What is Alkene? (Orangic Chemistry)

 

                                            A 3D model of ethylene, the simplest alkene.

In organic chemistry, an alkene is an unsaturated hydrocarbon that contains at least one carbon–
carbon double bond.^[1] The words alkene, olefin, and olefine are used often interchangeably (see nomenclature section below). Acyclic alkenes, with only one double bond and no other functional groups, known as mono-enes, form a homologous series of hydrocarbons with the general formula C_nH_2n.^[2] Alkenes have two hydrogen atoms less than the corresponding alkane (with the same number of carbon atoms). The simplest alkene, ethylene (C₂H₄), with the International Union of Pure and Applied Chemistry (IUPAC) name ethene, is the organic compound produced on the largest scale industrially.^[3] Aromatic compounds are often drawn as cyclic alkenes, but their structure and properties are different and they are not considered to be alkenes.^[2]

Structure

Bonding

Ethylene (ethene), showing the pi bond in green.

 

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)^[1] and also shorter, with an average bond length of 1.33 Angstroms (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 half on the other.
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 are slightly different in their chemical and physical properties.
A 90° twist of the C=C bond (which may be determined by the positions of the groups attached to the carbons) requires less energy than the strength of a pi bond, and the bond still holds. This contradicts a common textbook assertion that the p orbitals would be unable sustain such a bond. In truth, the misalignment of the p orbitals is less than expected because pyramidalization takes place (See: pyramidal alkene). Trans-Cyclooctene is a stable strained alkene and the orbital misalignment is only 19° with a dihedral angle of 137° (normal 120°) and a degree of pyramidalization of 18°.^[4] The trans isomer of cycloheptene is stable only at low temperatures.

Shape

As predicted by the VSEPR model of electron pair repulsion, the molecular geometry of alkenes includes bond angles about each carbon in a double bond of about 120°. The angle may vary because of steric strain introduced by nonbonded interactions between functional groups attached to the carbons of the double bond. For example, the C-C-C bond angle in propylene is 123.9°.
For bridged alkenes, Bredt's rule states that a double bond cannot occur at the bridgehead of a bridged ring system unless the rings are large enough (8 or more atoms).