Editing Alkene (section)

===Bonding===
[[Image:Ethylene 3D.png|200px|thumb|Ethylene (ethene), showing the pi bond in green]]
A carbon–carbon double bond consists of a [[sigma bond]] and a [[pi bond]]. This double bond is stronger than a single [[covalent bond]] (611&nbsp;[[joule|kJ]]/[[Mole (unit)|mol]] for C=C vs. 347&nbsp;kJ/mol for C–C),<ref name="Wade"/> but not twice as strong. Double bonds are shorter than single bonds with an average [[bond length]] of 1.33 [[ångström|Å]] (133 [[picometre|pm]]) vs 1.53 Å for a typical C-C single bond.<ref>{{March6th|page=23}}</ref>

Each carbon atom of the double bond uses its three sp<sup>2</sup> [[orbital hybridization|hybrid orbitals]] to form sigma bonds to three atoms (the other carbon atom and two hydrogen atoms). The unhybridized 2p atomic orbitals, which lie perpendicular to the plane created by the axes of the three sp<sup>2</sup> hybrid orbitals, combine to form the pi bond. This bond lies outside the main C–C axis, with half of the bond on one side of the molecule and a half on the other.  With a strength of 65 kcal/mol, the pi bond is significantly weaker than the sigma bond.

Rotation about the carbon–carbon double bond is restricted because it incurs an energetic cost to break the alignment of the [[p orbital]]s on the two carbon atoms. Consequently ''cis'' or ''trans'' isomers interconvert so slowly that they can be freely handled at ambient conditions without isomerization. More complex alkenes may be named with the [[E-Z notation|''E''–''Z'' notation]] for molecules with three or four different [[substituent]]s (side groups). For example, of the [[Butene#Isomers|isomers of butene]], the two methyl groups of (''Z'')-but-2[[-ene]] (a.k.a. ''cis''-2-butene) appear on the same side of the double bond, and in (''E'')-but-2-ene (a.k.a. ''trans''-2-butene) the methyl groups appear on opposite sides. These two isomers of butene have distinct properties.
<!-- this section seems esotericTwisting to a 90° dihedral angle between two of the groups on the carbons requires less energy than the strength of a [[pi bond]], and the bond still holds. The carbons of the double bond become [[pyramidalization|pyramidal]], which allows preserving some [[p orbital]] alignment—and hence pi bonding. The other two attached groups remain at a larger dihedral angle. This contradicts a common textbook assertion that the two carbons retain their planar nature when twisting, in which case the p orbitals would rotate enough away from each other to be unable to sustain a pi bond. In a 90°-twisted alkene, the p orbitals are only misaligned by 42° and the strain energy is only around 40 kcal/mol.  In contrast, a fully broken pi bond has an energetic cost of around 65 kcal/mol.<ref name=":0">{{cite journal|last1=Barrows|first1=Susan E.|last2=Eberlein|first2=Thomas H.|year=2005|title=Understanding Rotation about a C=C Double Bond|journal=J. Chem. Educ.|volume=82|issue=9|pages=1329|bibcode=2005JChEd..82.1329B|doi=10.1021/ed082p1329}}</ref>

Some [[pyramidal alkene]]s are stable. For example, ''trans''-[[cyclooctene]] is a stable strained alkene and the orbital misalignment is only 19°, despite having a significant [[dihedral angle]] of 137° (a planar system has a dihedral angle of 180°) and a degree of pyramidalization of 18°. Even ''trans''-[[cycloheptene]] is stable at low temperatures.<ref name=":0" />-->