Alkyne 2763 220147139 2008-06-18T14:25:45Z Thijs!bot 1392310 robot Adding: [[hu:Alkinek]] [[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]] [[ar:ألكاين]] [[bs:Alkini]] [[bg:Алкин]] [[ca:Alquí]] [[cs:Alkyny]] [[da:Alkyn]] [[de:Alkine]] [[et:Alküünid]] [[el:Αλκίνιο]] [[es:Alquino]] [[eo:Alkino]] [[fa:آلکین]] [[fr:Alcyne]] [[hr:Alkini]] [[id:Alkuna]] [[it:Alchini]] [[he:אלקין]] [[la:Alkinum]] [[lv:Alkīni]] [[hu:Alkinek]] [[mk:Алкин]] [[ms:Alkuna]] [[nl:Alkyn]] [[ja:アルキン]] [[no:Alkyner]] [[nn:Alkyn]] [[pl:Alkiny]] [[pt:Alcino]] [[ro:Alchină]] [[ru:Алкины]] [[sk:Alkín]] [[sl:Alkin]] [[sr:Алкин]] [[su:Alkuna]] [[fi:Alkyyni]] [[sv:Alkyn]] [[ta:ஆல்க்கைன்]] [[uk:Алкіни]] [[zh:炔烴]]