Alkynes

(unsaturated hydrocarbons have one or more carbon carbon triple bond. They have the general formula: CnH2n-2 )

Basic IUPAC nomenclature: (See the home page for parent root names, prefixes & suffixes. The following modifications are used for alkynes.)

1. Number the longest chain of carbon atoms that contains the triple bond in the direction that gives the carbons of the triple bond the lowest numbers. Although the two carbons define the location of the triple bond, only the first carbon atom is numbered-by definition, the next carbon atom must be part of the alkyne. For alkynes, use the infix -yn- and the parent name of the alkane.

So, this compound has 4 carbons and is a butane carbon chain, and inserting the infix -yn-  gives the name:  butyne , but the position of the beginning of the triple bond must be specified. The triple bond begins on C1 and, as is the case with alkenes, it is not necessary to use two numbers to specify its location, so the full, proper name is: 1-butyne. Because the triple bond is located on the end of the carbon chain, these are sometimes referred to, as "terminal alkynes."

This is an isomer of 1-butyne; it is named 2-butyne, because the triple bond begins on C2. Notice, that the compound can be numbered left to right or right to left and the name is still 2-butyne. Because the triple bond is located on the "interior" of the carbon chain, these are sometimes referred to, as "internal alkynes."

2. For alkynes, which have other substitutents, or branching of the carbon chain, the rules for alkenes apply: number the longest chain containing the triple bond, number, name & alphabetize the substitutents. Note: the triple bond has linear geometry, and as such, there is no possibility of cis or trans isomerism.

The parent name of this compound is octane and the triple bond begins on C3. So, the parent name of the compound is: 3-octyne. There is a methyl- group at C6, so the full name becomes: 6-Methyl-3-octyne.

3. Common names in alkynes. The smallest member of this family (C₂H₂) is the two carbon unit: ethyne. However, this is known, almost exclusively, by its common name: acetylene.  The common nomenclature uses acetylene as the parent name and the substitutents on either side are added, in front, as separate words. Using 2-butyne as the example, if the common name, acetylene,  were used, then each of the methyl groups would represent the two sides of the structure, and no numbering would be necessary. The common name would be: Dimethylacetylene. The compound below could be named, Methylacetylene.

EXERCISE: Write the IUPAC name for each of the following structures.

Structure		Name	Structure		Name
1			6
2			7
3			8
4			9
5			10

END OF SECTION

email questions & comments to: Dr. JA Colapret