E-Z Isomerism, not as EZ as you think…

There has been a change in the syllabus this year in the study of geometric isomerism. It is no longer studied as cis-trans, but E-Z isomerism. This is slightly more confusing than cis- and trans- forms and there has been many questions about it on student forums. So here’s my breakdown of it.

• E-Z isomerism is a type of stereoisomerism called geometric
• It occurs due to restricted rotation about the >C=C< and the fact that each C atom across the doule bond has two different atoms or groups attached.
• Stereoisomers are structural isomers with the same molecular and structural formula but different arrangement of C atoms in space
• E- means Entgegen (against in German) and Z- means Zusammen (together in German)
• There are priority rules for the groups, simple as AS, the higher the atomic number of the group attached to >C=C<, the higher the priority

Consider the isomers in the diagram

• The first unnamed isomer is 2-methylprop-1-ene. It is a structural isomer of the E-but-2-ene and Z-but-2-ene but does not show E-Z isomerism as, despite having a >C=C< with restricted rotation, C1 has two identical groups attached. This means that if the bond was broken and we rotated the groups, the product formed would be identical.
• Now consider the E-but-2-ene (ignore the number 2s in the names in the diagram at AS). As H has an atomic number of 1, and Carbon has an atomic number of 6, Carbon has a higher priority in the naming than Hydrogen. As the two highest priority groups are arranged against each other (Entgegen) they are given the notation E-.
• Now consider the Z-but-2-ene (ignore the number 2s in the names in the diagram at AS). As H has an atomic number of 1, and Carbon has an atomic number of 6, Carbon has a higher priority in the naming than Hydrogen. As the two highest priority groups are arranged together (Zusammen) they are given the notation Z-.
• Note that in the simple case of H and another group, the E- and Z- also match the old trans- and cis- resectively. Trans means across and cis means on the same side.
• so E-but-2-ene is trans-but-2-ene
• and Z-but-2-ene is cis-but-2-ene
• Now when we consider E-2-chlorobut-2-ene we see a difference to the old cis-trans naming (cis-2-chlorobut-2-ene). The reason it is E- is because Cl, with an atomic number, has a higher priority than C and so the two highest priority groups lie across from eachother
• Now when we consider Z-2-chlorobut-2-ene we see a difference to the old cis-trans naming (trans-2-chlorobut-2-ene). The reason it is Z- is because Cl, with an atomic number, has a higher priority than C and so the two highest priority groups lie together
• Using the same arguements, it can be seen that 3-methylpent-2-ene, there are differences in naming from the cis-trans format as the groups attached to the C, give a secondary level of priority to the naming (an ethyl has a higher priority than a methyl).

I hope that goes some way to clarify – I must say I’m bemused why this stuff is in AS as it is quite subtle, but I’m pretty sure it will come up as it is new and it is obviously a grade discriminator, so learn it! That said I doubt the examples will be too dificult and will probably be centered around but-2-ene isomers in my opinion.