3.3.7 Optical isomerism Flashcards
Define optical isomerism
form of stereoisomerism, they have the same structural formula but different arrangement of atoms in space
What is a chiral molecule
has 4 different groups attached to a carbon atom
Define enantiomer
2 different arrangements of the 4 different groups attached to the chiral carbon atom
these enantiomers are non-superimposable, mirror images of each other
What are optical isomers
mirror images of each other and are non-superimposable
What are optical isomers
mirror images of each other and are non-superimposable
How do you find enantiomers from a structural formula?
- find the carbon atom with 4 DIFFERENT GROUPS surrounding it (chiral atom)
- draw this in a tetrahedral 3D shape
- draw the mirror image of this molecule to find the 2nd enantiomer
How exactly do you find the difference between enantiomers?
- using plane polarised light
-optical active isomers will rotate plane polarised light
-standard light which oscillates in all directions is polarised with a polarising filter to produce a plain polarised light ( light oscillating on one plane only)
- plain polarised light is shone through the optically active compound
-depending on the enantiomer present, it will rotate the plain polarised light clockwise or anticlockwise (either way they will rotate by the same value of degrees)
What is a racemic mixture
when we have an equal amount of each enantiomer
Why do racemates not rotate plane polarised light despite being optically active isomers?
the 2 enetiomers rotate light in opposite directions by the same degree and they cancel out
When is a racemic mixture of a chiral general produced?
by reacting ACHIRAL substances together
Why are isomers typically made in equal quantities?
Because there is 50/50 chance of either hydrogen being replaced
Why are molecules with planar profiles such as c=c, c=o able to make racemic mixtures?
-these reactions occur where we have an attack on the carbonyl group of unsymmetrical ketones and aldehydes (c=0/c=c are planar)
- the nucleophiles have a 50/50 option of attacking from above or below the chiral centre/delta positive carbon atom hence forming even amounts of 2 different enantiomers = racemic mixture
