An alkyne is a type of hydrocarbon molecule that contains at least one carbon-carbon triple bond. These compounds follow the general molecular formula \(C_nH_{2n-2}\), where ‘n’ represents the number of carbon atoms. The presence of this triple bond gives alkynes distinct chemical properties and requires specific naming conventions. The International Union of Pure and Applied Chemistry (IUPAC) established a systematic method for naming them. This nomenclature system provides a clear, standardized language by translating the chemical structure directly into a unique name.
Establishing the Parent Chain and Suffix
The first step in naming an alkyne involves identifying the longest continuous chain of carbon atoms, which must include the carbon-carbon triple bond. This requirement overrides the simple search for the absolute longest carbon chain. The number of carbons in this principal chain determines the base name of the alkyne (e.g., five carbons is “pent-“). The presence of the triple bond is signaled by changing the standard alkane suffix “-ane” to the alkyne suffix “-yne”. Therefore, a five-carbon parent chain with a triple bond is named pentyne. This newly established parent name provides the foundational structure for the complete IUPAC name.
Locating the Triple Bond
Once the parent chain is established, the next step is to assign a numerical location to the triple bond within that chain. The parent chain must be numbered from the end that gives the carbons of the triple bond the lowest possible locants (numbers). This principle, known as the lowest number rule, prioritizes the triple bond over any attached alkyl substituents. The numbering ensures that the first carbon atom involved in the triple bond receives the smaller number. This single number is then placed directly before the “-yne” suffix in the parent name, which is the preferred modern IUPAC recommendation. For example, if the triple bond begins on the second carbon of a five-carbon chain, the name is written as pent-2-yne.
Incorporating Alkyl Substituents
After determining the parent chain and numbering the triple bond, any carbon groups attached to the main chain, known as alkyl substituents, must be incorporated into the name. These side chains are named by replacing the “-ane” ending of the corresponding alkane with “-yl” (e.g., methyl or ethyl). Each substituent must be assigned a locant number corresponding to the carbon atom on the parent chain to which it is attached.
These numbered substituents are cited alphabetically at the beginning of the full name, preceding the parent alkyne name. Alphabetization is based on the name of the alkyl group, ignoring prefixes like di-, tri-, or tetra-. If the same type of alkyl group appears more than once, prefixes such as di- or tri- are used to indicate the total number of identical groups. For instance, a molecule might be named 4-ethyl-5,5-dimethyl-2-heptyne.
Prioritizing Multiple Bonds
Complex molecules may contain more than one triple bond, or both a double bond and a triple bond. When a molecule contains two or three triple bonds, the parent name is modified using suffixes like “-adiyne” or “-atriyne” to indicate the number of triple bonds. The position of each triple bond is indicated by a locant (e.g., a six-carbon chain with two triple bonds is a hexadiyne).
When a molecule contains both a double bond (alkene) and a triple bond (alkyne), it is named as an “enyne,” combining the suffixes for both functional groups. The parent chain must be numbered to give the lowest possible locant to the first multiple bond encountered. If both multiple bonds are equidistant from the ends of the chain, the double bond takes precedence and receives the lower number. The name is written in the form “n-en-n-yne,” where the double bond’s location precedes the triple bond’s location.