Difference between revisions of "Alkyne" - New World Encyclopedia

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[[Image:Alkyne.svg|Alkyne|right|200px|thumb|The [[structural formula]] of [[2-butyne]], a simple alkyne-containing [[molecule]].]]
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[[Image:Alkyne.png|Alkyne|right|250px|thumb|The [[structural formula]] of [[2-butyne]], a simple alkyne-containing [[molecule]].]]
  
'''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.
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'''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.
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{{toc}}
  
 
==Chemical properties==
 
==Chemical properties==
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Unlike [[alkane]]s and to a lesser extent, [[alkene]]s, alkynes are unstable and very reactive. 1-Alkynes are fairly acidic and have [[Acid dissociation constant|pK<sub>a</sub>]] values (25) between that of [[ammonia]] (35) or [[ethanol]] with 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 a s orbital benefit from closer proximity to the positively charged atom nucleus and are therefore lower in energy.
  
Unlike [[alkane]]s and to a lesser extent, [[alkene]]s, alkynes are unstable and very reactive. 1-Alkynes are fairly acidic and have [[Acid dissociation constant|pK<sub>a</sub>]] values (25) between that of [[ammonia]] (35) or [[ethanol]] with 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 a s orbital benefit from closer proximity to the positively charged atom nucleus and therefore lower in energy.
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A terminal alkyne with a [[strong base]] such as [[sodium]], [[sodium amide]], [[n-butyllithium]] or a [[Grignard reagent]] gives the [[anion]] of the terminal alkyne (a ''metal acetylide''):
 
 
A terminal alkyne with a [[strong base]] such as [[sodium]], [[sodium amide]], [[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>
 
:2 RC≡CH + 2 Na → 2 RC≡CNa + H<sub>2</sub>
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The acetylide anion is synthetically useful because as a strong [[nucleophile]], it can participate in C−C bond forming reactions.
 
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.
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It is also possible to form [[copper]] and [[silver]] alkynes, from this group of compounds [[silver acetylide]] is an often used example.
  
 
== Structure==
 
== Structure==
 
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The carbon atoms in an alkyne bond are [[sp hybridized]]&mdash;they each have two [[p orbital]]s and two [[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&mdash;in acetylene, the H-C-C [[bond angle]]s are 180°. Because a total of two electrons take part in bonding this triple bond it is very strong with a [[bond strength]] of 837 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 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 837 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
 
The simplest alkyne is [[ethyne]] ([[acetylene]]): H-C≡C-H
  
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[[File:Propyne-2D-flat.png|200px|thumb|1-propyne]]
 
==Terminal and internal alkynes==
 
==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).
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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).
 
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[[File:2-pentyne-3D-balls.png|thumb|200px|2-pentyne]]
[[Image:1-propyne.png|center]]
 
 
 
 
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.
 
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.png|center]]
 
  
 
== Synthesis ==
 
== 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 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]].  
  
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==Reactions==
 
==Reactions==
 
 
Alkynes are involved in many [[organic reaction]]s.
 
Alkynes are involved in many [[organic reaction]]s.
  
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** addition of [[halogen]]s to give the vinyl halides or alkyl halides
 
** 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
 
** addition of [[hydrogen halide]]s to give the corresponding [[vinyl halide]]s or [[alkyl halide]]s
** 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>:
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** 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. and K. Utimoto. "Effective transformation of unactivated alkynes into ketones or acetals with a gold(III) catalyst." ''J. Org. Chem.'' ''56'', 3729–3731, 1991. {{DOI|10.1021/jo00011a058}} Retrieved September 17, 2007.</ref> or (Ph<sub>3</sub>P)AuCH<sub>3</sub> <ref>Mizushima, E., D.M. Cui, D.C.D. Nath, T. Hayashi, and M. Tanaka. "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] Retrieved September 17, 2007.</ref>:
 
*:[[Image:AlkyneHydrolysis.png|400px|Alkyne hydrolysis]]
 
*:[[Image:AlkyneHydrolysis.png|400px|Alkyne hydrolysis]]
 
* [[Cycloaddition]]s
 
* [[Cycloaddition]]s
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==See also==
 
==See also==
 
 
* [[Alkane]]
 
* [[Alkane]]
 
* [[Alkene]]
 
* [[Alkene]]
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== References ==
 
== References ==
 
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* McMurry, John. ''Organic Chemistry''. 6th ed. Belmont, CA: Brooks/Cole, 2004. ISBN 0534420052
* McMurry, John. 2004. ''Organic Chemistry''. 6th ed. Belmont, CA: Brooks/Cole. ISBN 0534420052.
+
* Morrison, Robert T., and Robert N. Boyd. ''Organic Chemistry''. 6th ed. Englewood Cliffs, NJ: Prentice Hall, 1992. ISBN 0-13-643669-2
 
+
* Solomons, T.W. Graham, and Fryhle, Craig B. ''Organic Chemistry''. 8th ed. Hoboken, NJ: John Wiley, 2004. ISBN 0471417998
* Morrison, Robert T., and Robert N. Boyd. 1992. ''Organic Chemistry''. 6th ed. Englewood Cliffs, NJ: Prentice Hall. ISBN 0-13-643669-2.
 
