Alkyne
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2008-06-18T14:25:45Z
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[[Image:Acetylene-3D-vdW.png|Alkyne|right|thumb|A 3D model of [[acetylene]], the simplest alkyne]]
'''Alkynes''' are [[hydrocarbon]]s that have at least one [[triple bond]] between two [[carbon]] atoms, with the formula '''C<sub>n</sub>H<sub>2n-2</sub>'''. The alkynes are traditionally known as '''acetylenes''' or the '''acetylene series''', although the name ''acetylene'' is also used to refer specifically to the simplest member of the series, known as [[ethyne]] (C<sub>2</sub>H<sub>2</sub>) using formal [[IUPAC]] nomenclature.
==Chemical properties==
Unlike [[alkane]]s, and to a lesser extent, [[alkene]]s, alkynes are unstable and reactive. [[Terminal alkynes]] and [[acetylene]] are fairly acidic and have [[Acid dissociation constant|pK<sub>a</sub>]] values (25) between that of [[ammonia]] (35) and [[ethanol]] (16). This acidity is due to the ability for the negative charge in the acetylide [[conjugate base]] to be stabilized as a result of the high s character of the sp orbital, in which the electron pair resides. [[Electron]]s in an s orbital benefit from closer proximity to the positively charged atom nucleus, and are therefore lower in energy. This can also be thought of in terms of [[electronegativity]]: electrons in an [[hybrid orbital]] with high s character reside closer to the nucleus. The closer proximity of the electrons to the nucleus allows an acetylinic carbon to have a greater amount of [[electronegative character]]. As a result, a proton is more easily removed from the carbon as electrons flow more willingly to a more electronegative atom.
A terminal alkyne with a [[strong base]] such as [[sodium]], [[sodium amide]], [[n-butyllithium|''n''-butyllithium]] or a [[Grignard reagent]], gives the [[anion]] of the terminal alkyne (a '''metal acetylide'''):
:2 RC≡CH + 2 Na → 2 RC≡CNa + H<sub>2</sub>
More generally:
:RC≡CH + B → RC≡C<sup>−</sup> + HB<sup>+</sup>, where B denotes a strong base.
The acetylide anion is synthetically useful because as a strong [[nucleophile]], it can participate in C−C bond forming reactions.
It is also possible to form copper and silver alkynes, from this group of compounds [[silver acetylide]] is an often used example.
{{see also|Metal acetylide}}
== Structure==
The carbon atoms in an alkyne bond are [[sp hybridized]]: they each have 2 [[p orbital]]s and 2 [[Orbital hybridisation|sp hybrid orbitals]]. Overlap of an sp orbital from each atom forms one sp-sp [[sigma bond]]. Each p orbital on one atom overlaps one on the other atom, forming two [[pi bond]]s, giving a total of three bonds. The remaining sp orbital on each atom can form a sigma bond to another atom, for example to hydrogen atoms in the parent compound [[acetylene]]. The two sp orbitals on an atom are on opposite sides of the atom: in acetylene, the H-C-C [[bond angle]]s are 180°. Because a total of 6 electrons take part in bonding this triple bond is very strong with a [[bond strength]] of 839 kJ/mol. The sigma bond contributes 369 kJ/mol, the first pi bond contributes 268 kJ/mol and the second pi bond is weak with 202 kJ/mol bond strength. The CC bond distance with 121 [[picometer]]s is also much less than that of the [[alkene]] bond which is 134 pm or the alkane bond with 153 pm.
The simplest alkyne is [[ethyne]] ([[acetylene]]): H-C≡C-H
==Terminal and internal alkynes==
Terminal alkynes have a hydrogen atom bonded to at least one of the sp hybridized carbons (those involved in the triple bond. An example would be [[methylacetylene]] (1-propyne using IUPAC nomenclature).
[[Image:1-propyne.svg|frame|center|1-propyne]]
Internal alkynes have something other than hydrogen attached to the sp hybridized carbons, usually another carbon atom, but could be a heteroatom. A good example is 2-pentyne, in which there is a methyl group on one side of the triple bond and an ethyl group on the other side.
[[Image:2-pentyne.svg|frame|center|2-pentyne]]
The terminal Hydrogen atom is weakly acidic, and can be removed by a very strong base, to yield a salt. This property can be used as a chemical test to distinguish terminal alkynes from others, or the salt may be used to make larger alkyne molecules. A few drops of diamminesilver(I) hydroxide (Ag(NH3)2+ -OH or Ag(NH3)2OH)) solution are added to samples of a non-terminal alkyne and also a terminal alkyne. No reaction occurs for the non-terminal, but the terminal alkyne forms a characteristic white precipitate. This is the insoluble silver salt of the terminal alkyne:
R-C≡CH + Ag(NH3)2+ -OH → R-C≡C- Ag+ + NH4+ + NH3 (R = general alkyl group)
Warning: transition metal salts of terminal alkynes (metal; acetylides) can be explosive when
dry.
