Isomers - Geometric (Edexcel International A Level Chemistry): Revision Note
Isomers - Geometric
One of the features of alkenes is their stereoisomerism or geometric isomerism, which can be classed as E / Z or cis / trans
E / Z isomerism is an example of stereoisomerism where different atoms or groups of atoms are attached to each carbon atom of the C=C bond
Cis / trans isomerism is a special case of E / Z isomerism where two of the atoms or groups of atoms attached to each carbon atom of the C=C bond are the same
Cis / trans isomers
In saturated compounds, the atoms / functional groups attached to the single, σ-bonded carbons are not fixed in their position due to the free rotation about the C-C σ-bond
In unsaturated compounds, the groups attached to the C=C carbons remain fixed in their position
This is because free rotation of the bonds about the C=C bond is not possible due to the presence of a π bond
Cis / trans nomenclature can be used to distinguish between the isomers
Cis isomers have two functional groups on the same side of the double bond / carbon ring, i.e. both above the C=C bond or both below the C=C bond
Trans isomers have two functional groups on opposite sides of the double bond / carbon ring, i.e. one above and one below the C=C bond
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The presence of a π bond in unsaturated compounds restricts rotation about the C=C bond forcing the groups to remain fixed in their position and giving rise to the formation of certain configurational isomers
Deducing E / Z Isomers
Naming cis / trans isomers
For cis / trans isomers to exist, we need two different atoms or groups of atoms on either side of the C=C bond
This means that 2-methylpropene cannot have cis / trans isomers as the methyl groups are both on the same side of the C=C bond:
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2-methylpropene molecules do not have cis / trans isomers
However, moving one of the methyl groups to the other side of the C=C bond causes cis / trans isomerism:
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But-2-ene does have cis / trans isomers
The atoms or groups of atoms on either side of the C=C bond do not have to be the same for cis / trans isomers:
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1-chloroprop-1-ene also shows cis / trans isomerism
However, the cis / trans naming system starts to fail once we have more than one atom or group of atoms on either side of the C=C bond
The cis / trans naming system can still be used with three atoms / groups of atoms but only if:
Two of the three atoms or groups of atoms are the same
These two atoms or groups of atoms are on opposite sides of the double bond
1,2-dichloropropene can be named using cis / trans
The cis / trans naming system cannot be used with three atoms / groups of atoms when they are all different
This requires the use of the E / Z naming system
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1-bromo-2-chloropropene cannot be named using cis / trans
E / Z isomers
To discuss E / Z isomers, we will use an alkene of the general formula C2R4:
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The general alkene, C2R4
When the groups R1, R2, R3 and R4 are all different (i.e. R1 ≠ R2 ≠ R3 ≠ R4), we have to 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:
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1-bromo-1-propen-2-ol (compounds A and B)
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
More complicated E / Z isomers
Compound X exhibits E / Z isomerism:
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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
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