Among organic compounds, alkenes are one of the most difficult ones to name. This difficulty arises from the ‘geometrical isomers’ formation ability of alkenes, which is tackled by a traditional method i.e. cis/trans nomenclature. However, ambiguous cases require alternate solutions, so, a nomenclature exists, which works where cis/trans nomenclature fails. Such an efficient method of naming alkenes is termed E and Z nomenclature of alkenes.
Normally for geometrical isomers, the cis/trans system works well. But, for some alkenes, this nomenclature is unable to name them with non-similar groups surrounding double-bonded carbons. For example, 1-bromo-2-chloro-2-fluoro-1-iodoethene, etc.
“E and Z configuration of alkene based on the priority of the groups attached to double-bonded carbon atoms.”
On the basis of priority of functional groups present on the same or opposite sides, we decide whether the configuration of an alkene is E or Z. Here, E stands for the German word, ‘entgegen’ meaning ‘opposite’, and Z for the German word, ‘zusammen’ meaning ‘together’ or ‘same’.
E and Z configuration is an entirely different, although similar method of naming alkenes than the cis/trans nomenclature. E/Z naming system is applicable to complicated and special structures of alkenes.
It is important to note here that there is ‘No specific relationship’ between cis/trans and E/Z nomenclature systems.
In lieu of which, it is safe to say that, not every ‘cis’ isomer is necessarily a ‘Z’ isomer, and not every ‘trans’ isomer gets to be an ‘E’ isomer. This is true in only some cases, otherwise, they are related.
Priority Rules: The CIP Rules
Cahn-Ingold-Prelog (CIP) priority rules are used to give priority to the atoms attached to double-bonded (alkene) carbon atoms. These rules contribute to naming organic and organometallic stereoisomers.
The CIP rules were designed by and named after, R. S. Cahn, Sir Christopher Ingold, and Vladimir Prelog in 1965. They are used to assign absolute configurations to organic compounds with stereogenic centers such as R/S compounds. This configuration is further applied to E and Z compounds for priority ruling.
To apply the CIP rules on E/Z alkenes, the R/S configuration priority first needs to be clarified.
- First of all, determine the substituents attached to the double-bonded carbon atoms. For E/Z alkenes, both carbon atoms need to be separately analyzed.
- Now, start ranking the substituent on the basis of atomic numbers. For E/Z compounds, this priority is the basis of assigning numbers.
- If the first atoms of substituents are similar, proceed to the next atom of substituent until the first point of difference arrives.
- If multiple bonds are present in substituents, they are considered two or three times, as the bonding suggests.
The CIP priority rules help in naming E/Z alkenes as a secondary objective, with the primary one being, categorizing stereogenic centers of single-bonded carbon atoms.
E and Z Nomenclature
After the priority ruling by Cahn, Ingold, and Prelog rules, E/Z compounds can be named as:
- Add the ‘Z’ configuration before the systematic name of alkene, if higher priority groups on both double-bonded carbon atoms are on the same side.
- Add the ‘E’ configuration before the systematic name of alkene, if higher priority groups on both double-bonded carbon atoms are on the opposite sides.
E/Z Cycloalkenes
The E and Z isomerism occurs due to the restriction in carbon-carbon bond i.e. double bond, etc. It is also evident from the hybridization status of double bonds, i.e. sp2 hybridization, that E and Z nomenclature applies to planar molecules. Cycloalkanes, similar to straight-chain alkenes, have priority groups on the same and opposite sides, making them Z and E cycloalkenes respectively.
For example,
- (Z) 2/3-dimethylcyclohexenol is one of such cycloalkenes with exhibit Z-configuration.
- Similarly, (Z)1-chloro-3-methylcyclopentene is a Z-cycloalkene.
- (E) 2-chloro-3-methylhexenol, on the other hand, is an E-cycloalkene.
Stability of E and Z Isomers
Generally, Z isomers are less stable than E isomers because the higher priority groups are on the same side in Z isomers. This results in a steric hindrance and makes the energy of Z-isomers high.
Whereas, in E isomers both high priority groups are on opposite sides, decreasing the overall steric hindrance. So, they are low-energy molecules with high stability.
Key Takeaway(s)
- E refers to entgegen, meaning opposite sides while Z refers to zusammen, meaning same sides.
E and Z Alkenes are stereoisomers to each other
- E and Z configurations are stereoisomers because they have the same molecular formula but different arrangements of atoms in space.
- More specifically E and Z configurations are ‘diastereomers’ because they are non-superimposable and non-mirror images of each other.
- As diastereoisomers have different physical properties, they can be separated from each other.
Concepts Berg
What is the relationship between E and Z alkene?
E and Z are stereoisomers. In such geometrical isomers, molecules have a similar molecular formula but a different arrangement of atoms in space due to restrictions in bond rotations.
How do you know if alkene is E or Z?
If high priority groups are on the same side of a double-bonded carbon atom then it is called Z configuration. Whereas, if high priority groups are on the different sides of a double-bonded carbon atom, it is called an E configuration.
What do E and Z mean in organic chemistry?
- E means entgegen which is german for the opposite (sides).
- Z means zusammen which is german for the same (sides).
What is a cis alkene?
A cis alkene is the one with similar groups on the same side of a double bond. It is usually also the Z-isomer, but not always.
Is there a difference between cis/trans and E/Z molecules?
Cis and trans isomers are only applicable where alkenes have two different groups attached to double-bonded carbon atoms and both carbon atoms have at least one similar group.
Whereas, E and Z isomerism is also applicable to alkene where cis-trans isomerism doesn’t work. It is even applicable to alkenes in which double-bonded carbon atoms are attached to all four different types of groups.
How to determine E/Z isomerism for cycloalkenes?
E/Z isomerism in cycloalkenes is just similar to that of straight-chain alkenes. See examples in the above section.
References
- Organic Chemistry by Joseph M. Hornback (University of Denver)
- Introduction to Stereochemistry [essentials]: For Students and Trainees by Torsten Schmiermund (Frankfurt, Hesse, Germany)
- E- and Z-nomenclature of alkenes (University of Calgary)
