Chapter 13: Alkynes

1
Chapter 13 Alkynes

• Lectures 13 and 14

2
Preparation of Alkynes by Double Elimination
13-4
Alkynes are prepared from dihaloalkanes by
elimination.
3
For a terminal alkyne, three equivalents of base
are required A synthetic sequence called
halogenation-double dehydrohalogenation can be
used to convert alkenes into the corresponding
alkynes.
4
Haloalkenes are intermediates in alkyne synthesis
by elimination.
With diastereomerically pure vicinal
dihaloalkones, a single haloalkene product is
formed due to the anti elimination reaction
mechanism.
5
Preparation of Alkynes from Alkynyl Anions
13-5
Terminal alkynyl anions behave like strong
nucleophiles The reaction of this anion with a
primary halide provides a route for C-C bond
formation.
Reaction with secondary and tertiary halides
leads to E2 products.
6
Other reactions of alkynyl anions
7
Reduction of Alkynes The Relative Reactivity of
the Two p Bonds
13-6
Alkynes can undergo addition reactions, such as
hydrogenation and electrophilic attack.
8
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.
9
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.
10
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.
11
The second electron transfer takes place faster
than any cis/trans equilibrium of the alkenyl
radical.
12
The final alkene is stable to further reduction
by this reagent.
13
Electrophilic Addition Reactions of Alkynes
13-7
Addition of hydrogen halides forms haloalkenes
and geminal dihaloalkanes.
14
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 Markovnikovs rule.
15
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.
16
Mercury-catalyzed hydration of alkynes yields
ketones
17
Hydration follows Markovnikovs rule Terminal
alkynes give methyl ketones
18
Symmetrical internal alkynes give a single
carbonyl compound, but unsymmetrical systems give
a mixture of products
19
Aldehydes result from hydroboration of terminal
alkynes. This reaction occurs in an
anti-Markovnikov fashion The less hindered
carbon is attacked by the boron. In the case of
borane, BH3, both pi bonds react. To stop at the
alkenyl-borane stage, a bulky borane reagent must
be used