that have at least one triple bond
between two carbon
atoms, with the formula CnH2n-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
) using formal IUPAC
, and to a lesser extent, alkenes
, alkynes are unstable and reactive. Terminal alkynes
are fairly acidic and have pKa
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
- 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.
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.
The terminal Hydrogen atom is weakly acidic, and can be removed by a very strong base, to yield a salt. This property can be used as a chemical test to distinguish terminal alkynes from others, or the salt may be used to make larger alkyne molecules. A few drops of diamminesilver(I) hydroxide (Ag(NH3)2+ -OH or Ag(NH3)2OH)) solution are added to samples of a non-terminal alkyne and also a terminal alkyne. No reaction occurs for the non-terminal, but the terminal alkyne forms a characteristic white precipitate. This is the insoluble silver salt of the terminal alkyne:
R-C≡CH + Ag(NH3)2+ -OH → R-C≡C- Ag+ + NH4+ + NH3 (R = general alkyl group)
Warning: transition metal salts of terminal alkynes (metal; acetylides) can be explosive when
Alkynes are generally prepared by dehydrohalogenation
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.
Alkynes are involved in many organic reactions