page_title: Alkene
section_title: Bonding
hierarchical_section_title: Alkene / Structure / Bonding
caption_attribution_description: English: Ethylene (ethene) - 3D model with p orbitals. Čeština: Ethylen (ethen) - 3D model s p orbitaly.
context_page_description: In chemistry, an alkene is a hydrocarbon that contains a carbon–carbon double bond.
  The term is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds. However, the IUPAC recommends using name "alkene" only for acyclic hydrocarbons with just one double bond; alkadiene, alkatriene, etc., or polyene for acyclic hydrocarbons with two or more double bonds; cycloalkene, cycloakadiene, etc. for cyclic ones; and "olefin" for the general class — cyclic or acyclic, with one or more double bonds.
  Acyclic alkenes, with only one double bond and no other functional groups form a homologous series of hydrocarbons with the general formula CₙH₂ₙ with n being 2 or more. When n is four or more, there are multiple isomers with this formula, distinguished by the position and conformation of the double bond.
  Alkenes are generally colorless apolar compounds, somewhat similar to alkanes but more reactive. The first few members of the series are gases or liquids at room temperature. The simplest alkene, ethylene is the organic compound produced on the largest scale industrially.
context_section_description: Like a single covalent bond, double bonds can be described in terms of overlapping atomic orbitals, except that, unlike a single bond (which consists of a single sigma bond), a carbon–carbon double bond consists of one sigma bond and one pi bond. This double bond is stronger than a single covalent bond (611 kJ/mol for C=C vs. 347 kJ/mol for C–C) and also shorter, with an average bond length of 1.33 ångströms (133 pm).
  Each carbon of the double bond uses its three sp² hybrid orbitals to form sigma bonds to three atoms (the other carbon and two hydrogen atoms). The unhybridized 2p atomic orbitals, which lie perpendicular to the plane created by the axes of the three sp² 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 orbitals on the two carbon atoms. As a consequence, substituted alkenes may exist as one of two isomers, called cis or trans isomers. More complex alkenes may be named with the E–Z notation for molecules with three or four different substituents (side groups). For example, of the 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.
  Twisting 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 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.
  Some pyramidal alkenes 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.
caption: Ethylene (ethene), showing the pi bond in green.
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