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.

Hydration follows Markovnikov’s rule—terminal alkynes give methyl ketones:

Symmetrical internal alkynes give a single carbonyl compound; unsymmetrical systems give a mixture of products:

Anti-Markovnikov Additions to Triple Bonds13-8

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.

Chemistry of Alkenyl Halides13-9

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, they 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.

Organometallic derivatives of alkenyl halides can react to produce a variety of specifically substituted alkenes:

Ethyne as an Industrial Starting Material13-10

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.

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.

2-Butyne-1,4-diol is the precursor for the production of oxacyclopentane (tetrahydrofuran):

Polymers of acrylonitrile (acrylic fibers) include clothing (Orlon), carpets, and insulation.

Copolymers of acrylonitrile and 10–15% vinyl chloride are used in children’s sleepwear due to their fire resistant properties.

Naturally Occurring and Physiologically Active Alkynes

13-11

Chamomile flower

Chrysanthemum

Lower Amazon Indian arrowhead poison

Poison Arrow Frog toxin

Nonprescription hypnotic Naturally occurring antibiotic-antitumor agents