Naming Alkynes - Online Tutor, Practice Problems & Exam Prep

Alkynes are hydrocarbons characterized by a carbon-carbon triple bond, with naming conventions similar to alkenes. The suffix changes from -ane to -yne, indicating the presence of the triple bond. Unlike alkenes, alkynes do not exhibit cis or trans isomerism. When naming, it is essential to specify the location of substituents and the triple bond within the structure. Understanding these principles is crucial for mastering organic chemistry and the behavior of functional groups like alkynes.

Rules for Naming Alkynes Concept 1

Now recall that an alkyne possesses a carbon-carbon triple bond. And we're going to say here that the set of rules for naming alkynes are very similar to the ones for alkenes. Here though, the modifier is we're going to change the ending from '-ane', coming from an alkane, to '-yne', because we're dealing with an alkyne. Now, a good thing here is that alkynes, because they have a triple bond, do not possess cis or trans isomers. Here in terms of our naming convention, we'd still have to give the location, the numerical location of substituents; we'd have to give the location of our triple bond in our structure, and again we'd have our modifier where we change the ending from 'ane' to 'yne'. So keep that in mind when we start naming different types of alkynes.

Naming Alkynes Example 1

It says, "Determine the systematic name of the following alkyne." So to do that, we're going to employ the following steps. Step 1, we're going to find the longest carbon chain, which represents our parent chain, and assign a name according to the prefixes and modifier. Now, the parent chain should include a triple bond, and have the greatest number of carbons. If there is a tie along this chain, choose the chain with more substituents. Step 2, you would assign names to all the substituents.

Alright. So first, let's look at what the longest chain could possibly be. I need to make sure that the triple bond is within that longest chain. So we have here, what? 1, 2, 3, 4. Yes. There's a carbon here and a carbon here next to the triple bond. 5, and then we could go 6, 7. If we did that, that would give us what? That would give us 2 substituents. But is there another way we could number this? Could we go 1, 2, 3, 4, 5, 6, 7? That is also a 7 chain, alkyne. 7 Carbon Chain Alkyne. But look, how many substituents do we have now? 1, 2, 3. This way gives us more substituents, and it also gives us the longest carbon chain as well. So this is the correct way to go. So we're gonna say that our longest carbon chain that includes the triple bond and gives us the most substituents would be this way. Alright. And we've determined who the substituents are. There's this one carbon here which is a methyl. This one carbon here which is also a methyl. And then 2 carbons here which is ethyl.

Start numbering the chain from the end closest to the triple bond. Assign the location to the first triple-bonded carbon. Alright. So if I go 1, 2, 3, 4 to reach the triple bond, or go from this end, 1, 2, 3, and I get to the triple bond. So this is the way I should number it. Alright. So we've figured out our substituents. We've determined our longest carbon chain that includes the triple bond. Step 4 through 6 is just using the rules that we've employed basically in other topics, namely alkanes with substituents. If you haven't watched that video, make sure you go back and take a look. Because in those videos, we talk about where we have to name the substituents alphabetically, and we also have to give numerical locations for them.

Alright. So 7 carbons is heptane. But remember, we change the ending from 'ane' to 'yne.' So here comes heptyne. The triple bond starts on carbon number 3, so 3-heptyne. Now we have to give the numerical locations of the substituents, and name them alphabetically. E comes before M. So we'd say that this would be, 5-ethyl, and then we have methyls on carbons 2 and 6, so 2,6-dimethyl. And then dash-three heptyne. So the name of this structure will be 5-ethyl-2,6-dimethyl-3-heptyne.

Give a systematic name for this molecule.

• A. 10-methyl-8-propyl-4-undecyne
• B. 10-methyl-8-propyl-4-undecane
• C. 8-sec-butyl-4-undecene
• D. 8-tert-butyl-4-undecene

Draw a structure for 4-ethyl-7-phenyl-2-heptyne.

• A.
• B.
• C.
• D.

Here’s what students ask on this topic:

What is the general rule for naming alkynes?

The general rule for naming alkynes involves identifying the longest carbon chain that includes the triple bond and changing the suffix from -ane (used for alkanes) to -yne. Number the chain from the end nearest the triple bond to give the lowest possible number to the triple bond. If there are substituents, their positions and names should be specified, and the location of the triple bond should be indicated by the number of the first carbon involved in the bond. For example, 2-butyne indicates a four-carbon chain with a triple bond starting at the second carbon.

How do you determine the position of the triple bond in an alkyne?

To determine the position of the triple bond in an alkyne, identify the longest carbon chain that includes the triple bond. Number the chain from the end closest to the triple bond to ensure the triple bond gets the lowest possible number. The position of the triple bond is indicated by the number of the first carbon involved in the bond. For example, in 3-hexyne, the triple bond starts at the third carbon of a six-carbon chain.

Why don't alkynes exhibit cis or trans isomerism?

Alkynes do not exhibit cis or trans isomerism because the carbon-carbon triple bond creates a linear structure, leaving no room for the geometric variation seen in alkenes. In alkenes, the double bond allows for different spatial arrangements of substituents (cis or trans). However, the triple bond in alkynes forces the bonded carbons and their substituents into a straight line, eliminating the possibility of such isomerism.

What is the difference between naming alkanes, alkenes, and alkynes?

The primary difference in naming alkanes, alkenes, and alkynes lies in the suffix used and the type of bond present. Alkanes have single bonds and use the suffix -ane. Alkenes have at least one double bond and use the suffix -ene. Alkynes have at least one triple bond and use the suffix -yne. Additionally, for alkenes and alkynes, the position of the double or triple bond must be indicated by numbering the carbon chain to give the lowest possible number to the bond.

How do you name an alkyne with multiple substituents?

To name an alkyne with multiple substituents, first identify the longest carbon chain that includes the triple bond and number it from the end nearest the triple bond. Name and number each substituent according to its position on the chain. List the substituents in alphabetical order, and indicate the position of the triple bond by the number of the first carbon involved in the bond. For example, 4-methyl-2-pentyne indicates a five-carbon chain with a triple bond starting at the second carbon and a methyl group attached to the fourth carbon.