How does the structure and bonding of alkanes differ to that of alkenes?
In alkanes, such as propane, the carbon atoms are sp3 hybridised, meaning that each carbon atom has 4 sp3 molecular orbitals associated with it available for covalent bonding. These form 4 sigma bonds (bonds formed along the direction of the bonding plane) with additional carbon and hydrogen atoms in a tetrahedral structure with bond angles of 109.5 degrees. In alkenes, such as propene, at least 2 carbon atoms are sp2 hybridised And so use 2 of their 3 available p orbitals to form 3 molecular orbitals, each of which forms a sigma bond with additional hydrogen and carbon atoms with bond angles of 120 degrees. The remaining p orbital lies orthogonal to the plane of sigma bonds, and overlaps with the adjacent carbon atoms’ p orbitals to form a pi bond between 2 carbon atoms leading to the characteristic unsaturated structure of these molecules. All other carbon atoms are sp3 hybridised as in alkanes.
