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Naming Straight-Chain Alkynes

Key Concepts

  • Alkynes are hydrocarbons in which there is a triple bond between two carbon atoms1:

    C≡C

  • The systematic IUPAC2 name of all simple straight-chain alkynes ends in "yne".

  • The systematic IUPAC name of a straight-chain alk-n-yne is made up of three parts3:

    (i) A prefix which tells us how many carbon atoms are in the chain (alk).

    No. carbon atoms 1 2 3 4 5 6 7 8 9 10
    Prefix meth eth prop but pent hex hept oct non dec

    (ii) An infix which is a number that tells us the location of the triple bond (-n-)

    (ii) The suffix yne telling us that a triple bond is present within the chain.

  • Note that the preferred IUPAC name4 may not be the same as the systematic IUPAC name:

    molecular formula structure systematic IUPAC name trivial name Preferred IUPAC Name
    C2H2
    H-C C-H
    ethyne acetylene acetylene

Naming Straight-Chain Alkynes:

  • Identify the longest carbon chain containing the triple bond.

  • Determine the prefix for the name of the alkyne based on the number of carbon atoms in the chain.

  • Number each carbon atom along the longest carbon chain so that the triple bonded carbon atoms have the lowest possible number.

  • Determine the infix for the name of the alkyne based on the location of the triple bond (use the lowest number out of the two carbon atoms joined by the triple bond).

    Note that the infix may not be required if the longest carbon chain contains only two or three carbon atoms.

  • Determine the suffix for the name of the alkyne. All straight chain alkynes containing one triple bond will end in "yne".

  • Write the name for the alkyne in the form of prefix-infix-suffix

Example: Naming a Straight-Chain Alkyne

Give the systematic IUPAC name for this alkyne:

H- H
|
C
|
H
- C C - H
|
C
|
H
-H
  • Identify the longest carbon chain containing the triple bond.

    H- H
    |
    C
    |
    H
    - C C - H
    |
    C
    |
    H
    -H

  • Determine the prefix for the name of the alkyne based on the number of carbon atoms in the chain.

    Longest carbon chain contains four carbon atoms.

    prefix is but

  • Number each carbon atom along the longest carbon chain so that the triple bonded carbon atoms have the lowest possible number.

    numbering from left to right   numbering from right to left
    H- H
    |
    C1
    |
    H
    - C2 C3 - H
    |
    C4
    |
    H
    -H
    OR
    H- H
    |
    C4
    |
    H
    - C3 C2 - H
    |
    C1
    |
    H
    -H
    location of triple bond: C2   location of triple bond: C2

    In this case the triple bond occurs between C2 and C3 in both cases.

  • Determine the infix for the name of the alkyne based on the location of the triple bond (use the lowest number out of the two carbon atoms joined by the triple bond).

    H- H
    |
    C
    |
    H
    - C2 C - H
    |
    C
    |
    H
    -H

    infix is -2-

  • Determine the suffix for the name of the alkyne.

    All straight chain alkynes containing one double bond will end in "yne".

    Suffix is yne

  • Write the systematic IUPAC name for the alkyne in the form of prefix-infix-suffix

    Systematic IUPAC name is but-2-yne

    Another systematic name is 2-butyne

Drawing the structure5 of straight-chain alkynes:

  • Break the systematic IUPAC name of the alkyne into its three parts:

    alk -n- yne
    prefix infix suffix

  • Determine the number of carbon atoms in the longest carbon chain using the prefix.

  • Draw a chain of carbon atoms of the required length using dashes to represent a single covalent bond between each pair of carbon atoms.

  • Number the carbon atoms in the chain from left to right.

  • Determine the location of the triple bond using the infix.
    We know there is a triple bond because the molecule's name has the suffix yne.

  • Draw a second and then a third dash, a second and third covalent bond, between the carbon with the same number as the infix, and the carbon atom with a number equal to the infix + 1 (that is, the adjacent carbon atom on the right hand side if you have numbered your carbon chain from left to right).

  • Draw dashes around each carbon atom in the chain such that each carbon atom makes 4 bonds.
    Note that the carbon atoms involved in the triple bond have already used 3 out of the 4 possible bonds in making the triple bond!

