Alkenes

What an alkene is

• Alkenes are a family of hydrocarbons that contain at least one carbon–carbon double bond, written as C=C and drawn as two parallel lines between two adjacent carbons
• The C=C double bond is the alkenes' functional group — the feature that fixes their chemistry and tells them apart from alkanes
• A molecule with a C=C double bond is described as unsaturated: the two carbons of the double bond are not joined to as many hydrogens as they possibly could be. Open the double bond and each carbon could accept one extra atom

General formula

• The general formula of the alkene series (with one C=C double bond) is:

C_nH_2n

• where n is the number of carbon atoms (n ≥ 2; methene does not exist because one carbon cannot form a double bond with itself)
• Compare with alkanes (C_nH_2n+2): an alkene has two fewer hydrogen atoms than its alkane of the same chain length, because two of the bonds that would have held hydrogens now sit between the two C=C carbons

The first four alkenes

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