How to name alkenes and alkynes (IUPAC)

The five steps

1. Find the parent chain. Most courses teach: choose the longest chain that contains the C=C. (The 2013 IUPAC recommendations rank chain length first, but exam boards and most textbooks still use this rule, and it gives the same answer for simple molecules.)
2. Number from the end nearer the double bond. The C=C must get the lowest possible locant. Substituents only decide the numbering if the double bond is the same distance from both ends.
3. Change -ane to -ene and put the locant of the first carbon of the C=C just before it: but-2-ene, not 2-butene (the older style you may still see).
4. Add substituents as prefixes in alphabetical order, each with its locant: 3-methylbut-1-ene.
5. Add E or Z in front if the double bond can have two arrangements: (E)-but-2-ene.

Worked examples

More than one double bond

Use -diene, -triene and so on, and add an extra "a" to the stem so the name is easier to say: buta-1,3-diene, not but-1,3-diene. Number the chain so the set of locants for the double bonds is as low as possible.

Alkynes and enynes

Alkynes use -yne in the same way: but-1-yne, pent-2-yne. When a molecule has both a C=C and a C≡C, the ending is -en-…-yne, with "en" first. Number to give the multiple bonds the lowest locants as a set. If there's a tie, the double bond gets the lower number.

Rings

In a cycloalkene the two carbons of the C=C are always C1 and C2, so you don't write the "1". Then number in the direction that gives substituents the lowest locants.

Common mistakes

• Numbering from the end nearer a substituent instead of the end nearer the double bond.
• Choosing a longer chain that misses the C=C (if your course uses the "chain must contain the C=C" rule).
• Writing the locant of the second carbon of the double bond. The locant is always the lower-numbered carbon.
• Forgetting E/Z when each carbon of the C=C has two different groups. See E/Z vs cis/trans.