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.
Unlike alkanes, and to a lesser extent, alkenes, alkynes are unstable and 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 an s orbital benefit from closer proximity to the positively charged atom nucleus, and are therefore lower in energy.
- 2 RC≡CH + 2 Na → 2 RC≡CNa + H2
- 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.
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.
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.
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 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
- 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) or (Ph3P)AuCH3 :
- 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
- nucleophilic substitution reactions of metal acetylides
- new carbon-carbon bond formation with alkyl halides
- nucleophilic addition reactions of metal acetylides
- 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.
- 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
- 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). Link.
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