Functional Classes II, Reactions - Organic Chemistry Concepts: An EFL Approach - page 110

7.4 ALKYNES
Alkynes are related in structure to alkenes, and the triple bond is a region of high
electron density. For this reason, the addition chemistry of alkynes is similar
to alkenes. A substitution reaction can occur with a terminal alkyne. Terminal
means that the alkyne is at the end of the chain. One end of the alkyne is then
an unsubstituted C—H group.
7.4.1 Alkyne Addition Chemistry
The electrophilic addition of hydrogen halides and halogens is as expected.
Because there are two π-bonds, addition can occur two times, as shown in Figure
7.11 . For unsymmetrical situations, Markovnikov's rule is followed.
Hydrogenation gives complete saturation by using two equivalents of H 2 . Reduc-
tion can be stopped at the alkene stage using the Lindlar palladium catalysts, Pd-
BaCO 3 or Pd-CaCO 3 .
Hydration , with the help of a catalyst, initially gives alcohol products which
have the hydroxy group directly attached to a C ] C. This structure is the enol,
and it quickly tautomerizes to the more stable ketone, as seen in Chapter 6.
FIGURE 7.11
Alkyne addition reactions.
7.4.2 Terminal Alkyne Carbanions
Chapter 6 showed that sp -hybridization gives a carbon which is more electro-
negative. As a result, a terminal alkyne C-H bond is more polarized and has a
higher acidity (p K a ≈ 25). Strong bases such as sodium amide (NaNH 2 ) can react
with these protons.
This reaction gives an organometallic compound which has both organic and
metallic parts. This alkynide is also commonly called an acetylide. These carb-
anions are good nucleophilic reagents which react with electrophiles. If an alkyl
halide is used as the electrophile, it gives a very useful C-C bond-forming pro-
cess as shown in Figure 7.12 . This second step is an example of the S N 2 nucleo-
philic substitution reaction shown in Figure 7.16 .