Alkenes and alkynes
Naming alkenes and alkynes: Practice exercises
How to name alkenes and alkynes: the -ene and -yne endings, numbering the double and triple bonds, dienes and rings, with exercises and worked answers.
Alkenes contain a C=C double bond and are named with the ending -ene; alkynes contain a C≡C triple bond and are named with -yne. A locant in front of the ending says where the multiple bond starts.
The rules you need
- Choose the parent chain: the longest chain; if two are equally long, the one with more multiple bonds. Older rules, still used in many textbooks, took the chain with the most multiple bonds first. Both give the same chain whenever the longest chain contains every double and triple bond, as in the examples below.
- Number from the end that gives the multiple bonds the lowest locants, double and triple bonds together. The locant is the lower-numbered carbon of the bond: in pent-2-ene the double bond joins C2 and C3. Substituents decide only when the multiple bonds tie.
- Two double bonds give -diene, with an a added to the stem: buta-1,3-diene, hexa-1,5-diene.
- A double and a triple bond together give -en-…-yne, with the double bond named first: hex-1-en-4-yne. If both ends give the same locants, the double bond gets the lower number.
- Rings: the double bond of a cycloalkene joins C1 and C2, so cyclohexene needs no locant, and substituents are numbered from there: 3-methylcyclohex-1-ene.
Two names, read step by step
2-Methylpent-1-ene
Two chains have five carbons, but only one of them contains the double bond, so that one is the parent: a pentene. Numbering from the CH2= end gives the double bond the locant 1 (from the other end it would be 4), and the methyl group is then on C2.
Hex-1-en-4-yne
Six carbons with one double and one triple bond. From the left end the multiple bonds get the locants 1 and 4; from the right end, 2 and 5. The lower set wins: the double bond starts at C1 and the triple bond at C4. The final e of -ene is dropped before -yne.
Watch out for
- The multiple bond is numbered first, even when a substituent then gets a higher locant: 3-methylbut-1-ene, not 2-methylbut-3-ene.
- The two carbons of a triple bond and the atoms next to them lie on a straight line; draw them that way.
- The geometry of a double bond, E or Z, is a separate topic: see the guide to E/Z and R/S.
Draw from the name: 18 exercises
Each name opens its exercise. Draw the structure and check it: any correct drawing is accepted, and hints and a worked answer help when you are stuck.
- but-1-ene Beginner
- pent-1-yne Beginner
- but-2-yne Beginner
- cyclohexene Beginner
- buta-1,3-diene Beginner
- 2-methylbut-2-ene Intermediate
- 2-methylbuta-1,3-diene Intermediate
- 2,3-dimethylbut-2-ene Intermediate
- 4-methylpent-2-yne Intermediate
- hexa-1,5-diene Intermediate
- 1-methylcyclohex-1-ene Intermediate
- 3-ethylpent-2-ene Intermediate
- 3,3-dimethylbut-1-yne Intermediate
- pent-1-en-4-yne Advanced
- 3,3-dimethylcyclohex-1-ene Advanced
- 3-methylcyclohex-1-ene Advanced
- 2-methylhex-1-en-3-yne Advanced
- 1,3-dimethylcyclohex-1-ene Advanced
Name the structure
18 structures of this family to name, each with four names to choose from and an explanation of every wrong choice.
Printable worksheet
For practice on paper: names to draw and structures to name, with an answer key to check your work.
The full guide
The guide Alkenes and alkynes explains these names in more detail, with more worked examples and the most common mistakes.