Alkyne

Alkynes are hydrocarbons 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 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 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 picometers is also much less than that of the alkene bond which is 134 pm or the alkane bond with 153 pm.

Chemical properties

Unlike alkanes, alkynes are unstable and very reactive. This gives rise to the intense heat (>3000 °C) 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 pK_a 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. Electrons 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 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 or the Seyferth-Gilbert homologation.

Reactions

Alkynes are involved in many organic reactions.

electrophilic addition reactions
- addition of hydrogen to the alkene or the alkane
- addition of halogens to the vinyl halides or alkyl halides
- addition of hydrogen halides to the corresponding vinyl halides or alkyl halides
- addition of water to the carbonyl compound (often through the enol intermediate), for example the hydrolysis of 1-phenylacetylene to acetophenone with sodium tetrachloroaurate in water/methanol Effective transformation of unactivated alkynes into ketones or acetals with a gold(III) catalyst

Yukitoshi Fukuda and Kiitiro Utimoto J. Org. Chem.; 1991; 56(11) pp 3729 - 3731; :

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
^*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.

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

References

Alkynes | Hydrocarbons | functional groups

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