Structure of alkenes:
Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond.
The trigonal planar structure of alkenes is due to sp2 hybridization.
What is sp2 hybridization?
sp2 Hybridization definition:
The hybridization in which one s-orbital and two p-orbitals are mixed to form three identical hybrid orbitals is called sp2 hybridization.
The three sp2 hybrid orbitals degenerate (have equal energy) and they are directed in space to three corners of a trigonal planar structure having an angle of 120o. They are coplanar i.e. lying in the same plane. While the third un-hybridized p-orbital is perpendicular to the plane of hybridized orbitals. Each sp2 hybrid orbital has 33% s-character and 67% p-character.
Example of Ethylene or Ethene (C2H4):
In ethene the doubly bonded carbon atoms are sp2 hybridized. The electronic configuration of carbon is 1s2, 2s2, 2p2. Its valence shell has 2 electrons in s-orbital and one electron in each of two p-orbitals while the third p-orbital is lying vacant.
By absorption of energy one electron of 2s promotes to vacant p-orbital and the atom becomes an excited state, which is not a stable state. Thus to gain a state of stability mixing of atomic orbitals take place and energy is lowered. One s-orbital and two p-orbitals are mixed together and give three sp2 hybridized orbitals. These three sp2 hybrid orbitals are identical in shape and energy and each has a single electron in it. Also, the unhybridized p-orbital has a single electron. So carbon is tetravalent and not divalent.
Two half-filled s-orbitals of two hydrogen atoms overlap with half-filled sp2 hybrid orbitals of one carbon while half-filled s-orbitals of two other hydrogen atoms overlap with half-filled sp2 hybrid orbitals of second carbon forming a total of four C-H sigma-bonds, two with each carbon. As overlapping have taken place on the bond axis so four C-H sigma bonds are produced. These bonds are formed due to sp2-s overlapping.
The remaining third sp2 hybrid orbital of one carbon overlaps with the third sp2 hybrid orbital of other carbon on the bond axis thus forming a C-C sigma bond. This bond is formed due to sp2-sp2 overlapping.
The un-hybridized p-orbitals (one of each carbon) are parallel to each other so they overlap side wisely (i.e. laterally) and form a pi-bond. Pi-bond is a weak bond and is easy to break hence ethene is reactive.
Bond angle between any two C-H bonds is 116.7o. Bond angle between C=C and C-H bond is 121.6o. The C-H bond length is 1.10Í10-10 m or 1.10 Ao. While C=C bond length is 1.34Í10-10m or 1.34 Ao.