleepwear.

In Summary Ethyne was once, and may again be in the future, a valuable industrial feedstock because of its ability to react with a large number of substrates to yield useful monomers and other compounds having functional groups. It can be made from coal and H2 at
high temperatures, or it can be prepared from calcium carbide by hydrolysis. Some of the
industrial reactions that it undergoes are carbonylation, addition of formaldehyde, and addition reactions with HX.

Poly(vinyl chloride) is extensively
used in the construction industry
for water and sewer pipes.