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.
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.
Unlike
alkanes, alkynes are unstable and very reactive. This gives rise to the intense heat (>3000 °C) of the acetylene flame used in welding.
The simplest alkyne is
ethyne (
acetylene):
 |
Ethyne.png |
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.
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
pKa 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.
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.
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) catalystYukitoshi 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
** [2+2+1]cycloaddition of an alkyne,
alkene and
carbon monoxide in the
Pausonâ€"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
