7.1 : Structure et liaison des alcènes

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 overlap is shorter than that derived from sp³–sp³ overlap in propane.

Deviation of H–C–C bond angles from the expected value of 120° is due to the strain created by repulsive nonbonding interactions of substituents across the double bond.

A carbon–carbon double bond exerts an electron-withdrawing effect. The electron density in the unhybridized 2p orbital is unevenly distributed and hence does not screen the nuclear charge on the carbon as effectively as the hybrid orbital does. Thus, some alkenes, although weakly, exhibit dipole moments.

Due to the absence of strong dipoles, the key attractive forces between alkene molecules are London dispersion forces. These forces get stronger with increasing molecular mass and larger surface area. Therefore, at room temperature, small alkenes are gaseous, and those with more than four carbons are liquids with increasing boiling points.

Being nonpolar, alkenes are miscible in nonpolar solvents and immiscible in water.