Nomenclature of Alkynes

As with alkanes and alkenes, there are two approaches to naming alkynes: IUPAC nomenclature and Common nomenclature.

The IUPAC names of alkynes bear the suffix "-yne" to indicate the presence of carbon-carbon triple bonds.

Let's work out the IUPAC name of the alkyne shown here.

First, identify the parent chain, which is the longest carbon chain containing the triple bond. Here, the carbon chain has seven carbon atoms; hence the parent name is heptyne.

Secondly, number the carbon atoms in the parent chain such that the triply bonded carbon atoms get the lowest number. Place the triple bond locant before the parent name.

Next, name the substituents. List the numbered substituents before the parent name in alphabetical order.

The IUPAC name for this alkyne is four-ethyl-five-five-dimethyl-two-heptyne.

When alkynes contain more than one triple bond, the names end with "-diyne," "-triyne," and so on. Thus, the IUPAC name here is nine-chloro-four-methoxy- two-six-decadiyne.

In compounds with double and triple bonds, numbering begins from the end closest to the multiple bonds. Here, both suffixes, "-ene" and "-yne," are included in the parent name in alphabetical order. The IUPAC name for this alkyne is seven-bromo-oct five-ene-two-yne.

If the double and triple bonds are equidistant on the carbon chain, the double bond receives the lowest number giving two-bromo-oct-two-ene-six-yne.

When an alkyne contains a principal functional group, the latter receives the lowest possible number. The name bears suffixes of the alkyne followed by that of the principal group. Here, the IUPAC name is five-methyl-two-hexyn-one-ol.

For common names, alkynes adopt the parent name like acetylene, with the substituents' names added as a prefix. Thus, two-butyne becomes dimethylacetylene, and two-five-dimethyl-three-hexyne would be diisopropyl acetylene.

Acyclic alkynes are classified as terminal and internal based on the location of the triple bond. Terminal alkynes are monosubstituted acetylenes, while internal alkynes are disubstituted acetylenes.