3.3.7 Optical Isomerism Flashcards
What is a chiral (or asymmetric) carbon?
Carbon atom that 4 different groups has attached to it
What are optical isomers called?
enantiomers
Enantiomers are _______ ____ ____
non-superimposed mirror images
Draw 2-hydroxypropanoic acid and indentify its chiral centre
Draw the 2 enantiomers of 2-hydroxypropanoic acid
Plane-polarised light only vibrates in ___ direction
one
Optical isomers are optically active. What is meant by this?
They rotate plane-polarised light
Describe what happens when enantiomers rotate plane-polarised light
- One enantiomer rotates it in a clockwise direction
- & other rotates it in an anticlockwise direction
What does a racemic mixture contain?
Racemic mixture contains equal quantities of each enantiomer of an optically active compound
Racemates don’t show any ____ ______
optical activity
Why don’t racemates show any optical activity?
2 enantiomers cancel each other’s light-rotating effect
How do chemists get a racemic mixture of a chiral product? Explain why this works
- React 2 achiral things together
- ∵ when 2 molecules react there’s an equal chance of forming each of enantiomers
Explain how reacting butane and chlorine (2 achiral compounds) yields a racemic mixture of a chiral product
- Cl atom replaces on the H atoms
- Either H atom can be replaced so reaction produces mixture of 2 possible enantiomers
- Each H has 50-50 chance of being replaced = 2 optical isomers formed in equal amounts
What is the disadvantage of using chemical methods to modify a reaction to produce a single enantiomer?
It’s difficult and expensive
Doubles bonds (C=O and C=C) are _____
planar (flat)
When is a racemate formed in a reaction mechanism?
When reactant or intermediate has a trigonal planar group in molecule is approached from both sides by an attacking species
e.g. Nucleophilic addition of HCN to aldehydes and ketones (unsymmetrical)
Draw the 1st stage of the mechanism for the reaction of propanal with acidified potassium cyanide (KCN)
When propanal is reacted with acidified potassium cyanide (KCN), why is there an equal amount of each enantiomer formed?
C=O bond is planar = equal chance that nucleophile will attack from above the plane of the molecule or from below it
Why does the product(s) of nucleophilic addition of HCN to symmetrical ketones not display optical isomerism?
Make a product that doesn’t have a chiral centre
Name another mechanism where a racemate can be formed
electrophilic addition of HBr to an unsymmetrical alkene
