Optical Isomerism
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What is an isomer?
Same molecular formula but different structure.
What are the three types of structural isomers?
Position isomer Chain isomer Functional group isomer
What is a position isomer?
Compounds with the same molecular formula but different structures due to different positions of the same functional group on the same carbon skeleton.
What is a chain isomer?
Compounds with the same molecular formula but different structures of the carbon skeleton.
What is a functional group isomer?
Compounds with the same molecular formula but with atoms arranges to give different functional groups
What is stereoisomerism?
Same structural formula but different arrangement of atoms in space.
What are the two types of stereoisomers?
E-Z isomers Optical isomers
When does optical isomerism occur?
When there are four different substituents attached to one carbon atom.
What does optical isomerism result in?
Two isomers that are non-superimposable mirror images of one another.
Why are they called optical isomers?
They rotate plane polarised light by 90 degrees in opposite directions.
What are two optical isomers called?
Enantiomers
What is the chiral centre/asymmetric carbon atom?
The carbon bonded to four different groups. REMEMBER ASYMMETRIC CARBON IS A CHIRAL CARBON!
What is the chiral centre represented by?
*
What is a racemic mixture?
A mixture containing a 50/50 mixture of the two isomers
Why is a racemic mixture not optically active?
The effects of the two isomers cancel out.
How is an racemate formed from a carbonyl containing compound? What kind of reaction is this?
Add Hydrogen Cyanide across the C=O bond. Nucleophilic addition reaction.
What happens after the racemate is formed in the nucleophilic addition reaction? What kind of reaction is this?
The optical isomer (nitrile) reacts with water acidified with dilute HCl and the CN group is replaced by a COOH group. NH4Cl is formed. Hydrolysis reaction.
What are the similarities between two optical isomers?
Same atoms and bonds. Non-superimposable mirror images of one another. Rotate plane of polarisation by the same angle but in different directions.
What are the differences between two optical isomers?
Not identical in chemical properties Differ in the way they rotate plane polarised light.
Give an example of a chiral molecule.
Alpha amino acids e.g. lactic acid (2-hydroxypropanoic acid)
How is light polarised?
By passing it through a polaroid filter so oscillations are only in one plane.
What effect does the racemic mixture have on plane polarised light?
None as the rotation by each enantiomer cancels out to nothing.
What effect does the positive isomer have on plane polarised light?
Rotates plane of polarisation by x degrees clockwise.
What effect does the negative isomer have on plane polarised light?
Rotates plane of polarisation by x degrees anticlockwise.