Alkynes, also known as acetylenes, are compounds that contain a carbon to carbon triple bond within the organic molecule. Overall alkynes share the same rules for naming compounds except with a couple minor changes.
First, the compound being named will have a –yne ending instead of the –ene ending found with alkenes or –ane for alkanes. An example of an eight chain carbon with the triple bond between the second and third carbons would be: 2-octyne.
Second, if there is a double bond within the molecule, this will take priority in naming over the triple bond. For a chain that is eight carbons long with a double bond between the second and third carbons and a triple bond between the fifth and sixth carbons would be: (cis/trans)-2-octen-5-yne.
Two things to note from this are:
- The placement of first (or higher priority bond) can be moved in naming. An example is 2-heptene and hept-2-ene are both the same compound.
- When there are both double and triple bonds in the compound, the last e on the –ene for the double bond ending is dropped.
Another important piece of information is that triple bonds do not have any type of cis/trans characteristic to them like double bonds do. This is because there is only one thing attached to either side of the triple bond.
In order to create a carbon to carbon triple bond on a molecule, there are some synthesis that can be conducted. The most effective way is to use remove two adjacent halides with two equivalents (Eq) of a strong base. The halides may be vicinal or geminal. Two recommended strong bases are sodium amide (NaNH2) and Lithium diisopropylamide (LDA).
In regards to what can be done with alkynes is much broader. Important reactions that can be conducted are:
- Forming an organic halide: HX(g) or X2(g) with energy can be applied to an alkyne in one equivalent to form an alkene with halide(s) attached across the bond in the trans configuration. This can be done once more to add another set of halide(s) across the bond. If HX(g) was used both times, then the vicinal halide will be formed.
- Forming a ketone: To add the double bonded oxygen to the most substituted carbon in the bond sulfuric acid (H2SO4), mercury II sulfate (HgSO4), and water is used in the synthesis. To add the doubled bonded oxygen to the least substituted carbon, first BH3 is add followed by H2O2.
- Forming an alkene (Dissolving Metal Reaction/Birch Reduction): an alkali metal (lithium, sodium, or potassium) and liquid ammonia (NH3 (L)) is used to reduce the triple bond to a double bond. A majority of the product will be in the trans configuration.
There are many more reactions that can occur to an alkyne group to form other functional groups. More reactions involving alkynes will be covered in different topics.