3.3.7-Optical isomerism (PAPER 2)
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What is optical isomerism?
A form of stereoisomerism- where isomers have same structural formula but different arrangement of atoms in space. Isomers are mirror images of eachother and have chiral carbon atom.
What is a chiral molecule? enantiomers
One with 4 different groups attached to a C atom. These groups can be arranged in 2 different ways forming 2 different molecules- enantiomers.
What are enantiomers?
Mirror images of eachother which are non-superimposable- regardless of how you turn them they will not overlap.
How to draw a molecule as an enantiomer?
Find the chiral centre- the C atom with 4 different groups surrounding it Draw this using the tetrahedral 3D shape Draw the mirror image to show both enantiomers- a reflection on the right with functional groups inverted
What are optically active enantiomers?
These rotate plane polarised light- which is a method of detecting optically active compounds.
Describe plane polarised light
Standard light oscillates in all directions so we pass the light through a polaroid filter to produce plain polarised light, this light only oscillates in one direction.
How to use plane polarised light to determine enantiomers?
Optically active compounds rotate the plane polarised- one enantiomer rotates the light clockwise, the other rotates it anticlockwise by the same degree.
What is a racemic mixture?
An equal amount of each enantiomer.
Why do racemates not rotate plane polarised light- Why are they optically inactive?
The 2 enantiomers rotate light equally in opposite directions so cancel out.
How to make a racemic mixture of a chiral product?
Reacting achiral (not chiral) substances together, making an even chance of forming each enantiomer.
Draw the 2 possible enantiomers when butane reacts with bromine
2 different structures formed where different H has been replaced by Br. H H3C C Br C2H5 Br H3C C H C2H5
Which molecules can make racemic products?
Molecules with planar profiles such as double bonds in C=C and C=O
Draw the reaction between propanal (C3H6O) and acidified potassium cyanide KCN- attack on carbonyl group (C=O)
Draw the C3H6O molecule, with aldehyde group C=O part is planar so CN- ion can attack delta positive C from either above or below forming 2 different enantiomers. Even chance of nucleophile attacking from top and bottom: 50/50 mixture of both enantiomers so racemic mixture of products. OH H C CN C2H5 OH NC C H C2H5