Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Carbon Functional Groups and Organic Nomenclature
2.9 Structure and Nomenclature of Alkynes

For the nomenclature of alkynes, there is only a minor change in rules that were discussed in Section 2.7 for the nomenclature of alkenes.

Rule #1 Find the longest continuous chain that contains the alkyne functionality.

Rule #2 Assign numbers to each carbon of this longest continuous chain that contains the triple bond. Start the assignment so that the end closest to the triple bond has number 1.

Rule #3 Use Table 2.3 to determine the name of the parent or root name and specify the position of the triple bond with a number. Based on the number of carbons that are present in the compound, a root or parent name is assigned by changing the -ane of the corresponding alkane to -yne. Thus, a compound that has six carbons and has a carbon—carbon triple bond is named as a hexyne, and the position of the triple bond is specified with a number as illustrated in the name of the molecule below.

The rest of the rules are the same as those discussed for the naming of alkanes and alkenes. For compounds that have two triple bonds, the name of the compound is a diyne, and numbers are used to indicate the positions of the triple bonds as shown in the example below.

If there are other functional groups present, such as shown in the molecule below, which has both an alkene and an alkyne functionality identify the longest chain that contains both functionalities.

Start numbering at the end closest to either the double or triple bond, in this case the triple bond. For this molecule, the alkyne will become the main root, heptyne, and more specific, 2-heptyne with the alkene functionality in position #4. The combined root name becomes: 4-hepten-2-yne (also can be written as hep-4-en-2-yne). Since there is a methyl group in position #6, the complete name is shown below.

If the double and triple bonds are equidistant from the ends, the alkene functionality will become the root name to determine the numbering.

Problem 2.17

i. Give structures of the following molecules.

1. 5-Chloro-4-methyl-2-hexyne

2. 5,5-Dimethyl-3-heptyne

3. 4-Bromo-5-chloro-2-octyne

4. 5,5-Dichloro-2-heptyne

ii. Give the IUPAC names for the compounds shown below