Alkyne

  Alkynes are hydrocarbons that have at least one triple bond between two carbon atoms, with the formula C_nH_2n-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.

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Chemical properties

Unlike alkanes, and to a lesser extent, alkenes, alkynes are unstable and reactive. Terminal alkynes and acetylene are fairly acidic and have pK_a 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.

See also: Metal acetylide

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.

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.

• 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)^[1] or (Ph₃P)AuCH₃ ^[2]:

• 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 in the Favorskii reaction.
• hydroboration of alkynes with organoboranes 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 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.

References

Further reading

• -yne
• cycloalkyne