Alkenes: structure, names and isomerism

Alkenes (also called olefins) have one C=C double bond, and the general formula CₙH₂ₙ: ethene C₂H₄, propene C₃H₆… Each carbon of the C=C is sp² (planar, 120°). The double bond is one σ + one π bond, 134 pm long, with a bond enthalpy of 681 kJ mol⁻¹. It is stronger and shorter than a C–C single bond, but its π part is weak and exposed, so alkenes are much more reactive than alkanes.

Naming: the name ends in -ene. The main chain must contain the double bond, numbered to give it the lowest number: 2,8-dimethyldeca-3,6-diene has two double bonds, starting at carbons 3 and 6. In any open-chain molecule, the number of σ bonds = number of atoms – 1.

Structural isomers of alkenes can differ in the chain or in the position of the double bond. C₄H₈ gives three: but-1-ene, but-2-ene and 2-methylpropene.

Because the π bond prevents rotation about C=C (like two boards nailed together with two nails instead of one), a new kind of isomerism appears. If each carbon of the double bond carries two different groups, there are two geometrical isomers:

• cis: the similar groups are on the same side of the double bond.
• trans: they are on opposite sides.

If either carbon carries two identical groups, swapping them changes nothing, so there are no cis and trans forms.

cis and trans isomers have different properties. cis isomers are often polar (cis-but-2-ene has a dipole moment of 0.33 D; trans-but-2-ene, 0), so they have higher boiling points. trans isomers are more symmetrical, pack better in a solid, and usually have higher melting points.