 
 
* Solomons, T.W. Graham, and Fryhle, Craig B. 2004. ''Organic Chemistry''. 8th ed. Hoboken, NJ: John Wiley. ISBN 0471417998.
 
  
 
{{Functional Groups}}
 
{{Functional Groups}}

Latest revision as of 23:36, 5 March 2019

The structural formula of 2-butyne, a simple alkyne-containing molecule.

Alkynes are hydrocarbons that have at least one triple bond between two carbon atoms, with the formula CnH2n-2. 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 (C2H2) using formal IUPAC nomenclature.

Chemical properties

Unlike alkanes and to a lesser extent, alkenes, alkynes are unstable and very reactive. 1-Alkynes are fairly acidic and have pKa values (25) between that of ammonia (35) or ethanol with 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. Electrons in a s orbital benefit from closer proximity to the positively charged atom nucleus and are therefore lower in energy.

A terminal alkyne with a strong base such as sodium, sodium amide, n-butyllithium or a Grignard reagent gives the anion of the terminal alkyne (a metal acetylide):

2 RC≡CH + 2 Na → 2 RC≡CNa + H2

More generally:

RC≡CH + B → RC≡C + HB+, 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.

Structure

The carbon atoms in an alkyne bond are sp hybridized—they each have two p orbitals and two 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 bonds, 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 angles are 180°. Because a total of two electrons take part in bonding this triple bond it is very strong with a bond strength of 837 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 picometers 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

1-propyne

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).

2-pentyne

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.

Synthesis

Alkynes are generally prepared by dehydrohalogenation of vicinal alkyl dihalides or the reaction of metal acetylides with primary alkyl halides. In the Fritsch-Buttenberg-Wiechell rearrangement an alkyne is prepared starting from a vinyl bromide.

Alkynes can be prepared from aldehydes using the Corey-Fuchs reaction or the Seyferth-Gilbert homologation.

Reactions

Alkynes are involved in many organic reactions.

  • electrophilic addition reactions
    • addition of hydrogen to give the alkene or the alkane
    • addition of halogens to give the vinyl halides or alkyl halides
    • addition of hydrogen halides to give the corresponding vinyl halides or alkyl halides
    • 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)[1] or (Ph3P)AuCH3 [2]:
    Alkyne hydrolysis
  • Cycloadditions
    • Diels-Alder reaction with 2-pyrone to an aromatic compound after elimination of carbon dioxide
    • Azide alkyne Huisgen cycloaddition to triazoles
    • 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
  • 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 acetylides
    • reaction with carbonyl compounds to an intermediate alkoxide and then to the hydroxyalkyne after acidic workup.
  • hydroboration of alkynes with organoboranes to vinylic boranes
  • oxidative cleavage with potassium permanganate to the carboxylic acids
  • 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.

See also

Notes

  1. Fukuda, Y. and K. Utimoto. "Effective transformation of unactivated alkynes into ketones or acetals with a gold(III) catalyst." J. Org. Chem. 56, 3729–3731, 1991. DOI:10.1021/jo00011a058 Retrieved September 17, 2007.
  2. Mizushima, E., D.M. Cui, D.C.D. Nath, T. Hayashi, and M. Tanaka. "Au(I)-Catalyzed hydratation of alkynes: 2,8-nonanedione." Organic Syntheses. Vol. 83, p.55, 2005. Link Retrieved September 17, 2007.

References
ISBN links support NWE through referral fees

  • McMurry, John. Organic Chemistry. 6th ed. Belmont, CA: Brooks/Cole, 2004. ISBN 0534420052
  • Morrison, Robert T., and Robert N. Boyd. Organic Chemistry. 6th ed. Englewood Cliffs, NJ: Prentice Hall, 1992. ISBN 0-13-643669-2
  • Solomons, T.W. Graham, and Fryhle, Craig B. Organic Chemistry. 8th ed. Hoboken, NJ: John Wiley, 2004. ISBN 0471417998


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