  • Complete the structure by placing a hydrogen atom (H) at the end of all the vacant dashes.

Example: Drawing the Structure of a Straight-Chain Alkyne

Draw the structure for the molecule with the systematic IUPAC name of hex-2-yne.
  • Break the systematic IUPAC name of the alkene into its three parts:

    hex -2- yne
    prefix infix suffix

  • Determine the number of carbon atoms in the longest carbon chain using the prefix.

    Prefix is hex so there are 6 carbon atoms in the longest carbon chain.

  • Draw a chain of carbon atoms of the required length using dashes to represent a single covalent bond between each pair of carbon atoms.
    C - C - C - C - C - C

  • Number the carbon atoms in the chain from left to right.
    C1 - C2 - C3 - C4 - C5 - C6

  • Determine the location of the triple bond using the infix.
    We know there is a triple bond because the molecule's name has the suffix yne.

    Infix is 2, so this is the location of the triple bond.

    C - C2 - C - C - C - C

  • Draw a second and a third dash, a second and third covalent bond, between the carbon with the same number as the infix, and the carbon atom with a number equal to the infix + 1 (that is, the adjacent carbon atom on the right hand side if you have numbered your carbon chain from left to right).

    The triple bond will occur between C2 and C3 (C2+1=3).

    C - C2 C3 - C - C - C

  • Draw dashes around each carbon atom in the chain such that each carbon atom makes 4 bonds.
    Note that the carbon atoms involved in the triple bond have already used 3 out of the 4 possible bonds in making the triple bond!

      |           |   |   |  
    - C - C C - C - C - C -
      |           |   |   |  

  • Complete the structure by placing a hydrogen atom (H) at the end of all the vacant dashes.

      H
    |
              H
    |
      H
    |
      H
    |
     
    H- C - C C - C - C - C -H
      |
    H
              |
    H
      |
    H
      |
    H
     

Molecular formula of straight-chain alkynes:6

  • Draw the structure of the straight-chain alkyne.

  • Write a skeletal molecular formula using the symbols for carbon and hydrogen:
    C   H

  • Count the number of carbon atoms in total in the structure.
    total number of carbon atoms = n

  • Write the number of carbon atoms as a subscript number to the right of the symbol for carbon
    CnH

  • Count the number of hydrogen atoms in total in the structure.
    total number of hydrogen atoms = y

  • Write the number of hydrogen atoms as a subscript number to the right of the symbol for carbon
    CnHy

  • The general molecular formula for a straight-chain alkyne is CnH2n-2

        where n = number of carbon atoms in the carbon chain

Example: Writing the Molecular Formula of a Straight-Chain Alkyne

Write the molecular formula for the molecule with the systematic IUPAC name of but-1-yne.

  • Draw the structure of the straight-chain alkyne.

              H
    |
      H
    |
     
    H- C C - C - C -H
              |
    H
      |
    H
     

  • Write a skeletal molecular formula using the symbols for carbon and hydrogen:
    C   H

  • Count the number of carbon atoms in total in the structure.

              H
    |
      H
    |
     
    H- C1 C2 - C3 - C4 -H
              |
    H
      |
    H
     

    total number of carbon atoms = n = 4

  • Write the number of carbon atoms as a subscript number to the right of the symbol for carbon
    C4H

  • Count the number of hydrogen atoms in total in the structure.

              H2
    |
      H3
    |
     
    H1- C C - C - C -H4
              |
    H5
      |
    H6
     

    total number of hydrogen atoms = y = 6

  • Write the number of hydrogen atoms as a subscript number to the right of the symbol for carbon
    C4H6

  • The general molecular formula for a straight-chain alkyne is CnH2n-2

        where n = number of carbon atoms in the carbon chain

    When n = 4 = number of carbon atoms in alkyne, then, the number of hydrogen atoms in the alkyne will equal 2 x 4 -2 = 8 - 2 = 6

Animated Tutorial

Summary Table for Naming Straight-Chain Alkynes

no. C atoms
(n)
Systematic IUPAC Name Other Names Molecular Formula Structure
2 ethyne acetylene
(preferred IUPAC name)
C2H2
H- C C -H

