Alkenes are hydrocarbons that contain a double bond between two carbons, and are sometimes referred to by their old name, “olefin”. You might see this in some older, American textbooks.
Double bonded carbons show sp2 hybridisation, rather than sp3, (alkanes). In sp2 atoms, the s orbital has hybridised with 2 of the 3 available p orbitals, forming a trigonal planar arrangement of 3 sp2 orbitals.
This arrangement leaves a p orbital unhybridised, which sits perpendicular to the plane of the sp2 orbitals. This p orbital forms the pi bond of the double bond.
The nomenclature of alkenes is much the same as alkanes:
- Find the longest carbon chain, which contains the double bond and name it as though it were an alkane, but change the end from –ane to –ene.
- Number the chain so as to include both carbon atoms of the double bond, and begin numbering at the end of the chain nearer the double bond. Use the number of the first carbon atom in the double bond to designate its position.
- Number the other substituents
- If you have a cycloalkene, number the carbons so the double bond has positions 1 and 2.
- Designate the geometry of the double bond with cis/trans or E/Z.
Name the following alkenes:
Cis and trans can be used to designate the geometry of an alkene, provided it is disubstitued.
As soon as the alkene is tri or tetra substituted, cis and trans become meaningless, so E/Z is more correctly used for alkenes. (note: cis/trans is used for cycloalkanes).
E and Z designation follow the Cahn-Ingold-Prelog rules for determining priority of a substituent. The priority of each group on one carbon atom of the double bond is determined, then that is repeated for the second carbon atom. If the higher priority on both carbon atoms are on the same side of the double bond, (see cis), then it is Z (zusammen, German for together). If they are on opposite sides, (see: trans), it is E (entgegen, German for opposite).
Name the following alkenes, including E/Z and R/S geometries where required: