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Geometric isomerism
Stereoisomerism
Structural isomerism are compounds with the same molecular formula but with a different structural formulae. We are now going to look at stereoisomerism.
Stereo is a reference to three dimensions.
Stereoisomers have the same structural formula but differ from each other because their atoms or groups are arranged differently in three dimensions.
Stereoisomerism can be classified into two types optical isomerism and geometric isomerism.
All the types of isomers you need to know:
Structural isomerism are compounds with the same molecular formula but with a different structural formulae. We are now going to look at stereoisomerism.
Stereo is a reference to three dimensions.
Stereoisomers have the same structural formula but differ from each other because their atoms or groups are arranged differently in three dimensions.
Stereoisomerism can be classified into two types optical isomerism and geometric isomerism.
All the types of isomers you need to know:
Geometric isomerism
Geometric isomers differ from each other because their atoms or groups are attached at different positions on opposite sides of a C=C double bond.
Will take But-2-ene as an example
CH3 - CH = CH - CH3
This structural formula doesn't tell us much about the geometric isomerism within the compound. We need to show the correct bond angles of 120 degrees.
Geometric isomers differ from each other because their atoms or groups are attached at different positions on opposite sides of a C=C double bond.
Will take But-2-ene as an example
CH3 - CH = CH - CH3
This structural formula doesn't tell us much about the geometric isomerism within the compound. We need to show the correct bond angles of 120 degrees.
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The left hand structure shows the two CH3 groups further apart from each other. they are across the molecule, so we add the abbreviation trans- (Latin for across) to the begining of the name to indicate this. Think of transatlantic, which means opposite ends of the Atlantic ocean.
We call this structure trans-but-2-ene
The right hand structure shows the two CH3 groups on the same side above or below the C=C double bond. We add the term cis-(latin for 'on this side') to the name of this compound.
We call this structure cis-but-2-ene
This type of isomerism can only exist in alkenes, as they need a C=C double bond.
We call this structure trans-but-2-ene
The right hand structure shows the two CH3 groups on the same side above or below the C=C double bond. We add the term cis-(latin for 'on this side') to the name of this compound.
We call this structure cis-but-2-ene
This type of isomerism can only exist in alkenes, as they need a C=C double bond.
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E-Z naming system
The only problem with cis-trans naming system is that it only works with some compounds.
When there are different atoms attached to the C=C, the idea of two identical groups or atoms being in a cis- or trans- arrangement cannot work. So we use the E-Z system. For example
The only problem with cis-trans naming system is that it only works with some compounds.
When there are different atoms attached to the C=C, the idea of two identical groups or atoms being in a cis- or trans- arrangement cannot work. So we use the E-Z system. For example
So how do we decide if it's a E or Z isomer?
1. Work out the name using nomenclature rules 1-bromo-2-chloro-1-fluoroethene
2. Use the priority rule check which atom on each side of the C=C has the highest atomic number.
In this case it's bromine on the left hand side (Br = 35 and F = 9)
on the right hand side it's Chlorine (H = 1 and Cl = 17)
3. Now decide where the two atoms with the highest priority are in relation with each other. If both are above the double bond then it's a Z-Isomer. If one is above and the other below it's a E-isomer
1. Work out the name using nomenclature rules 1-bromo-2-chloro-1-fluoroethene
2. Use the priority rule check which atom on each side of the C=C has the highest atomic number.
In this case it's bromine on the left hand side (Br = 35 and F = 9)
on the right hand side it's Chlorine (H = 1 and Cl = 17)
3. Now decide where the two atoms with the highest priority are in relation with each other. If both are above the double bond then it's a Z-Isomer. If one is above and the other below it's a E-isomer
How to remember them:
E = Enemies - as far apart as possible
E = Enemies - as far apart as possible
