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3.9: Alkynes

  • Page ID
    221779
  • Learning Objectives

    • Describe the general physical and chemical properties of alkynes.
    • Name alkynes given formulas and write formulas for alkynes given names.

    The simplest alkyne—a hydrocarbon with carbon-to-carbon triple bond—has the molecular formula C2H2 and is known by its common name—acetylene (Figure \(\PageIndex{1}\)). Its structure is H–C≡C–H.

    Figure \(\PageIndex{1}\): Ball-and-Spring Model of Acetylene. Acetylene (ethyne) is the simplest member of the alkyne family.

    Acetylene is used in oxyacetylene torches for cutting and welding metals. The flame from such a torch can be very hot. Most acetylene, however, is converted to chemical intermediates that are used to make vinyl and acrylic plastics, fibers, resins, and a variety of other products.

    Alkynes are similar to alkenes in both physical and chemical properties. For example, alkynes undergo many of the typical addition reactions of alkenes.

    Nomenclature

    The International Union of Pure and Applied Chemistry (IUPAC) names for alkynes parallel those of alkenes, except that the family ending is -yne rather than -ene. The IUPAC name for acetylene is ethyne. The names of other alkynes are illustrated in the following exercises.

    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.

    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.

    Examples:

     

    Alkynes IUPAC.png

    Concept Review Exercises

    1. Briefly identify the important differences between an alkene and an alkyne. How are they similar?
    2. The alkene (CH3)2CHCH2CH=CH2 is named 4-methyl-1-pentene. What is the name of (CH3)2CHCH2C≡CH?
    3. Do alkynes show cis-trans isomerism? Explain.

    Answers

    1. Alkenes have double bonds; alkynes have triple bonds. Both undergo addition reactions.
    2. 4-methyl-1-pentyne
    3. No; a triply bonded carbon atom can form only one other bond. It would have to have two groups attached to show cis-trans isomerism.

    Key Takeaway

    • Alkynes are hydrocarbons with carbon-to-carbon triple bonds and properties much like those of alkenes.

    Exercises

    1. Draw the structure for each compound.

      1. acetylene
      2. 3-methyl-1-hexyne
    2. Draw the structure for each compound.

      1. 4-methyl-2-hexyne
      2. 3-octyne
    3. Name each alkyne.

      1. CH3CH2CH2C≡CH
      2. CH3CH2CH2C≡CCH3

    Answers

      1. H–C≡C–H
      2. 1b.jpg
     
      1. 1-pentyne
      2. 2-hexyne

      Attributions and citations

      Naming Alkynes. (2021, February 15). Retrieved May 20, 2021, from https://chem.libretexts.org/@go/page/31481

      Alkynes. (2020, August 17). Retrieved May 20, 2021, from https://chem.libretexts.org/@go/page/16058

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