Structure and Synthesis of Alkenes

Introduction to Alkenes
?Alkenes: Hydrocarbons with C=C double bonds.
?Alkenes are unsaturated.
?Alkenes also called olefins, meaning “oil-forming gas.”
?Alkene FG is the reactive C=C double bond.
The Geometry of Alkenes
? In C=C bonds, sp2 hybrid orbitals are formed by the
carbon atoms, with one electron left in a 2p orbital.
? During hybridization, two of the 2p orbitals mix with
the single 2s orbital to produce three sp2 hybrid
orbitals. One 2p orbital is not hybridized and remains
unchanged.
? sp2 hybrid orbitals have more s character than the sp3
hybrid orbitals.
The Geometry of Alkenes (continued)
1. One bond (sigma, ?) is formed by
overlap of two sp2 hybrids.
2. Second bond (pi, ?) is formed by
connecting the electrons from 2
unhybridized p orbitals.
3. Trigonal planar molecular
geometry.
Planar geometry of the sp2hybrid orbitals and pi bond locks the C=C bond firmly in place.
?No rotation around the carbon–carbon bond is possible without breaking the pi bond (264 kJ/mole).
?Cisand trans isomers cannot be interconverted