9.2
Nomenclature of Alkynes
Alkynes are unsaturated hydrocarbons characterized by the presence of carbon-carbon triple bonds and have a general formula CnH2n-2. The nomenclature…
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.
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Q1: What suffix do alkynes use in IUPAC nomenclature?
Alkynes use the suffix "-yne" in IUPAC nomenclature to indicate the presence of carbon-carbon triple bonds. When multiple triple bonds are present, the suffix changes to "-diyne," "-triyne," and so on. This distinguishes alkynes from alkanes, which use "-ane," and alkenes, which use "-ene."
Q2: How do you number the carbon chain when naming an alkyne?
Number the carbon chain from the end that gives the triple bond the lowest possible number. Place this locant before the parent name. When a molecule contains both double and triple bonds, numbering begins from the end closest to the multiple bonds, with the double bond receiving priority if both are equidistant.
Q3: What is the difference between terminal and internal alkynes?
Terminal alkynes are monosubstituted acetylenes with the triple bond located at the end of the carbon chain, such as 1-pentyne. Internal alkynes are disubstituted alkynes where the triple bond is positioned at the center or near the center of the chain but not at the end, such as 3-heptyne.
Q4: How are substituents named and ordered in alkyne nomenclature?
Substituents are named and listed before the parent name in alphabetical order, with their position numbers included. For example, in 4-ethyl-5,5-dimethyl-2-heptyne, the ethyl group at carbon 4 and the two methyl groups at carbon 5 are listed alphabetically. Hyphens separate numbers from letters, and commas separate numbers from each other.
Q5: What happens when an alkyne contains a principal functional group?
When an alkyne contains a principal functional group like a hydroxyl group, the principal group receives the lowest possible number. The IUPAC name bears the suffix of the alkyne followed by that of the principal group. For example, 5-methyl-2-hexyn-1-ol places the hydroxyl group at position 1, giving it priority over the triple bond.
Q6: How do common names differ from IUPAC names for alkynes?
Common names for alkynes adopt the parent name acetylene with substituent names added as prefixes. For instance, 2-butyne becomes dimethylacetylene, and 2,5-dimethyl-3-hexyne becomes diisopropylacetylene. This alternative naming system is less systematic than IUPAC nomenclature but remains useful in organic chemistry practice.
Q7: Why are cyclic alkynes with fewer than eight carbons unstable?
Cyclic alkynes with fewer than eight carbon atoms are unstable because the carbon-carbon triple bond must deviate significantly from its preferred 180-degree angle to form smaller rings, creating highly strained structures. The smallest stable cyclic alkyne isolated is cyclooctyne, which contains eight carbon atoms.