Bonding in Alkenes

The C=C double bond in an alkene is made of two different kinds of covalent bond: one σ (sigma) bond and one π (pi) bond.A σ bond forms by the end-on (direct) overlap of orbitals, so its electron density lies directly between the two carbon nuclei. Every single covalent bond is a σ bond.A π bond forms by the sideways overlap of two p orbitals, one on each carbon atom. Its electron density sits in two regions, one above and one below the plane of the molecule.Because the p orbitals overlap less effectively than in a σ bond, the π bond is weaker. This is why the mean bond enthalpy of C=C (612 kJ mol⁻¹) is less than twice that of C-C (347 kJ mol⁻¹), and why alkenes are much more reactive than alkanes. The exposed π electrons form a region of high electron density that other species can attack.Each carbon of the C=C has three regions of electron density around it, so the arrangement is trigonal planar with bond angles of 120°. The two carbons and the four atoms attached to them all lie in one plane.The π bond also means there is restricted rotation about the C=C bond: turning one end would pull the p orbitals out of line and break the π bond. This is what makes E/Z isomerism possible.