Alkenes cannot rotate through their C=C double bond, it locks the structure of the molecule in place at that point. As a result, if we have different groups bonded to the carbon atoms either side of the double bond, we have pairs of stereoisomers – molecules with the same molecular formula but a different structural arrangement.
You can read more about single and double bonds here and if you need a refresher on hybrid orbitals then check out valence theory.
Isomers will have slightly different physical (and sometimes chemical) properties.
Naming these isomers so that we can easily tell one from the other has evolved over the years …
- cis / trans isomers
The trouble with this naming system is that it only worked for simple alkenes.
- E/Z isomers – this is a systematic system that assigns priority to each of the groups / atoms bonded to the carbon atoms either side of the double bond. The system is based on the rules devised by Robert Cahn, Christopher Ingold and Vladimir Prelog for naming enantiomers (optical isomers), widely known as the Cahn, Ingold, Prelog or CIP rules.
E is for the isomer with high priority groups on opposite sides of the C=C bond (from ‘etgegen’ meaning opposite) …. think ‘trans’
Z is for the isomer with high priority groups on the same side of the C=C bond (from ‘zusammen’ meaning together) …. think ‘cis’
It will all make sense in a minute, honest!
- Priority is based on the atomic number of the atom directly bonded to the carbon of the C=C bond. The higher the atomic number, the higher the priority.
I > Br > Cl > F > O > N > C > H
- If the same atoms are bonded directly to the carbon of the C=C bond (e.g. a methyl group and an ethyl group), then we rank the 2nd / 3rd / 4th atom until there is a difference.
- If the groups we are trying to assign priority to contain double or triple bonds then we assume that the carbon is bonded to two or three of that atom.
In summary, we work out which is the high and the low priority group bonded to each carbon of the C=C bond, and then we decide whether we have the E or the Z isomer …
Practice questions
- How many sigma and pi bonds are there in the structure below:
- Name the compound below:
- Describe the arrangement around each of the double bonds in this molecule using the Cahn, Ingold, Prelog rules:
Answers
- 12 sigma bonds and 2 pi bonds.
- (E)-2-bromobut-2-ene
- There is an (E) arrangement around both double bonds – taking each double bond in turn, the highest priority groups are opposite each other.