Structure and Bonding of Alkenes

Alkenes, or olefins, represented by the general formula C_nH_2n, are hydrocarbons containing a carbon–carbon double bond. The cyclic analogs, enclosing a double bond in a ring, are called cycloalkenes.

Exemplified by the simplest alkene, ethylene, the carbon atoms across the double bond are sp² hybridized. Head-to-head overlap of hybrid orbitals forms a sigma bond, and sideways overlap of the two p orbitals leads to a π bond, which, according to molecular orbital theory, has the electron density concentrated above and below the molecular plane.

Although a double bond between sp² carbons is significantly shorter and stronger than the single bond between sp³ carbons, a π bond is weaker than a σ bond. This is indicated by the carbon–carbon double bond energy, which is not twice that of a single bond. The less effective overlap of participating orbitals and higher energy of 2p electrons compared to sp² accounts for the weaker π bond.

Because sp² orbitals have more s character compared to sp³, the carbon–hydrogen bonds formed by sp²–s overlap in ethylene are shorter than those created by sp³–s overlap in ethane.

Likewise, in propylene, the carbon–carbon single bond resulting from sp²–sp³