Alkenes: double bond, names and isomers

Alkenes contain at least one C=C and have general formula CₙH₂ₙ. They are also called olefins (oil-forming), because ethene gives an oily liquid with chlorine.

The double bond is one σ bond (about 397 kJ mol⁻¹) from head-on overlap of sp² orbitals and one weaker π bond (about 284 kJ mol⁻¹) from sideways overlap of 2p orbitals. The whole C=C (681 kJ mol⁻¹) is stronger than the C–C of ethane (348 kJ mol⁻¹) and shorter (134 pm against 154 pm).

The loosely held π electrons make alkenes targets for electrophiles (electron-seeking reagents), so alkenes are less stable than alkanes and readily add reagents across the double bond.

IUPAC naming: take the longest chain containing the double bond, number from the end nearer to it, and replace -ane by -ene. Ethene (common name ethylene) is the first stable member; CH₂ (methene) is too short-lived.

Structural isomers of C₄H₈: but-1-ene and but-2-ene are position isomers; each is a chain isomer of 2-methylprop-1-ene. C₅H₁₀ has five alkene structural isomers.

Rotation about C=C is restricted, so when each doubly bonded carbon carries two different groups, two geometrical isomers exist: cis (identical groups on the same side) and trans (on opposite sides). Types XYC=CXY, XYC=CXZ and XYC=CZW all show it.

cis and trans forms differ in melting point, boiling point, dipole moment and solubility. cis-But-2-ene has μ = 0.33 D; in trans-but-2-ene the two C–CH₃ dipoles cancel, so μ is zero. Among solids, the trans isomer usually melts higher.

A compound with two identical groups on one doubly bonded carbon, such as CH₂=CBr₂ or (CH₃)₂C=CH–C₂H₅, shows no cis-trans isomerism.