Naming Alkynes: Videos & Practice Problems

Alkynes are hydrocarbons characterized by a carbon-carbon triple bond, with naming conventions similar to alkenes. The suffix changes from -ane to -yne to indicate the presence of the triple bond. Alkynes do not exhibit cis or trans isomerism, simplifying their structural representation. When naming, it is essential to specify the location of substituents and the triple bond within the molecule. Understanding these principles is crucial for mastering organic chemistry and the behavior of aliphatic hydrocarbons.

Rules for Naming Alkynes

Alkynes are hydrocarbons characterized by the presence of a carbon-carbon triple bond, which significantly influences their chemical properties and nomenclature. The naming conventions for alkynes closely resemble those for alkenes, with a key distinction in the suffix used. Instead of the suffix -ane found in alkanes, alkynes adopt the suffix -yne. This change reflects the presence of the triple bond in the molecular structure.

One important aspect of alkynes is that they do not exhibit cis or trans isomerism due to the linear geometry around the triple bond. When naming alkynes, it is essential to indicate the position of the triple bond and any substituents present in the molecule. This is done by using numerical locants that specify the carbon atom to which the substituent or the triple bond is attached. For example, in a compound like 2-butyne, the '2' indicates that the triple bond starts at the second carbon in the longest carbon chain.

In summary, when naming alkynes, remember to replace the -ane ending of alkanes with -yne, provide the location of the triple bond, and include the positions of any substituents. This systematic approach ensures clarity and consistency in organic chemistry nomenclature.

Naming Alkynes Example

To determine the systematic name of an alkyne, follow a structured approach that involves identifying the longest carbon chain and naming the substituents appropriately. The first step is to locate the longest carbon chain that includes the triple bond, ensuring it has the maximum number of carbon atoms. If there is a tie in length, the chain with more substituents should be selected.

For example, consider a carbon chain where you can count the carbons as follows: 1, 2, 3, 4, 5, 6, and 7. This chain includes a triple bond and has substituents that need to be identified. In this case, there are two methyl groups and one ethyl group as substituents. The methyl groups are located on carbons 2 and 6, while the ethyl group is on carbon 5.

Next, number the carbon chain starting from the end closest to the triple bond. This ensures that the position of the triple bond is indicated correctly. In this example, the triple bond starts at carbon 3, leading to the designation of the parent chain as heptyne, which is derived from heptane by changing the suffix from "-ane" to "-yne."

When naming the substituents, they should be listed in alphabetical order, regardless of their numerical positions. In this case, the ethyl group (E) comes before the methyl groups (M). Therefore, the final name of the compound is structured as follows: 5-ethyl-2,6-dimethyl-3-heptyne. This systematic naming convention ensures clarity and consistency in organic chemistry nomenclature.

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

Answer: (A)

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

• A.
• B.
• C.
• D.

Answer: (B)

Here’s what students ask on this topic:

What is the general rule for naming alkynes?

The general rule for naming alkynes is similar to that of alkenes, with a few modifications. Alkynes possess a carbon-carbon triple bond, and the suffix '-ane' from alkanes is changed to '-yne' to indicate the presence of the triple bond. The numerical location of the triple bond must be specified in the name, and the longest carbon chain containing the triple bond is chosen as the parent chain. Additionally, the positions of any substituents on the carbon chain must also be indicated with numbers.

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

To determine the position of the triple bond in an alkyne, you need to identify the longest carbon chain that includes the triple bond. Number the carbon atoms in the chain starting from the end closest to the triple bond. The position of the triple bond is indicated by the number of the first carbon atom involved in the triple bond. For example, in 2-butyne, the triple bond starts at the second carbon atom in a four-carbon chain.

Why don't alkynes possess cis or trans isomers?

Alkynes do not possess cis or trans isomers because of the linear geometry of the carbon-carbon triple bond. The triple bond forces the two carbon atoms and the atoms directly attached to them into a straight line, leaving no room for the different spatial arrangements that give rise to cis or trans isomerism. This linear structure makes it impossible for alkynes to have the same kind of geometric isomerism seen in alkenes.

What is the significance of the suffix '-yne' in alkyne nomenclature?

The suffix '-yne' in alkyne nomenclature is significant because it indicates the presence of a carbon-carbon triple bond in the molecule. This suffix differentiates alkynes from alkanes (which have single bonds) and alkenes (which have double bonds). The '-yne' suffix is essential for correctly identifying and naming compounds with triple bonds, ensuring clear communication and understanding in chemical nomenclature.

How do you name an alkyne with multiple substituents?

To name an alkyne with multiple substituents, follow these steps: 1) Identify the longest carbon chain containing the triple bond and number the chain starting from the end closest to the triple bond. 2) Indicate the position of the triple bond with the appropriate number. 3) Identify and number the positions of all substituents on the carbon chain. 4) List the substituents in alphabetical order, each with its corresponding position number, before the base name of the alkyne. 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.