9.1: Naming Alkynes (2024)

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Objectives

After completing this section, you should be able to

provide the correct IUPAC name of an alkyne, given its Kekulé, condensed or shorthand structure.
provide the correct IUPAC name of a compound containing both double and triple bonds, given its Kekulé, condensed or shorthand structure.
draw the structure of a compound containing one or more triple bonds, and possibly one or more double bonds, given its IUPAC name.
name and draw the structure of simple alkynyl groups, and where appropriate, use these names as part of the IUPAC system of nomenclature.

Study Notes

Simple alkynes are named by the same rules that are used for alkenes (see Section 7.3), except that the ending is -yne instead of -ene. Alkynes cannot exhibit E,Z (cis‑trans) isomerism; hence, in this sense, their nomenclature is simpler than that of alkenes.

Alkynes are organic molecules made of the functional group carbon-carbon triple bonds and are written in the empirical formula of \(\ce{C_{n}H_{2n-2}}\). They are unsaturated hydrocarbons. Like alkenes have the suffix –ene, alkynes use the ending –yne; this suffix is used when there is only one alkyne in the molecule.

Introduction

Here are the molecular formulas and names of the first ten carbon straight chain alkynes.

Name	Molecular Formula
Ethyne	C₂H₂
Propyne	C₃H₄
1-Butyne	C₄H₆
1-Pentyne	C₅H₈
1-Hexyne	C₆H₁₀
1-Heptyne	C₇H₁₂
1-Octyne	C₈H₁₄
1-Nonyne	C₉H₁₆
1-Decyne	C₁₀H₁₈

The more commonly used name for ethyne is acetylene, which used industrially.

Naming Alkynes

Like previously mentioned, the IUPAC rules are used for the naming of alkynes.

Rule 1

Find the longest carbon chain that includes both carbons of the triple bond.

Rule 2

Number the longest chain starting at the end closest to the triple bond. A 1-alkyne is referred to as a terminal alkyne and alkynes at any other position are called internal alkynes.

For example:

Rule 3

After numbering the longest chain with the lowest number assigned to the alkyne, label each of the substituents at its corresponding carbon. While writing out the name of the molecule, arrange the substituents in alphabetical order. If there are more than one of the same substituent use the prefixes di, tri, and tetra for two, three, and four substituents respectively. These prefixes are not taken into account in the alphabetical order.

For example:

If there is an alcohol present in the molecule, number the longest chain starting at the end closest to it, and follow the same rules. However, the suffix would be –ynol, because the alcohol group takes priority over the triple bond.

When there are two triple bonds in the molecule, find the longest carbon chain including both the triple bonds. Number the longest chain starting at the end closest to the triple bond that appears first. The suffix that would be used to name this molecule would be –diyne.

For example:

Rule 4

Substituents containing a triple bond are called alkynyl.

For example:

Here is a table with a few of the alkynyl substituents:

Name	Molecule
Ethynyl	-C≡CH
2- Propynyl	-CH₂C≡CH
2-Butynyl	-CH₃C≡CH₂CH₃

Rule 5

A molecule that contains both double and triple bonds is called an alkenyne. The chain can be numbered starting with the end closest to the functional group that appears first. For example:

If both functional groups are the exact same distance from the ending of the parent chain, the alkene takes precedence in the numbering.

Exercise \(\PageIndex{1}\)

Name the following compounds:

Answer

3,6-diethyl-4-octyne
3-methylbutyne
4-ethyl-2-heptyne
cyclodecyne

Exercise \(\PageIndex{2}\)

How many isomers are possible for C₅H₈? Draw them.

Answer

2 possible isomers

Exercise \(\PageIndex{3}\)

Draw the following compounds:

4,4-dimethyl-2-pentyne
3-octyne
3-methyl-1-hexyne
trans 3-hepten-1-yne

Answer

Exercise \(\PageIndex{4}\)

Do alkynes show cis-trans isomerism? Explain.

Answer: No. A triply bonded carbon atom can form only one other bond and has linear electron geometry so there are no "sides". Allkenes have two groups attached to each inyl carbon with a trigonal planar electron geometry that creates the possibility of cis-trans isomerism.

Reference

Vollhardt, Peter, and Neil E. Schore. Organic Chemistry: Structure and Function. 5th Edition. New York: W. H. Freeman & Company, 2007.