3 prop-1-yne 1-propyne
propyne
C3H4
          H
|
 
H- C C - C -H
          |
H
 

4 but-1-yne 1-butyne C4H6
          H
|
  H
|
 
H- C C - C - C -H
          |
H
  |
H
 
but-2-yne 2-butyne C4H6
  H
|
          H
|
 
H- C - C C - C -H
  |
H
          |
H
 

5 pent-1-yne 1-pentyne C5H8
          H
|
  H
|
  H
|
 
H- C C - C - C - C -H
          |
H
  |
H
  |
H
 
pent-2-yne 2-pentyne C5H8
  H
|
          H
|
  H
|
 
H- C - C C - C - C -H
  |
H
          |
H
  |
H
 

6 hex-1-yne 1-hexyne C6H10
          H
|
  H
|
  H
|
  H
|
 
H- C C - C - C - C - C -H
          |
H
  |
H
  |
H
  |
H
 
hex-2-yne 2-hexyne C6H10
  H
|
          H
|
  H
|
  H
|
 
H- C - C C - C - C - C -H
  |
H
          |
H
  |
H
  |
H
 
hex-3-yne 3-hexyne C6H10
  H
|
  H
|
          H
|
  H
|
 
H- C - C - C C - C - C -H
  |
H
  |
H
          |
H
  |
H
 

7 hept-1-yne 1-heptyne C7H12
          H
|
  H
|
  H
|
  H
|
  H
|
 
H- C C - C - C - C - C - C -H
          |
H
  |
H
  |
H
  |
H
  |
H
 
hept-2-yne 2-heptyne C7H12
  H
|
          H
|
  H
|
  H
|
  H
|
 
H- C - C C - C - C - C - C -H
  |
H
          |
H
  |
H
  |
H
  |
H
 
hept-3-yne 3-heptyne C7H12
  H
|
  H
|
          H
|
  H
|
  H
|
 
H- C - C - C C - C - C - C -H
  |
H
  |
H
          |
H
  |
H
  |
H
 

8 oct-1-yne 1-octyne C8H14
          H
|
  H
|
  H
|
  H
|
  H
|
  H
|
 
H- C C - C - C - C - C - C - C -H
          |
H
  |
H
  |
H
  |
H
  |
H
  |
H
 
oct-2-yne 2-octyne C8H14
  H
|
          H
|
  H
|
  H
|
  H
|
  H
|
 
H- C - C C - C - C - C - C - C -H
  |
H
          |
H
  |
H
  |
H
  |
H
  |
H
 
oct-3-yne 3-octyne C8H14
  H
|
  H
|
          H
|
  H
|
  H
|
  H
|
 
H- C - C - C C - C - C - C - C -H
  |
H
  |
H
          |
H
  |
H
  |
H
  |
H
 
oct-4-yne 4-octyne C8H14
  H
|
  H
|
  H
|
          H
|
  H
|
  H
|
 
H- C - C - C - C C - C - C - C -H
  |
H
  |
H
  |
H
          |
H
  |
H
  |
H
 

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1 We are ignoring the possibility of more than one triple bond existing in the carbon chain during this discussion.

2IUPAC is the abbreviation for the International Union of Pure and Applied Chemistry

3The preferred IUPAC systematic name places the infix for the locant immediately before that part of the name to which it relates, except when the preferred IUPAC name is the traditional contracted name in which case the infix is placed at the front of the name.
For the simple straight chain alkynes we are discussing, either nomenclature is acceptable since the name in each case is unambiguous.

4The systematic IUPAC name is derived from a set of general "rules" designed to ensure that each organic molecule can be given an unambiguous name.
The rules for naming organic compounds are still being developed. The most recent document for referral is "Preferred names in the nomenclature of organic compounds" (Draft 7 October 2004).

5"Structure" here will refer to a valence structure, which can be used to represent the 2-dimensional structural formula.

Once you have drawn the valence structure or 2-dimensional structural formula you can use this to draw

a condensed (semi) structural formula

or a skeletal structure

6We are going to ignore the possibility of cyclic compounds here as well as the possibility of there being more than triple bond.

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