Alkynes are organic or carbon-based compounds that contain a triple bond between two carbon atoms. This triple bond ensures these compounds have different structural and chemical properties from compounds with double-bonded carbons, which are called alkenes, or alkanes that have single-bonded carbons only. Although alkynes are uncommon in nature, one of them is important in industry and some naturally occurring compounds do contain triple-bonded carbons.
Ethyne aka Acetylene
Ethyne is the simplest of the alkynes; its molecular formula is C2H2, and it consists of two carbon atoms triple-bonded to each other with a hydrogen atom bonded to each of the carbons. At room temperature it's a colorless gas. The short, electron-rich triple bond has a high energy content and thus burning ethyne releases a lot of energy, which makes it a popular fuel in welding torches and other similar applications. Most welders refer to it by its common name, acetylene. In the early 20th century it was also an important starting material for synthesis of industrial chemicals, although its popularity waned as oil-based starting materials took its place.
Ethinyl Estradiol is a synthetic compound similar in structure to the naturally occurring compound estradiol, which is an important estrogen hormone in the female body. Like estradiol, it acts as an estrogen but it's extraordinarily potent and long lasting -- and that's why it's administered as an active ingredient in oral contraceptives. Introducing this compound into the body changes the body's hormone balance so that the ovaries do not ovulate and the lining of the womb becomes thinner.
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Some organisms produce poisonous alkyne compounds to help them deal with predators or prey. One example is ichthyothereol, a highly toxic alkyne found in the leaves of a small herb named Ichthyothere terminalis in Brazil. Indians living in the region once used this poison to kill fish. Another example is histrioconicotoxin, an alkyne compound found in the skin of the poison arrow frog from the same region. As the name suggests, its historical use was as a coating for arrowheads.
A few compounds of medical interest aside from ethinyl estradiol also contain alkyne units. One example is the compounds calicheamicin and esperamicin. Both are found in nature and have similar structures. They are extremely toxic to cells and work by breaking double-stranded DNA. Unfortunately, they are so toxic it's not possible to use them untethered as cancer drugs because they're too good at killing healthy cells. Some researchers, however, have sought to attach calicheamicin to antibodies that specifically target cancer cells so the poison will be delivered to the cancer cells only. One such drug called gemtuzumab ozogamicin was approved by FDA for treatment of acute myelogenous leukemia in 2000, but was withdrawn from the U.S. market in 2010.