13.2 Stereoisomerism
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What are stereoisomers?
They have the same structural formula but different arrangement of the atoms in space
Two types of isomerism
E/Z isomerism Optical isomerism
When does E/Z isomerism take place?
Occurs in compounds with a C=C double bond
Why does stereoisomerism arise?
Rotation around the double bond is restricted Groups attached to carbon atom are fixed relative to each other
What causes the rigidity between the C=C double bond?
The position of the pi bond’s electron density above and below the plane of the sigma bond
Conditions for E/Z isomerism
C=C double bond Different groups attached to each carbon atom of the double bond
Conditions for cis-trans isomers
C=C double bond Each Carbon must be attached to 2 different groups One of the groups on each Carbon atom of the double bond must be the same
Which isomer is cis and which is trans?
Cis isomer is the Z isomer Trans isomer is the E isomer
When can the cis-trans system of naming be used?
When each Carbon atom in the double bond is attached to a single hydrogen atom
Cahn-Ingold-Prelog rules
If the groups of higher priority are on the same side of the double bond, compound is the Z isomer If the groups of higher priority are diagonally placed across the double bond, compound is the E isomer
How is priority given?
Look at the atoms attached to the Carbon atoms and the one with higher atomic number is higher priority
If two atoms attached to a Carbon atom in a double bond are the same….
Find the first point of difference - the one with higher atomic number at this point is the higher priority group