AskDefine | Define alkyne

Dictionary Definition

alkyne n : a colorless flammable gas used chiefly in welding and in organic synthesis [syn: acetylene, ethyne]

User Contributed Dictionary

English

Noun

  1. A hydrocarbon containing at least one carbon-carbon triple bond.

Translations

hydrocarbon containing at least one carbon-carbon triple bond
  • Estonian: alküün
  • Finnish: alkyyni
  • Greek: αλκίνιο
  • Italian: alchino
  • Spanish: alquino

See also

Extensive Definition

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 reactive. Terminal alkynes and acetylene are fairly acidic and have pKa 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. Electrons 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 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 2 p orbitals and 2 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 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 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

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).
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.
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 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 and from aldehydes or ketones by the Seyferth-Gilbert homologation.

Reactions

Alkynes are involved in many organic reactions.

References

See also

alkyne in Arabic: ألكاين
alkyne in Bosnian: Alkini
alkyne in Bulgarian: Алкин
alkyne in Catalan: Alquí
alkyne in Czech: Alkyny
alkyne in Danish: Alkyn
alkyne in German: Alkine
alkyne in Estonian: Alküünid
alkyne in Modern Greek (1453-): Αλκίνιο
alkyne in Spanish: Alquino
alkyne in Esperanto: Alkino
alkyne in Persian: آلکین
alkyne in French: Alcyne
alkyne in Croatian: Alkini
alkyne in Indonesian: Alkuna
alkyne in Italian: Alchini
alkyne in Hebrew: אלקין
alkyne in Latin: Alkinum
alkyne in Latvian: Alkīni
alkyne in Macedonian: Алкин
alkyne in Malay (macrolanguage): Alkuna
alkyne in Dutch: Alkyn
alkyne in Japanese: アルキン
alkyne in Norwegian: Alkyner
alkyne in Norwegian Nynorsk: Alkyn
alkyne in Polish: Alkiny
alkyne in Portuguese: Alcino
alkyne in Romanian: Alchină
alkyne in Russian: Алкины
alkyne in Slovenian: Alkin
alkyne in Serbian: Алкин
alkyne in Sundanese: Alkuna
alkyne in Finnish: Alkyyni
alkyne in Swedish: Alkyn
alkyne in Tamil: ஆல்க்கைன்
alkyne in Ukrainian: Алкіни
alkyne in Chinese: 炔烴
alkyne in Slovak: Alkín
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