- If a hydrocarbon has a double bond, use the suffix -ene
- If a hydrocarbon has a triple bond, use the suffix -yne
The same rules that we used to name alkanes (repeated below) apply when multiple bonds are present in the hydrocarbon chain.
Review: Hydrocarbon nomenclature rules:
- Find the longest continuous chain of carbon atoms (parent chain). This chain determines the parent name (root) of the alkane.
- Identify the hydrocarbon suffix.
- Number the parent chain. For simple alkanes, begin the numbering at end of the chain that is nearest to a branch. For alkenes and alkynes, begin numbering at the end that is nearest to the multiple bond.
- Name the substituent groups (branches). Each substituent name has two components: a root and a suffix. Use the table above to determine the root name based on the longest chain of carbons in the branch. The suffix for alkyl substituent groups is “-yl”.
- If an alkyl substituent appears more than once, use the prefixes: di, tri, tetra, penta, hexa, etc.
- Complete the name. Use the numbers from step 3 to designate the location of the substituents groups. Numbers are separated from letters by a dash (-) and two numbers are separated by a comma (,). If two or more substituents are present on the same carbon atom, use the number twice. List the groups alphabetically (alphabetize based on the root group from step 4, not the prefixes from step 5).
In the molecule below, the root of the molecule is “but-”, and now that a double bond is present the hydrocarbon suffix is “-ene”. When naming this molecule, you need to indicate where the double bond starts by including the lowest carbon number involved in the double bond before the "-ene" suffix.
When there is more than one double bond, use the same prefixes as outlined in rule 5. For example, in the molecule below, there are two double bonds so the hydrocarbon suffix is “-diene”. When two consonants combine after the root, you need to include an “a”. For example, the molecule below is “2-methyl-1,3-butadiene” not “2-methyl-1,3-butdiene”.
When there are both double and triple bonds in the molecule, use the hydrocarbon suffix for both. Both alkenes and alkynes are treated as equal priority, so the focus is to minimize the overall numbering in the molecule. For the molecule below, numbering starts on the left because 2-methylbut-1-en-3-yne gives a lower number to the substituent than 3-methyl-but-3-en-1-yne.
Interactive: