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.
Encyclopedia
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 of the acetylene flame used in welding.
Examples
The simplest alkyne is
ethyne :
Terminal and internal alkynes
Terminal alkynes have a hydrogen atom bonded to at least one of the sp hybridized carbons
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 . Acetylenes are fairly acidic and have pK
a values between that of
ammonia 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.
| Triple point [i] || 13.8033 K, 7.042 kPa
...
to the
alkene or the
alkane- or AuCH3 .
...
or
ketoneReferences
