The alkynes
When dealing with molecules which have a triple carbon to carbon bond follow the following steps which are similar to alkenes.
1) Determine the longest chain of carbon atoms that includes the triple bond.
2) Name the carbon atoms so that the triple bond is on the lowest carbon.
3) Each of the substituents and the number of the carbon to which they are attached are placed in the name as prefixes in alphabetical order, followed by the longest
carbon chain and finally the location of the double bond.
Eg 5-methylhept-3-yne has a seven carbon backbone with a triple bond at carbon number 3 and a methyl group on carbon number 5.
**notice how in the suffix of the name the "ane" of heptane is replaced with "yne" for heptene.
Let's see an example.
The molecule above is given the name propyne. Numbering ( prop-1-yne) is not required here as there is no ambiguity. The triple bond can only go in one place.
In the presence of multiple triple bonds, however, the following steps are taken:
-
number of triple bonds is indicated by the appropriate suffix such as -diyne or -triyne. Numbers before the suffix give the carbon number that is relevant to the location of each carbon to carbon triple bond on the backbone or parent carbon chain.
-
only -ne is removed from the parent alkane chain name leaving an "a" in the name.
Let's see an example
Name the molecule shown on the left.
- select the longest chain that includes both triple bonds.
It is a buta-diyne
- number the carbons so that the are on the lowest possible numbered carbons.
buta-1,3-diyne (although butadiyne will suffice as there is no ambiguity. The triple bonds can only go on carbon 1 and 3)
- identify the substituent groups and the carbon number to which they are attached and add it to the name as a prefix.
2-bromo-4-chlorobuta-1,3-diyne
Name the molecule shown on the left.
- select the longest chain that includes both triple bonds.
It is a octa-diyne
- number the carbons so that the are on the lowest possible numbered carbons.
octa-2,6-diyne
Name the molecule shown on the left.
- select the longest chain that includes both triple bonds.
It is a hepta-diyne
- number the carbons so that the are on the lowest possible numbered carbons.
hepta-1,5-diyne
- identify the substituent groups and the carbon number to which they are attached and add it to the name as a prefix.
4-methylhepta-1,5-diyne
Name themolecule shown on the left.
- select the longest chain that includes the triple bond.
It is a heptyne
- number the carbons so that the are on the lowest possible numbered carbons.
hept-2-yne
- identify the substituent groups and the carbon number to which they are attached and add it to the name as a prefix.
7-bromo-4-methylhept-2-yne