Alkyne

Alkynes are ethyne (C₂H₂) using formal IUPAC nomenclature.

Chemical properties

Unlike 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 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 alkene bond which is 134 pm or the alkane bond with 153 pm.

The simplest alkyne is 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 vinyl bromide.

Alkynes can be prepared from Seyferth-Gilbert homologation.

Reactions

Alkynes are involved in many organic reactions.

electrophilic addition reactions
- addition of 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 acetophenone with sodium tetrachloroaurate in water/methanol (scheme shown below)^[1] or (Ph₃P)AuCH₃ ^[2]:

Cycloadditions
- carbon dioxide
- Azide alkyne Huisgen cycloaddition to triazoles
- aromatic compound
- aromatic compounds
- [2+2+1]cycloaddition of an alkyne, 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
metal acetylides
- reaction with Favorskii reaction.
organoboranes to vinylic boranes
- followed by reduction by oxidation with ketone
oxidative cleavage with carboxylic acids
migration of the alkyne along a hydrocarbon chain by treatment with a strong base
Cadiot-Chodkiewicz coupling, Glaser coupling and the Eglinton coupling.

References

^ 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.