Rules for Naming Alkenes and Alkynes

The rules for naming alkenes and alkynes are basically the same.

• Instead of ending the name of the molecule with -ane, change the ending to -ene for alkenes and -yne for alkynes

• The parent chain is the longest carbon chain containing the double or triple bond

• You need to indicate the carbon number at which the double or triple bond is located. Depending on which naming system you use, it can look 2 different ways. For example, a 4 carbon molecule with a double bond between the first and second carbons could be named 1-butene in the old system or but-1-ene in the new system.

• The parent chain needs to be numbered so that the double or triple bond has the lowest number possible.

Examples

Alkene Example

• The parent chain would be the six-carbon chain containing the double bond. So this molecule is a hexene.

• We’ll number from left to right to give the double bond the number 2.

  • Numbering right to left would give it the number 4.

• So, we have 2-hexene or hex-2-ene so far.

• Now, we have to name and number the substituent.

  • The substituent is a methyl group at carbon 3, so it would be named 3-methyl.

• Putting that together, we have 3-methyl-2-hexene or 3-methyl-hex-2-ene.

Alkyne Example

• The parent chain is the six-carbon chain containing the triple bond, so we have a hexyne.

• We’ll number from left to right to give the triple bond the number 1.

  • If we number from right to left, the triple bond would have the number 5.

• So, we have 1-hexyne or hex-1-yne so far.

• Now, we have to name and number the substituent.

• The substituent is a methyl group at carbon 3 (carbon 2 isn’t easy to see but it’s at the end of the triple bond), so we would name it 3-methyl.

• Putting it all together, we have 3-methyl-1-hexyne or 3-methyl-hex-1-yne.