| Alkynes |
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| alkynes | |
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Alkynes are Hydrocarbon s that have at least one Triple Bond between two Carbon atoms. The alkynes are traditionally known as '''acetylenes''', although the name acetylene is also used to refer specifically to the simplest member of the series, known officially as Ethyne . STRUCTURE The carbon atoms in an alkyne bond are 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. CHEMICAL PROPERTIES Unlike Alkane s, Alkyne s are unstable and very reactive. This gives rise to the intense heat of the acetylene flame used in welding. EXAMPLES The simplest alkyne is Ethyne ( Acetylene ): 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 under 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. METAL ACETYLIDES 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). Acetylenes are fairly acidic and have PKa values (25) between that of Ammonia (35) or Ethanol with 16. The explanation for this acidity is that the negative charge in acetylide is 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. SYNTHESIS Alkynes are generally prepared by Dehydrohalogenation of 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 Bromide . Alkynes can be prepared from Aldehyde s using the Corey-Fuchs Reaction or the Seyferth-Gilbert Homologation . REACTIONS Alkynes are involved in many Organic Reaction s.
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