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 of alkynes follows a set of rules similar to alkanes and alkenes; however, alkynes bear the suffix "-yne" instead of "-ane" or "-ene." There are two approaches to naming alkynes:
- • IUPAC names
- • Common names
The IUPAC naming system follows a systematic approach to deduce names of alkynes based on their chemical structure. Consider the following alkyne:
The steps involved in deducing the corresponding IUPAC name are summarized below:
1. The first step is to identify the parent chain, which is the longest carbon chain containing the triple bond. The suffix "-yne" is added to the parent name to indicate the presence of carbon-carbon triple bonds.
Here, the longest chain (red-colored bonds) contains seven carbon atoms; therefore, the parent name would be heptyne.
2. Next, number the parent chain from the end, which gives the triple bond the lowest possible number, and place the locant before the parent name. Here, there are two ways of numbering the chain:
Structure (b) gives the lowest number to the triple bond. Hence, the name 2-heptyne.
3. Now, name the substituents and list the numbered substituents before the parent name in alphabetical order. In this example, there are three substituents: one ethyl group at C4 and two methyl groups at C5.
4. Finally, combine steps 1-3 to deduce the alkyne name, which is 4-ethyl-5,5-dimethyl-2-heptyne. Note that the hyphen separates numbers from alphabets, and the comma separates numbers from each other.
Naming alkynes with more than one triple bond
In alkynes with more than one triple bond, the names end with "-diyne," "-triyne," and so forth; the other rules remain the same.
Naming alkynes with double and triple bonds
When a molecule contains both double and triple bonds, numbering begins from the end closest to the multiple bond. Here, the suffix “-ene” (for the double bond) and “-yne” (for the triple bond) are included in the parent name in alphabetical order.
If the numbering is ambiguous, i.e., if the double and triple bonds are equidistant on the carbon chain as shown below, the double bond gets the higher priority and the lowest number.
|Option (1)||Option (2)|
The correct IUPAC name would follow the option (2) numbering scheme, giving 2-bromo-oct-2-en-6-yne.
Naming alkynes with principal functional groups
Principal functional groups are substituents that take numerical precedence over double or triple bonds, as well as halogen and alkyl groups. The IUPAC name bears the suffix of the alkyne group, in this case, "-yn” instead of “-yne”, followed by that of the principal group. In the following example, the -OH group receives numerical precedence over the triple bond and the methyl substituent.
Naming cyclic alkynes
Cyclo alkynes are named by adding the prefix “cyclo-'' to the parent ring with the substituents encountered first receiving the lower number; the other conventions are similar to acyclic alkynes. For example,
Cyclic alkynes with less than eight carbon atoms are unstable and result in highly strained structures. This is because the carbon-carbon triple bond is expected to deviate significantly from 180° to form rings smaller than eight carbon atoms. The smallest cyclic alkyne isolated thus far is cyclooctyne.
An alternative approach to systematic naming is the common name system. Here, alkynes adopt the parent name like acetylene with the substituents’ name added as a prefix.
|IUPAC name: 2-butyne
Common name: dimethylacetylene
|IUPAC name: 2,5-dimethyl-3-hexyne
Common name: diisopropylacetylene
Terminal and internal alkynes
Linear alkynes can be classified as terminal and internal based on the location of the triple bond. Terminal alkynes are monosubstituted acetylenes where the triple bond is located at the end of the chain.
Internal alkynes are disubstituted alkynes. Here, the location of the triple bond is either at the center of the chain or close to it but not at the end.