Alkynes
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.)
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Ethyne (Acetylene), HC≡CH
(C2H2) |
3D |
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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. |
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1-Propyne, HC≡CCH3 (C3H4) |
3D |
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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) |
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1-Butyne, HC≡CCH2CH3 (C4H6) |
3D |
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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. |
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2-Butyne, CH3C≡CCH3 (C4H6) |
3D |
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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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Capillin |
3D |
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Capillin is an antifungal agent. |
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Parsalmide |
3D |
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Parsalmide is an analgesic, sold under the trade
names Parsal® and Sinovial®. |
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Pargyline |
3D |
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Pargyline is an antihypertensive agent, sold under
the trade names Eudatin® and Supirdyl®. |
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References
P. W. Atkins, Molecules, 2nd ed.
Cambridge: Cambridge University Press, 2003, p. 74.
Paula Yurkanis Bruice, Organic Chemistry,
4th ed. Upper Saddle River: Prentice Hall, 2004, p. 238-240.
Marye Anne Fox and James K. Whitesell,
Organic Chemistry, 3rd ed. Sudbury: Jones and Bartlett
Publishers, p. 75-78.
Maitland Jones, Jr., Organic Chemistry.
New York: W. W. Norton & Company, 1997, p. 142-145.
Richard J. Lewis, Sr., Hawley's Condensed
Chemical Dictionary, 13th ed. New York: Van Nostrand Reinhold,
1997.
G. Marc Loudon, Organic Chemistry, 4th
ed. New York: Oxford University Press, 2002, p. 606-610.
Robert Thornton Morrison and Robert Neilson
Boyd, Organic Chemistry, 6th ed. Englewood Cliffs: Prentice
Hall, 1992, p. 425-429.
D. W. A. Sharp, The Penguin Dictionary of
Chemistry, 2nd ed. London: Penguin Books, 1990.
Graham Solomons and Craig Fryhle, Organic
Chemistry, 7th ed. New York: John Wiley & Sons, Inc., 2000, p.
53.
L. G. Wade, Jr., Organic Chemistry, 5th
ed. Upper Saddle River: Prentice Hall, 2003, p. 370-374.
Martha Windholz (ed.), The Merck Index, 10th ed. Rahway: Merck & Co., Inc., 1983.