Alkynes are hydrocarbons which contain carbon-carbon triple bonds. Their general formula is CnH2n-2 for molecules with one triple bond (and no rings).
Alkynes undergo many of the same reactions as alkenes, but can react twice because of the presence of the two p-bonds in the triple bond.
Straight-chain and branched alkynes are named by using the stem that indicates the number of carbon atoms to which is added the suffix -yne. A locator number is placed immediately in front of the prefix to indicate which carbon number the double bond starts on. Thus, the word "1-butyne" indicates a chain of four carbons, with a triple bond between carbons 1 and 2; the word "2-butyne" indicates a chain of four carbons, with a triple bond between carbons 2 and 3.
Triple-bonded carbons are sp-hybridized, and have linear shapes, with the bonded atoms at angles of 180° to each other. Because of this linear shape, geometric isomerism does not occur in alkynes.
Alkynes are nonpolar, since they contain nothing but carbon and hydrogen, and so, like the alkanes and alkenes, they are not soluble in water, and are generally less dense than water.
As we organic chemists say, "It takes alkynes to make a world." (The jury is still out as to whether or not we should keep saying that.)
|Ethyne (Acetylene), HC≡CH (C2H2)||3D|
Ethyne is more commonly known under the trivial name acetylene. It is the simplest of the alkynes, consisting of two carbon atoms connected by a triple bond, leaving each carbon able to bond to one hydrogen atom. Since both carbon atoms are linear in shape, all four atoms lie in a straight line. Acetylene is a colorless, unpleasant-smelling gas (bp -28.1°C), which burns in air to produce a very sooty flame. In the presence of pure oxygen, however, it burns at very high temperatures (up to 2800°C), and is used in welding and cutting torches. Acetylene can be produced by the reaction of calcium carbide (CaC2) with water; this reaction was used to power light on headlamps used by miners, until safer methods of illumination were developed.
|1-Propyne, HC≡CCH3 (C3H4)||3D|
Increasing the length of the carbon chain by one brings us to 1-propyne (also known as methylacetylene); all three carbon atoms lie in a straight line. Alkynes which are located at the beginning of a carbon chain are known as terminal alkynes (although sometimes they recover — ha, ha). 1-Propyne burns more cleanly in air than acetylene does, and is also used in welding torches. It is often used in a mixture with 1,2-propadiene, which is known as MAPP® gas (methylacetylene-propadiene)
|1-Butyne, HC≡CCH2CH3 (C4H6)||3D|
An alkyne which contains four carbon atoms in the chain has two structural isomers: 1-butyne and 2-butyne. 1-Butyne is a terminal alkyne, with the first three carbon atoms in the chain adopting a linear shape because of the triple bond.
|2-Butyne, CH3C≡CCH3 (C4H6)||3D|
2-Butyne is a structural isomer of 1-butyne, having a carbon-carbon triple bond in the middle of the chain. All four carbon atoms lie in a straight line in this molecule.
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