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Naming Straight-Chain Alkenes

Key Concepts

  • Alkenes are hydrocarbons in which there is a double bond between two carbon atoms1:

    C=C

  • The systematic IUPAC2 name of all simple straight-chain alkenes ends in "ene".

  • The systematic IUPAC name of a straight-chain alk-n-ene 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 double bond (-n-)

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

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

    The preferred IUPAC name for the compound H2C=CH2 is ethylene rather than the systematic IUPAC name ethene.

Naming Straight-Chain Alkenes:

  • Identify the longest carbon chain containing the double bond.

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

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

  • Determine the infix for the name of the alkene based on the location of the double bond (use the lowest number out of the two carbon atoms joined by the double 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 alkene. All straight chain alkenes containing one double bond will end in "ene".

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

Example

Give the systematic IUPAC name for this alkene:

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

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

  • Determine the prefix for the name of the alkene 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 double bonded carbon atoms have the lowest possible number.

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

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

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

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

    infix is -2-

  • Determine the suffix for the name of the alkene.

    All straight chain alkenes containing one double bond will end in "ene".

    Suffix is ene

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

    Systematic IUPAC name is but-2-ene

    Another systematic name is 2-butene

Drawing the structure5 of straight-chain alkenes:

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

    alk -n- ene
    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 double bond using the infix.
    We know there is a double bond because the molecule's name has the suffix ene.

  • Draw a second dash, a second 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 double bond have already used 2 out of the 4 possible bonds in making the double bond!

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

Example:

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

    hex -2- ene
    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 double bond using the infix.
    We know there is a double bond because the molecule's name has the suffix ene.

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

    C - C2 - C - C - C - C

  • Draw a second dash, a second 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 double 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 double bond have already used 2 out of the 4 possible bonds in making the double 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
    |
      H
    |
     
    H- C - C = C - C - C - C -H
      |
    H
              |
    H
      |
    H
      |
    H
     

Molecular formula of straight-chain alkenes:6

  • Draw the structure of the straight-chain alkene.

  • 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 alkene is CnH2n

        where n = number of carbon atoms in the carbon chain

Example:

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

  • Draw the structure of the straight-chain alkene.

      H
    |
      H
    |
      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
    |
      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
    |
      H4
    |
      H5
    |
     
    H1- C = C - C - C -H6
              |
    H8
      |
    H7
     

    total number of hydrogen atoms = y = 8

  • Write the number of hydrogen atoms as a subscript number to the right of the symbol for carbon
    C4H8

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

        where n = number of carbon atoms in the carbon chain

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

Summary Table for the Nomenclature of Simple Straight-Chain Alkenes

no. C atoms
(n)
Systematic IUPAC Name Other Names Molecular Formula Structure
2 ethene ethylene
(preferred IUPAC name)
C2H4
  H
|
  H
|
 
  C = C  
  |
H
  |
H
 

3 prop-1-ene 1-propene
propene
propylene
C3H6
  H
|
      H
|
 
  C = C - C -H
  |
H
  |
H
  |
H
 

4 but-1-ene 1-butene C4H8
  H
|
      H
|
  H
|
 
  C = C - C - C -H
  |
H
  |
H
  |
H
  |
H
 
but-2-ene 2-butene C4H8
  H
|
  H
|
  H
|
  H
|
 
H- C - C = C - C -H
  |
H
          |
H
 

5 pent-1-ene 1-pentene C5H10
  H
|
      H
|
  H
|
  H
|
 
  C = C - C - C - C -H
  |
H
  |
H
  |
H
  |
H
  |
H
 
pent-2-ene 2-pentene C5H10
  H
|
  H
|
  H
|
  H
|
  H
|
 
H- C - C = C - C - C -H
  |
H
          |
H
  |
H
 

6 hex-1-ene 1-hexene C6H12
  H
|
      H
|
  H
|
  H
|
  H
|
 
  C = C - C - C - C - C -H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
 
hex-2-ene 2-hexene C6H12
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
 
H- C - C = C - C - C - C -H
  |
H
          |
H
  |
H
  |
H
 
hex-3-ene 3-hexene C6H12
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
 
H- C - C - C = C - C - C -H
  |
H
  |
H
          |
H
  |
H
 

7 hept-1-ene 1-heptene C7H14
  H
|
      H
|
  H
|
  H
|
  H
|
  H
|
 
  C = C - C - C - C - C - C -H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
 
hept-2-ene 2-heptene C7H14
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
 
H- C - C = C - C - C - C - C -H
  |
H
          |
H
  |
H
  |
H
  |
H
 
hept-3-ene 3-heptene C7H14
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
 
H- C - C - C = C - C - C - C -H
  |
H
  |
H
          |
H
  |
H
  |
H
 

8 oct-1-ene 1-octene C8H16
  H
|
      H
|
  H
|
  H
|
  H
|
  H
|
  H
|
 
  C = C - C - C - C - C - C - C -H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
 
oct-2-ene 2-octene C8H16
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
 
H- C - C = C - C - C - C - C - C -H
  |
H
          |
H
  |
H
  |
H
  |
H
  |
H
 
oct-3-ene 3-octene C8H16
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
 
H- C - C - C = C - C - C - C - C -H
  |
H
  |
H
          |
H
  |
H
  |
H
  |
H
 
oct-4-ene 4-octene C8H16
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
 
H- C - C - C - C = C - C - C - C -H
  |
H
  |
H
  |
H
          |
H
  |
H
  |
H
 

Animated Tutorial

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1 We are ignoring the possibility of more than one double 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 alkenes 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 double bond.

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