Question

State the name for each of the following alkynes: (a) ${\mathrm{CH}}_3-\mathrm{C}\equiv \mathrm{C}-{\mathrm{CH}}_2-{\mathrm{CH}}_3$ (b) $\mathrm{CH}\equiv \mathrm{C}-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_3$ (c) ${\mathrm{CH}}_3-{\mathrm{CH}}_2-\mathrm{C}\equiv \mathrm{C}-{\mathrm{CH}}_2-{\mathrm{CH}}_3$ (d) ${\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\mathrm{C}\equiv \mathrm{C}-{\mathrm{CH}}_3$

Short answer

(a) 2-pentyne, (b) 1-butyne, (c) 3-hexyne, (d) 1-hexyne.

Step-by-step solution

Identify the Longest Carbon Chain Containing the Triple Bond

To name an alkyne, we first identify the longest carbon chain that includes the triple bond. This chain determines the root name of the alkyne, which is based on the number of carbon atoms it contains. Remember to start numbering the chain from the end closest to the triple bond.

Number the Carbon Atoms in the Chain

The carbon atoms in the chain should be numbered from the end closest to the triple bond. This ensures that the triple bond gets the lowest possible number, which is used in the alkyne's name.

Name the Alkyne

Use the root name based on the longest chain, and indicate the position of the triple bond using the numbered location from Step 2. The suffix '-yne' is used to denote the presence of a triple bond.

Apply the Steps to Each Compound

We apply Steps 1-3 to each alkyne separately: (a) The longest chain includes 5 carbons and a triple bond starting on the second carbon, so it is named 2-pentyne. (b) The longest chain has 4 carbons with a triple bond on the first carbon, so it is named 1-butyne. (c) The longest chain has 6 carbons, with the triple bond starting on the third carbon; hence it is named 3-hexyne. (d) The longest chain has 6 carbons, with the triple bond on the first carbon, so it is named 1-hexyne.

Key concepts

Longest Carbon Chain

When naming alkynes, identifying the longest carbon chain that contains the triple bond is a primary step. This chain dictates the root name of the alkyne and is based on the number of carbon atoms involved.

Why is this important? First, the presence of the triple bond can significantly affect the molecule's properties as well as its name. For example, in compound (a) (${\mathrm{CH}}_3-\mathrm{C}\equiv \mathrm{C}-{\mathrm{CH}}_2-{\mathrm{CH}}_3$), the longest chain counting through the triple bond spans 5 carbon atoms. Hence, the base root name is "pent".

• Select the longest continuous chain containing the triple bond.
• The chain's length determines the base name:
  • 1 carbon = methane
  • 2 carbons = ethane
  • 3 carbons = propane, and so on.
• Ensure the triple bond is part of this longest chain.

Remember, if two chains are of the same length, choose the one with more branching or more attached functional groups. This is foundational not just for the alkyne nomenclature, but also for other classes of hydrocarbons.

Triple Bond Position

After identifying the longest carbon chain in an alkyne, the next step is determining the position of the triple bond. This requires numbering the carbon atoms in the chain.

To correctly number them, start from the end closest to the triple bond. This ensures that the triple bond gets the lowest possible numbering, which is crucial for proper naming.

• Because the position of the triple bond alters the compound's properties, accurate numbering is crucial.
• In compound (b) $(\mathrm{CH}\equiv \mathrm{C}-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_3)$, numbering the chain starting from the end closest to the triple bond gives the first carbon as the start point. This is why the compound is called 1-butyne.

Always remember, if there are ties in achieving the lowest numbering, move on to other criteria such as additional functional groups. For alkynes, this number will directly precede "-yne" in the name, clearly indicating the position of the triple bond.

Root Name

The root name of an alkyne is derived from the longest carbon chain that includes the triple bond, and this name is completed by adding a suffix. In the case of alkynes, this suffix is "-yne". This suffix identifies the presence of a carbon-carbon triple bond.

Here’s how to construct the full name:

• Identify the longest chain with the triple bond to get the root name (as previously discussed).
• The position number of the triple bond is combined with this root.
• The suffix "-yne" is appended to signal the presence of the triple bond.

For example, in compound (c) $({\mathrm{CH}}_3-{\mathrm{CH}}_2-\mathrm{C}\equiv \mathrm{C}-{\mathrm{CH}}_2-{\mathrm{CH}}_3)$, the root name is "hex" since there are six carbon atoms. With the triple bond starting at the third carbon, the name becomes 3-hexyne.

Consistently using these naming guidelines not only helps in correctly naming compounds but also gives insight into the compound’s structure based on its name.