Alkynes are unsaturated hydrocarbons containing at least one carbon-carbon triple bond in organic chemistry. Alkynes are generally hydrophobic like other hydrocarbons, but tend to be more reactive. Alkynes are more unsaturated than alkenes, but in some reactions, alkynes are less reactive than alkenes. For example, in a molecule having one alkenyl and one alkynyl group, addition occurs preferentially to alkenyl group. Simple alkyne compounds include acetylene, propyne and so on. Among them, acetylene is the most important alkyne, which can be used to weld and cut metal in the industry (oxyacetylene flame), and is also the basic raw material for manufacturing acetaldehyde, acetic acid, benzene, synthetic rubber and synthetic fiber.
Organic chemistry: Alkynes are "doubly unsaturated," which means that 2 equivalents of halogen or related HX reagent (X = halide, pseudohalide, and so on) can be added into per alkyne unit. Alkynes can undergo different cycloaddition reactions. The most notable reaction is the Diels-Alder reaction with 1,3-dienes to produce 1,4-cyclohexadiene. This general reaction has been widely developed, and electrophilic alkynes are particularly effective dienophiles. The cycloadduct obtained from the addition of alkynes to 2-pyrone eliminates carbon dioxide and gives an aromatic compound. Other specialized cycloaddition reactions include multi-component reactions such as alkyne trimerization to produce aromatics and [2+2+1]-cyclicaddition of alkynes, alkenes and carbon monoxide in the Pauson-Khand reaction. Cycloadditions involving alkynes are usually catalyzed by metals.
Nature product: In 1826, the first naturally occuring acetylenic compound, dehydromatricaria ester, was isolated from Artemisia species. Polyyne is a subset of this type of natural product that has been isolated from cultures of a wide variety of plant species, higher fungi, bacteria, marine sponges and corals. Some acids, such as tylonic acid, contain alkynyl groups. Other examples are cicutoxin, oantantoxin, falcarinol and carotatoxin, which are highly bioactive such as nematicides.
Medical chemistry: Alkynes appear in some medicines, including the noretynodrel pill. Carbon-carbon triple bonds exist in marketed drugs such as the antiretroviral efavirenz and the antifungal terbinafine. The molecule called ene-diyne is characterized as a ring containing alkane between two alkyne groups. These compounds, such as calicheamicin, are known as the most aggressive antitumor drugs, so that ene-diyne is sometimes referred to as a "warhead". The ene-diyne rearranges through Bergman's cyclization, resulting in a highly active free-radical intermediate that attacks the tumor's DNA.