As was the case for alkanes, three types of diagrams can be used to depict the structure of alkanes and alkenes: structural diagrams, condensed structural diagrams, and line diagrams.  You may be given any of these three types of diagrams and asked to assign an IUPAC name to an alkene or alkyne.

To name alkenes and alkynes:

1. Beginning at the end closest to the multiple bond, number the longest chain of carbons (otherwise known as the "parent chain"). This will give the multiple bond the lowest possible number.
   

   
   
   
   

   
   

   
   

   Learning Tip: When naming branched alkenes/alkynes, the position of the double bond takes priority, not the location of the branches.

    

   
   

   Fig. 1   pent-1-ene

   
   

    
   
   

2. Determine the appropriate prefix to use in the name, based on the number of carbons in the parent chain. You will use the same prefixes that are used to name the alkanes.

   
   

3. The ending of the name will be "ene" if there is a double bond or "yne" if there is a triple bond. If you have forgotten what these prefixes are, refer back to Table 3 on page 366 of your textbook. The prefixes are shown in red.

   
   
    
   

   Fig. 2  pent-2-ene

   
   

   
   

   Fig. 3  hept-3-yne

   
   
   
   

4. Next, identify the type and location of each branch.  List the branches in alphabetical order.

   
   

5. Now, write the IUPAC name using this format:

   
   

   (Branch location)-(branch name)(prefix)-(multiple bond location)-ene/yne

   
   

6. If the alkene or alkyne is in a closed ring, add "cyclo" to the front of the parent chain name.

   Remember that the double or triple bond should be assigned the lowest carbon number.  In other words, the carbons of the multiple bond in the cyclical structure automatically become carbons 1 and 2.
   

   Finally, number any branches to give them the lowest possible carbon numbers.

   
   

Check Your Understanding