Chapter 18 · Organic Chemistry
18.8Alkenes and Alkynes
10 min · two checks
Predict
How is the name of ethene related to ethane?
The idea
- Name simple alkenes and alkynes.
- Recognize that a double bond cannot rotate freely.
Replace -ane with -ene for a double bond and -yne for a triple bond. The chain is numbered so the multiple bond gets the lowest possible number: CH₃CH=CHCH₃ is but-2-ene, also written 2-butene. The double bond is rigid compared with a single bond. But-2-ene therefore exists as two stereoisomers, one with the two methyl groups on the same side of the double bond (cis) and one with them on opposite sides (trans). Those are different substances. A triple bond is linear at the two carbons and has no cis-trans choice.
Ethene (ethylene) and propene are the small alkenes industry uses to make polymers. Ethyne (acetylene) is the fuel of an oxyacetylene torch. Unsaturated fats are alkenes in their long chains; “cis fats” in nutrition are cis double bonds, which kink the chain. Hydrogenation adds H₂ across the double bond and saturates it.
Keep these
- -ene marks a double bond, -yne a triple bond. Number its location.
- Cis and trans are different compounds when each carbon of the double bond has two different attachments.
- A triple bond is linear and has no cis-trans forms.