Alkenes

The naming pattern is the same as the alkanes (eth-, prop-, but-, pent- for the carbon count) but the ending changes to -ene to signal the presence of a C=C double bond.

Naming alkenes with the C=C position

• For ethene and propene the C=C can only sit in one place, so no number is needed
• From butene onward, the position of the C=C must be shown with a number — counting from whichever end gives the lowest possible number
  • But-1-ene: C=C between carbons 1 and 2 (CH₂=CH−CH₂−CH₃)
  • But-2-ene: C=C between carbons 2 and 3 (CH₃−CH=CH−CH₃)
• A 5-carbon chain therefore gives two main alkenes — pent-1-ene and pent-2-ene — depending on where the double bond sits

Why alkenes are reactive

• The C=C double bond can break open: one of the two bonds between the carbons breaks, leaving a single bond between them and two spare bonds (one on each carbon) free to accept incoming atoms
• Each carbon now ends with four single bonds, which is the most stable arrangement for carbon
• This is why alkenes undergo addition reactions (section 2) so readily, while alkanes — with all single bonds already in place — do not