== Synthesis ==
Alkynes are generally prepared by [[dehydrohalogenation]] of [[Vicinal (chemistry)|vicinal]] alkyl [[dihalide]]s or the reaction of metal acetylides with primary [[alkyl halide]]s. In the [[Fritsch-Buttenberg-Wiechell rearrangement]] an alkyne is prepared starting from a [[vinyl|vinyl bromide]].
Alkynes can be prepared from [[aldehyde]]s using the [[Corey-Fuchs reaction]] and from aldehydes or [[ketone]]s by the [[Seyferth-Gilbert homologation]].
==Reactions==
Alkynes are involved in many [[organic reaction]]s.
* [[electrophilic addition]] reactions
** addition of [[hydrogen]] to give the [[alkene]] or the [[alkane]]
** addition of [[halogen]]s to give the vinyl halides or alkyl halides
** addition of [[hydrogen halide]]s to give the corresponding [[vinyl halide]]s or [[alkyl halide]]s
** [[Nicholas reaction]]
** addition of water to give the [[carbonyl]] compound (often through the [[enol]] intermediate), for example the [[hydrolysis]] of [[phenylacetylene]] to [[acetophenone]] with [[sodium tetrachloroaurate]] in water/methanol (scheme shown below)<ref>Fukuda, Y.; Utimoto, K. "Effective transformation of unactivated alkynes into ketones or acetals with a gold(III) catalyst". ''[[J. Org. Chem.]]'' '''1991''', ''56'', 3729–3731. {{DOI|10.1021/jo00011a058}}</ref> or (Ph<sub>3</sub>P)AuCH<sub>3</sub> <ref>Mizushima, E.; Cui, D.-M.; Nath, D. C. D.; Hayashi, T.; Tanaka, M. "Au(I)-Catalyzed hydratation of alkynes: 2,8-nonanedione". ''[[Organic Syntheses]]'', Vol. 83, p.55 (2005). [http://www.orgsynth.org/orgsyn/pdfs/v83p0055.pdf Link].</ref>:
[[Image:AlkyneHydrolysis.svg|center|400px|Alkyne hydrolysis]]
* [[Cycloaddition]]s
** [[Diels-Alder reaction]] with [[2-pyrone]] to an [[aromatic]] compound after elimination of [[carbon dioxide]]
** [[Azide alkyne Huisgen cycloaddition]] to [[triazole]]s
** [[Bergman cyclization]] of enediynes to an [[aromatic]] compound
** [[Alkyne trimerisation]] to [[aromatic]] compounds
** [2+2+1]cycloaddition of an alkyne, [[alkene]] and [[carbon monoxide]] in the [[Pauson–Khand reaction]]
* [[alkyne metathesis|Metathesis]]
** scrambling of alkynes in [[alkyne metathesis]] to new alkyne compounds
** reaction with alkenes to butadienes in [[enyne metathesis]]
* [[nucleophilic substitution]] reactions of metal acetylides
** new [[carbon-carbon bond]] formation with alkyl halides
* [[nucleophilic addition]] reactions of [[metal acetylide]]s
** reaction with [[carbonyl]] compounds to an intermediate [[alkoxide]] and then to the [[hydroxyalkyne]] after acidic workup in the [[Favorskii reaction]].
* [[hydroboration-oxidation reaction|hydroboration]] of alkynes with [[organoborane]]s to vinylic boranes
** followed by reduction by oxidation with [[hydrogen peroxide]] to the corresponding [[aldehyde]] or [[ketone]]
* oxidative cleavage with [[potassium permanganate]] to the [[carboxylic acid]]s
* migration of the alkyne along a hydrocarbon chain by treatment with a strong base
* [[Coupling reaction]] with other alkynes to di-alkynes in the [[Cadiot-Chodkiewicz coupling]], [[Glaser coupling]] and the [[Eglinton coupling]].
==References==
{{Reflist}}
==See also==
{{wiktionary}}
*[[-yne]]
*[[cycloalkyne]]
{{Alkynes}}
{{Functional Groups}}
{{BranchesofChemistry}}
[[Category:Alkynes| ]]
[[Category:Hydrocarbons]]
[[Category:functional groups]]
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