Stereoisomerism (Oxford AQA International A Level Chemistry)
Revision Note
Written by: Richard Boole
Reviewed by: Stewart Hird
Stereoisomerism
Stereoisomers are compounds that have the same atoms connected to each other, however the atoms are differently arranged in space
E/Z isomerism
E–Z isomerism is a specific type of stereoisomerism due to the restricted rotation about the planar carbon–carbon double bond
E / Z isomers have different atoms or groups of atoms attached to each carbon atom of a C=C bond
For example, 2-methylpropene cannot have E / Z isomers as the methyl groups are both on the same side of the C=C bond:
2-methylpropene
E / Z nomenclature is used to distinguish between the isomers
Z isomers have functional groups on the same side of the double bond/carbon ring
E isomers have functional groups on opposite sides of the double bond/carbon ring
To discuss E / Z isomers, we will use an alkene of the general formula C2R4:
The general alkene, C2R4
Cahn–Ingold–Prelog (CIP) priority rules
When the groups R1, R2, R3 and R4 are different (i.e. R1 ≠ R2 ≠ R3 ≠ R4), we use the E / Z naming system
This is based on Cahn-Ingold-Prelog (CIP) priority rules
To do this, we look at the atomic number of the first atom attached to the carbon in question
The higher the atomic number; the higher the priority
For example, 1-bromo-1-propen-2-ol has four different atoms or groups of atoms attached to the C=C bond
This means that it can have two different displayed formulae:
Compound A
Step 1: Apply the CIP priority rules
Look at R1 and R3:
Bromine has a higher atomic number than hydrogen so bromine has priority
Look at R2 and R4:
Oxygen has a higher atomic number than carbon so oxygen has priority
Step 2: Deduce E or Z
E isomers have the highest priority groups on opposite sides of the C=C bond, i.e. one above and one below
The E comes from the German word "entgegen" meaning opposite
Z isomers have the highest priority groups on the same side of the C=C bond, i.e. both above or both below
The Z comes from the German word "zusammen" meaning together
In compound A, the two highest priority groups are on opposite sides (above and below) the C=C bond
Therefore, compound A is E-1-bromo-1-propen-2-ol
Compound B
Step 1: Apply the CIP priority rules
Look at R1 and R3:
Bromine has a higher atomic number than hydrogen so bromine has priority
Look at R2 and R4:
Oxygen has a higher atomic number than carbon so oxygen has priority
Step 2: Deduce E or Z
In compound B, the two highest priority groups are on the same side (both below) the C=C bond
Therefore, compound B is Z-1-bromo-1-propen-2-ol
Worked Example
Name the following compound:
Answer:
The compound has a propene chain with a chlorine atom on carbon-1 and a bromine atom on carbon-2
2-bromo-1-chloropropene
The carbon on the left of the double bond:
Chlorine has a higher atomic number than hydrogen
Therefore, chlorine has a higher prioirity
The carbon on the right of the double bond:
Bromine has a higher atomic number than carbon
Therefore, bromine has a higher prioirity
The chlorine and bromine are on opposite sides (above and below) of the double bond, which means that this is an E isomer
Therefore, the overall name of the compound is E-2-bromo-1-chloro-1-propene
Examiner Tips and Tricks
You are expected to apply the CIP priority rules to naming E and Z isomers, as well as being able to draw the formulas of E and Z isomers.
Worked Example
Draw the displayed formula for E-2-bromobut-2-ene.
Answer:
The main carbon chain is 4 carbons long with a double bond between carbons 2 and 3
This is best shown as a carbon-carbon double bond with 2 methyl groups attached
There is also a bromine atom attached to carbon-2
On one side (left) of the double bond, the bromine atom has higher priority then the methyl group
On the other side (right) of the double bond, the carbon of the methyl group has higher priority than the hydrogen atom
Since this is an E-isomer the highest priority groups are on opposite sides (above and below) of the double bond
More complicated E / Z isomers
Compound X exhibits E / Z isomerism:
Compound X
Step 1: Apply the CIP priority rules
Look at R1 and R3:
Carbon is the first atom attached to the C=C bond, on the left hand side
Look at R2 and R4:
Carbon is the first atom attached to the C=C bond, on the right hand side
This means that we cannot deduce if compound X is an E or Z isomer by applying the CIP priority rules to the first atom attached to the C=C bond
Therefore, we now have to look at the second atoms attached
Look again at R1 and R3:
The second atoms attached to R1 are hydrogens and another carbon
The second atoms attached to R3 are hydrogens and bromine
We can ignore the hydrogens as both R groups have hydrogens
Bromine has a higher atomic number than carbon, so bromine is the higher priority
Therefore, the CH2Br group has priority over the CH3CH2 group
Look again at R2 and R4:
The second atoms attached to R2 are hydrogens
The second atoms attached to R3 are hydrogens and an oxygen
Oxygen has a higher atomic number than hydrogen, so oxygen is the higher priority
Therefore, the CH2OH group has priority over the CH3 group
Step 2: Deduce E or Z
In compound X, the two highest priority groups are on the same side (both below) the C=C bond
Therefore, compound X is the Z isomer
Examiner Tips and Tricks
In older exam papers and textbooks you may come across the terms:
Geometric isomer - this is an older term and should not be used
Cis / trans isomerism - this is still used but has been superceded by E / Z isomerism
Neither of these are required in exams.
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