Naming Linear Alkynes

Organic Nomenclature of Triple-Bonded Linear Hydrocarbons

Alkynes are a highly reactive class of organic compounds that contain carbon-carbon triple bonds. Acetylene, used in welding and cutting, is the most familiar alkyne.

Alkynes are hydrocarbons. They differ from alkanes and alkenes in that there is at least one pair of carbons connected to each other by a triple bond. The reactivity of alkynes is similar to that of alkenes possessing the same number of linear carbons. Although they are very weak acids, terminal alkynes (triple bond located between two end-chain carbons) are much more acidic than analogous single and double bonded hydrocarbons. Alkynes are highly flammable and make good starting materials for the synthesis of other compounds.

Because they are so reactive, naturally occurring alkynes are relatively rare. When they are found, they usually have functional groups attached and they are not purely hydrocarbons. One exception is acetylene, which can be isolated by distillation of crude oil in refineries (Figure 1). Large quantities of alkynes for industrial or pharmaceutical use are synthesized by dehydrogenation of alkenes or more commonly by dehydrohalogenation of alkyl halides (also known as haloalkanes).

Naming Linear Alkynes Containing One Triple Bond

Naming linear alkynes has much in common with the naming of linear alkenes. As is the case with alkenes, the first step in naming an alkyne is to count the carbons in the longest carbon chain. The name begins with a prefix based on the number of carbons. The common prefixes are:

1. meth (although there are no 1-carbon alkynes)
2. eth
3. but
4. prop
5. pent
6. hex
7. hept
8. oct
9. non
10. dec

The name of an alkyne begins with the numerical identifying prefix, and ends in "-yne."

The smallest and most familiar alkyne is ethyne (C≡C, hydrogens not shown), also called acetylene (Figure 2). Acetylene and oxygen are combined and ignited to perform oxyacetylene welding and cutting. There is only one three-carbon alkyne (C≡C–C), propyne (methylacetylene).

When the number of carbons exceeds three, a numbering system is used to locate the triple bond. The numbers are chosen to be as small as possible. For example, in the molecule C≡C-C-C, the triple bond is in the first bond position between the first two carbons in the chain, so this molecule is 1-butyne (also called ethylacetylene). A butyne with the triple bond at the second position (C–C≡C–C) is 2-butyne (or dimethylacetylene). To give another example, C-C-C≡C-C is 2-pentyne (not 3-pentyne).

Naming Alkynes Containing More Than One Triple Bond

Alkynes with two double bonds are alkadiynes (diynes), ones with three carbon double bonds are alkatriynes (triynes), and so on. A straightforward extension to the numbering system is used to properly name them. So, C≡C–C≡C–C is 1,3-pentadiyne. Other examples are 1,2-heptadiyne (C≡C=C–C–C–C–C), and 1,2,3-hexatriyne (C≡C≡C≡C–C–C).

Naming Branched Alkynes

Branched alkynes are named in a way similar to that of branched alkenes.

1. First, find the longest carbon chain. This chain must contain the maximum number of double and triple bonds. This is the main chain used to determine the name.

2. Assign numbers to the carbons where the side groups exist.

3. If there is more than one multiple bond (double or triple) the numbering begins at the end closest to multiple bond positions are named first, whether it is a double or triple bond locations.

4. Give the side group carbon numbers and names first, then the location of the double bonds, followed by numbers of the triple bonds.

Take as an example C–C–C≡CC–C, where the CC indicates a methyl group attached to C-2 (not C-4 because number should be chosen to be as small as possible). It may help to draw it to show the branched methyl group. The longest carbon chain has five carbons (-pent). The methyl group is on C-2. The name of this molecule is 2-methly-2-pentyne; C≡C–CCCl–C≡C–C is 3-methyl-3-chloro-1,4-hexadiyne (where v indicates a subscript).

Alkynes as Side Groups

Triple bonds can also appear on side groups, not part of the main chain. Common examples are the ethynly group –C≡C, and the propargyl group –C–C≡C.

An example including a double bond and a side chain triple bond is HCH=CHCH(C≡CH)CH=CHCH=HCH. In this case it is easier to keep track of what is bonded to what by showing the hydrogens. It will be helpful to draw it out. The longest chain contains 7 carbons and does not include the triple bond, so it is a heptene. There are three double bonds in the chain so it is a heptatriene. The double bonds are at positions 1, 3 and 6 (not 1, 4, 6), so it is a 1,3,6-heptatriene. The ethynyl group is attached to C-5 making the final name 5-ethynyl-1,3,6-heptatriene.

The copyright of the article Naming Linear Alkynes in Chemistry is owned by Philip McIntosh. Permission to republish Naming Linear Alkynes in print or online must be granted by the author in writing.

Space Filling Model of Acetylene	Figure 1. Petrochemical Refinery in Scotland	Figure 2. Diagramatic Representation of Acetylene
The Acetylene Torch is Used to Cut Metal